Technical Basis Document No. 8: Colloids Revision 2 September 2003 1. INTRODUCTION This technical basis document provides a summary of the conceptual understanding relevant to the projected behavior of radionuclides potentially attached to colloids as they are transported through the engineered and natural features of the Yucca Mountain repository system. This is one in a series of technical basis documents that are being prepared for the different components relevant to predicting the likely postclosure performance of the Yucca Mountain repository. The relationship of colloid formation, stability, and transport to the other components is illustrated in Figure 1-1. Figure 1-1. Components of the Postclosure Technical Basis for the License Application Colloids and colloidal transport processes are treated as an integrated component of the postclosure technical basis because aspects of their behavior could impact the performance of the various engineered and natural features of the repository system. Therefore, although colloids are not a feature of the repository system in the same way as the waste package, unsaturated zone and saturated zone are, treating colloids as an integrated component assures proper consistency of their treatment across these features. The information presented in this technical basis document, along with the associated references, provides a summary-level synthesis of the relevant aspects of colloid-facilitated radionuclide September 2003 1-1 No. 8: Colloids Revision 2 transport and forms an outline of the ongoing development of the postclosure safety analysis that will be included in the license application (LA). This information is also used to respond to open Key Technical Issue (KTI) agreements made between the U.S. Nuclear Regulatory Commission (NRC) and the U.S. Department of Energy (DOE). Placing the DOE responses to individual KTI agreement and additional information needed requests within the context of the overall colloid transport processes, allows for a more direct discussion of the relevance of the agreement. This technical basis document and appendices are responsive to agreements made between the DOE and the NRC during Technical Exchange and Management Meetings on Evolution of the Near-Field Environment (Reamer 2001a), Radionuclide Transport (Reamer and Williams 2000), and Total System Performance Assessment and Integration (Reamer 2001b). The appendices to this document are designed to allow for a transparent and direct response to each KTI agreement. This technical basis document presents a summary and synthesis of the detailed technical information presented in the analyses and model reports and other technical products that are used as the basis for the description of the waste form, invert, unsaturated zone, and saturated zone features and the incorporation of those features into the postclosure performance assessment. Several analyses, model reports, and other technical products support this summary: • Site-Scale Saturated Zone Transport (BSC 2003a) • Saturated Zone Colloid Transport (BSC 2003b) • Saturated Zone Flow and Transport Model Abstraction (BSC 2003c) • Saturated Zone In-Situ Testing (BSC 2003d) • Site-Scale Saturated Zone Flow Model (BSC 2003e) • EBS Radionuclide Transport Abstraction (BSC 2001a) • Radionuclide Transport Models Under Ambient Conditions (BSC 2001b) • Unsaturated Zone Flow Models and Submodels (BSC 2001c) • Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003f) • Dissolved Concentration Limits of Radioactive Elements (BSC 2003g) • In-Package Chemistry Abstraction (BSC 2003h) • Particle Tracking Model and Abstraction of Transport Processes (BSC 2003i) • In Situ Field Testing of Processes (BSC 2001d) • Advection Versus Diffusion in the Invert (BSC 2003j) • Engineered Barrier System: Physical and Chemical Environment Model (BSC 2003k). September 2003 1-2 No. 8: Colloids Revision 2 1.1 SUMMARY OF CONCEPTUAL UNDERSTANDING Colloids are very fine particles ranging in size from 1 nm to 10 µm in characteristic dimension. Colloid transport may affect the rate of migration of contaminants in the subsurface environment relative to the transport of the contaminants as dissolved species. Potentially faster movement of colloids is the result of complex physical, chemical, electrical, and hydrodynamic phenomena acting on the colloids. These are phenomena that typically have not been associated with the transport of dissolved species. The potentially faster transport characteristic of colloids is generally referred to as the hydrodynamic chromatography effect (Haber and Brenner 1993). Radioactive colloids (i.e., colloids that carry radionuclides) have been classified as either Type I colloids (either true or intrinsic) or Type II colloids (pseudocolloids). This classification was adopted to distinguish colloids resulting from nucleation or precipitation of dissolved radionuclides under chemically saturated conditions, which generate radionuclides in particulate form (true colloids), from colloids carrying radionuclides due to sorption of dissolved radionuclides onto natural or other colloids suspended in groundwater (pseudocolloids). This classification is used in this technical basis document. There are several potential sources of colloids in groundwater, such as clay minerals, metal oxides, viruses, bacteria, and humic macromolecules (Chrysikopoulos and James 2003). It has been suggested (Haber and Brenner 1993) that dissolved groundwater contaminants could have a large affinity to sorption onto the surface of suspended colloids. As a result, colloids can possibly act as fast carriers for contaminants in subsurface environments and can be significant contributors to the rate of migration of contaminants, such as radionuclides potentially released from a repository system. In order for colloids to provide a significant means for the transport of radionuclides from the waste form to the biosphere and, consequently, be a major contributor to dose, colloidal suspensions must: (1) contain high enough concentrations of colloids to carry a significant amount of radionuclides, either embedded in the colloids or attached on the surface of the colloids, relative to the concentration of dissolved radionuclides; (2) be stable for distances in the order of tens of kilometers and for time periods in the tens of thousands of years; and (3) not be appreciably filtered, reversibly or irreversibly, by the host rock. If one or more of these conditions are not met, colloids will not be significant to system performance of a high-level radioactive waste repository. The main conclusion of this technical basis document is that colloid-facilitated radionuclide transport is not an important contributor to predicted system performance compared to transport of dissolved radionuclides. This conclusion is consistent with the opinions expressed by the International Review Team, which conducted a peer review of the total system performance assessment (TSPA) for the site recommendation. In its report, the International Review Team concluded that overly conservative assumptions may have been used in the modeling of colloid transport through the unsaturated zone (OECD and IAEA 2002, p. 39) and the saturated zone (OECD and IAEA 2002, p. 43). As a result of the conservatisms used in the modeling of colloid transport of radionuclides, the International Review Team concluded that the importance of colloid transport’s contribution to dose could have been “over rated.” September 2003 1-3 No. 8: Colloids Revision 2 1.2 PURPOSE The purpose of this technical basis document is to present a screening of processes important to colloid-facilitated transport of radionuclides in the Yucca Mountain engineered and natural barrier subsystems that will demonstrate that the presence of colloids will not lead to the early and/or significant discharge of radionuclides to the biosphere, and hence, will not play a major role in the system performance. Screening arguments will be presented for each of the subsystems, from the waste forms within the waste package to the boundary between the saturated zone and the biosphere. The technical basis supporting the process screening comprises the general theory of colloid stability, filtration, and radionuclide partitioning as documented in the scientific literature, and the results of Yucca Mountain Project (YMP) science activities that have studied the relevant physical/chemical processes for colloid generation in the waste package and transport of colloids from the waste package through the invert, unsaturated, and saturated zones. 1.3 SUMMARY CONCLUSIONS To determine the importance of colloid-facilitated radionuclide transport to system performance, the pertinent processes regarding colloid transport in four repository subsystems are examined: waste package, invert, unsaturated zone and saturated zone. Some radionuclides (e.g., radionuclides of plutonium and americium) may be transported primarily in colloidal form during the 10,000-year regulatory period, while others are primarily transported as dissolved species (e.g., technetium, iodine, and neptunium). As supported by the discussions in Sections 3, 4, 5, and 6 of this technical basis document, different processes impact the concentration of colloids in different repository subsystems. Within the drift environment, the key controlling factors are those that determine the stability of the colloidal suspensions. These factors are related to chemical conditions (such as high ionic strengths for the smectite colloids and a pH-ionic strength combination for the iron oxyhydroxide colloids) that cause the suspensions of both types of those colloids to be unstable during the first 2,000 to 3,000 years following waste emplacement. The instability of the colloid suspensions results in the colloids settling out. These same factors will also be the main contributors to the instability of colloid suspensions in the invert and the perturbed near field. Within the unsaturated zone and the saturated zone, filtration mechanisms will be the major contributors to reducing the concentration of colloids. Within the unsaturated zone, the interface between adjacent geologic units with sharply contrasting hydrologic properties will act as a barrier inhibiting colloids from moving through the downstream unit. Figure 1-2 shows qualitatively the impact of the different phenomena that lead to a decrease in the number of colloids in the repository subsystems, from the waste package to the biosphere. The impact of the reducing concentration of colloids on radionuclide releases to the biosphere is shown qualitatively in Figure 1-3. Among all of the radionuclides, americium and plutonium are the ones of biggest concern with respect to colloidal transport because of their high affinity to attach to particle/solid surfaces. In Figure 1-3, the dissolved concentration of these two radionuclides is qualitatively compared to their concentration associated with colloids for the different repository subsystems, from the drift to the saturated zone. Because radionuclides that September 2003 1-4 No. 8: Colloids Revision 2 adsorb to natural colloids may also adsorb to immobile material coated with secondary minerals, these radionuclides are eventually removed from the transporting fluid. The colloids of concern represent only a small fraction of waste degradation or rock alteration products. Figure 1-3 shows that the concentration of smectite colloids qualitatively drops significantly within the waste package until it reaches a constant value. This constant value qualitatively represents the natural concentration of smectite colloids available for reversibly sorbing radionuclides. The drop in the concentration of smectite colloids within the waste package is due to higher temperature and high ionic strength rendering the colloids unstable. Mills et al. (1991) concluded that metals strongly adsorbing onto natural colloids may also strongly adsorb onto the immobile matrix. It is thus anticipated that dissolved radionuclides, such as technetium and iodine, which are highly soluble, will be significantly more important to system performance than americium or plutonium, which are strongly associated with colloids. The screening arguments discussed in Sections 3, 4, 5, and 6 of this technical basis document support the conclusion that colloid-facilitated transport of radionuclides during the regulatory period is not as significant as transport of dissolved species for each of the four subsystems discussed. Consequently, colloid-facilitated radionuclide transport will not be significant to system performance. The discussion in this technical basis document comprises a screening argument for processes governing colloid-facilitated transport of radionuclides at the Yucca Mountain repository. However, it is not suggested that colloid-facilitated transport be excluded from the TSPA. Rather, it is concluded that, on a risk basis, colloid-facilitated transport is not a significant contributor to system performance; therefore, uncertain technical issues associated with this transport mode are of relatively low importance compared to uncertain technical issues related to radionuclide transport as dissolved species. It is also concluded that, for purposes of the TSPA, the current treatment of colloid-facilitated transport of radionuclides is adequate. 1.4 ORGANIZATION OF TECHNICAL BASIS DOCUMENT In addition to this introduction, this technical basis document consists of the following sections: • Section 2—Colloid Stability, Filtration, and Radionuclide Partitioning: General Theory • Section 3—Waste Package • Section 4—Invert • Section 5—Transport of Colloids in Unsaturated Zone • Section 6—Colloid Transport in the Saturated Zone • Section 7—Summary • Section 8—References • Appendix A—Exclusion of Entrained Colloids in Thermal-Chemical Alteration (Response to ENFE 1.06, ENFE 4.04 and GEN 1.01 (Comment #35)) September 2003 1-5 No. 8: Colloids Revision 2 • Appendix B—Sensitivity Analysis of Colloid Transport Parameters (Response to ENFE 4.06 AIN-1 and GEN 1.01 (Comments 35 and 37)) • Appendix C—Screening Out Coupled Thermal-Hydrologic-Chemical Effects (Response to ENFE 4.03 and GEN 1.01 (Comments 35 and 37)) • Appendix D—Contrasting Colloid Concentrations in the Engineered Barrier System and Saturated Zone (Response to TSPAI 3.30 and GEN 1.01 (Comments 43 and 46)) • Appendix E—Sensitivity Studies to Test Importance of Colloid Transport Parameters and Models (Response to RT 3.07 and GEN 1.01 (Comments 35, 43, and 46)) • Appendix F—Transport of Dissolved and Colloidal Radionuclides Through Invert (Response to TSPAI 3.17 and GEN 1.01 (Comments 36 and 38)) • Appendix G—Screening Criteria for Attachment of Radionuclides to Colloids (Response to RT 1.03 AIN-1, ENFE 3.05 AIN-1 and ENFE 4.05 AIN-1) • Appendix H—Changes in Colloid Concentrations due to Shifts in pH and Ionic Strength (Response to TSPAI 3.42) 1.5 NOTE REGARDING THE STATUS OF SUPPORTING TECHNICAL INFORMATION This document was prepared using the most current information available at the time of its development. This technical basis document and its appendices providing KTI agreement responses that were prepared using preliminary or draft information reflect the status of the Yucca Mountain Project’s scientific and design basis at the time of submittal. In some cases, this involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the AMRs and other references will be reflected in the LA as the approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either this technical basis document or its KTI agreement appendices to reflect changes in the supporting references prior to submittal of the LA. September 2003 1-6 No. 8: Colloids Revision 2 September 2003 Figure 1-2. Processes Impacting Colloid Concentration from Drift to Biosphere Resulting in a Consistent Decrease in Colloid Concentration 1-7 No. 8: Colloids Figure 1-3. No. 8: Colloids Revision 2 Schematic Representation of Radionuclide Concentrations (Plutonium and Americium) as Dissolved Species and Colloids from Drift to Biosphere September 2003 1-8 Revision 2 2. COLLOID STABILITY, FILTRATION, AND RADIONUCLIDE PARTITIONING: GENERAL THEORY In this section, the general theory on the stability, transport and filtration, and sorption of radionuclides onto colloids is presented. A summary section is included presenting an overall picture of the potential impact of colloids on the performance of the repository system. This Section 2 is primarily based on a review of publications in the open literature. As appropriate, relevant information from YMP science activities is included to support the conclusions presented. It should be noted that the open literature, as well as that pertaining to repository systems has dealt mainly with colloid transport through saturated porous and fractured media. 2.1 STABILITY OF COLLOIDAL SUSPENSIONS A key condition for colloid-facilitated transport of radionuclides to be a significant contributor to repository performance is the ability of the colloidal suspension to remain stable for distances of tens of kilometers and over tens of thousands of years. The stability of colloidal suspensions against flocculation (also referred to as agglomeration or coagulation) of the colloids and deposition is governed by intercolloidal repulsive electrical forces, and these are a function of colloid number, density, size, composition, and surface chemistry (Haber and Brenner 1993), and attractive London-van der Waals forces that depend on the intercolloid separation distance and colloid size (Guzy et al. 1983; Elimelech and O’Melia 1990). In the 1940s and 1950s, the Derjaguin and Landau, and Verwey and Overbeek (Elimelech and O’Melia 1990) theory was used to explain colloid stability. The Derjaguin, Landau, Verwey and Overbeek theory defined the total interaction energy between colloids as a function of the separation distance between colloids. The total interaction energy is the sum of the energy of the electric double layer and the London-van der Waals energy. In addition, physical effects, such as flow rate and gravitational forces, and chemical conditions (the latter determining the colloid surface charge) also influence the intercolloid interactions that could lead to flocculation and gravitational settling of the particles (Guzy et al. 1983; Elimelech and O’Melia 1990). While gravitational settling is directly proportional to the third power of particle size, larger colloids tend to form more stable suspensions in natural systems due to size-exclusion effects (Haber and Brenner 1993). However, this should not be interpreted to mean that colloids, particularly in the higher end of the size spectrum (i.e., about 10 µm in diameter), will be stable over the temporal and spatial scales of relevance to repository performance. Several researchers have concluded that it is difficult to envision colloidal suspensions remaining stable, and therefore mobile, for large distances and long times (Mills et al. 1991; van der Lee et al. 1994). Elimelech and O’Melia (1990) have concluded that even for very stable colloidal suspensions, the attachment of the colloids to the wall of the flow channel may be significant. The interaction between colloids is governed by the sum of the electric repulsion force due to identical surface charges (known as the electric double-layer force) and the attractive London-van der Waals force (Elimelech and O’Melia 1990). The electric double-layer force stems from approaching colloids having equal surface charges. Each colloid is surrounded by a layer of oppositely charged ions that counterbalances the colloid surface charge. The September 2003 2-1 No. 8: Colloids Revision 2 London-van der Waals attractive force is a function of distance between approaching colloids and the colloids’ size. It has been shown that the strength of the London-van der Waals force is inversely proportional to the distance between the colloids and the colloid size (Guzy et al. 1983). That is, the closer the colloids are to each other and the smaller they are, the higher the strength of the attractive force. There are several factors that affect the strength of the repulsive electric double-layer force; the zeta potential (a measure of the surface charge), colloid size, and ionic strength and temperature. Ionic strength and temperature have a considerable impact on the thickness of the electrical double layer (Guzy et al. 1983). The double-layer thickness is inversely proportional to the square root of temperature and to the square root of the ionic strength (Guzy et al. 1983, Equations 47 and 48). Thus, higher ionic strengths and higher temperatures lead to thinner electric double layers and hence weaker repulsive forces between colloids. As temperature and ionic strengths increase, the repulsive force decreases allowing the attractive London-van der Waals forces to dominate colloidal particle interactions. These results are consistent with others reported in the literature. For example, Langmuir (1997, p. 439) states that a colloidal suspension’s stability ratio is inversely proportional to temperature. In Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003f), experimental results demonstrate that colloid suspensions become more unstable with increasing ionic strength. As discussed in Section 3, during the first one-third of the 10,000-year postclosure regulatory period, the high temperature will result in water evaporation and increased ionic strengths rendering colloids important to radionuclide transport unstable. This will decrease the concentration of colloids either carrying or able to carry radionuclides for the waste package. Even after 10,000 years, the temperature within the emplacement drifts is expected to be 20° to 25°C higher than the host-rock ambient temperature (BSC 2003l, Figure 6.5.2-1). 2.2 COLLOID FILTRATION As colloids travel along the flow path from the repository to the biosphere, they will undergo interactions with the host rock. Larger colloids are likely to be physically trapped within the pores in the rock matrix. If colloids are bigger than the pore size through which they are attempting to pass, the particles will not be able to get past the pore entrance. Smaller colloids that can enter the pore are likely to come in very close proximity with each other to overcome any possible repulsive forces between the colloids and flocculate, thus, forming larger particles that will then get trapped within the pores. The smaller colloids (i.e., diameter less than 1 µm) will be subjected to Brownian deposition onto the rock matrix (Haber and Brenner 1993), thus, reducing the number of colloids entrained in the flow. Colloid filtration is also called colloid attachment or colloid deposition. There are many different studies regarding the transport of and deposition of colloidal particles through fractures under saturated flow conditions; some examples are Haber and Brenner (1993), van der Lee et al. (1994), Chrysikopoulos and James (2003), Chrysikopoulos and Abdel-Salam (1997), and James and Chrysikopoulos (1999, 2000, 2003). There have been other studies aimed at understanding the kinetics of colloid interactions leading to deposition on the surfaces of immobile phases in September 2003 2-2 No. 8: Colloids Revision 2 porous media. Some examples are Guzy et al. (1983), Elimelech and O’Melia (1990), and Adamczyk et al. (1992a, 1992b). Even though the latter works were for porous media, the kinetic models apply equally to the interactions between colloids and fracture surfaces because (1) the ratio of the characteristic dimension of the colloids to the characteristic dimension of the immobile phase was considered to be much smaller than one, and (2) the interactions between colloids and the immobile phases are only important at separation distances much smaller than the colloid characteristic dimension. Consequently, as the colloids approach the immobile phase, the immobile phase looks and behaves like a planar surface from the perspective of the colloids. As colloids move down a fracture, they will be affected by (1) hydrodynamic and physical forces in the directions parallel to the fracture walls and (2) external physical, chemical, and electrical forces in the direction normal to the fracture walls. Hydrodynamic forces account for the drag that the flow exerts on the colloids leading to entrainment of the colloids in the flow and gravitational effects. External forces include gravitational effect, London-van der Waals attractive force, and electric double-layer force. In addition, the external forces account for lubrication effects that colloids are subjected to as they approach the immobile phase. Finally, colloids are subjected to Brownian motion, which is typically modeled using an equivalent diffusivity. The Brownian diffusivity is directly proportional to temperature and inversely proportional to colloid size (Guzy et al. 1983). The effects of Brownian motion are more pronounced for colloids of less than 1 µm in characteristic dimension. The theoretical studies regarding colloid transport through porous and fractured media are generally consistent regarding the effect of colloid size on transport times. Larger colloids are transported at a faster rate than smaller ones, with the latter showing a larger propensity to deposit onto the immobile rock. Comparisons of experimental and theoretical results for colloid deposition reported by Elimelech and O’Melia (1990) show increased deposition rates measured experimentally than predicted theoretically. Those experimental results also suggest that in natural waters colloid deposition is independent of size. Elimelech and O’Melia (1990) attribute this discrepancy to failure of the Derjaguin, Landau, Verwey and Overbeek theory to account for the dynamic interactions between colloids and the immobile phase. Colloid filtration will be an important phenomenon resulting in the decrease of the colloid concentration, particularly within the unsaturated zone directly below the repository. 2.3 RADIONUCLIDE PARTITIONING BETWEEN COLLOIDS AND DISSOLVED SPECIES Radionuclides can potentially be transported from the waste form to the biosphere as either dissolved species or in colloidal form. In order for colloid transport to be a significant contributor to the discharge of radionuclides to the biosphere, colloids must carry a significant mass of radionuclides. As discussed in Section 3, the most likely forms of radioactive colloids under the expected conditions at the Yucca Mountain repository are: (1) colloids with embedded radionuclides resulting from the degradation of defense high-level radioactive waste glass, referred to as “waste-form colloids”; (2) colloids formed as a result of corrosion of steel components within the September 2003 2-3 No. 8: Colloids Revision 2 repository, referred to as “iron oxyhydroxide colloids”; and (3) natural groundwater colloids, referred to as “smectite colloids.” The formation of colloids due to hydrolysis or polymerization of dissolved actinides (referred to as “true” or “intrinsic colloids”) is not considered because recent experiments have demonstrated that these colloids dissolve under the expected repository conditions (see Section 3 of this technical basis report). First, in the tests performed at Argonne National Laboratory for degradation of defense high-level radioactive waste glass (Ebert 1995, Sections 6.2.1 and 6.2.2), no evidence was found of the formation of true colloids. Second, degradation of commercial spent nuclear fuel (CSNF) may form true colloids close to the fuel surface where the fluid may be saturated with respect to uranium, but these true colloids are likely to dissolve in unsaturated fluid. Besides the mass of radionuclides that can be embedded in waste-form colloids from the degradation of defense high-level radioactive waste glass, dissolved radionuclides can be adsorbed onto the surface of colloids to form “pseudocolloids.” Only plutonium and americium are expected to be embedded in colloids resulting from the degradation of defense high-level radioactive waste glass. Thus, the discussion herein focuses on the sorption of dissolved radionuclides onto the surface of available colloids. A key consideration in the assessment of colloid contribution to radionuclide transport is determining the partition coefficients between interstitial solvent (i.e., dissolved in the groundwater), the immobile phase of the host rock (i.e., sorption within the porous rock and the fracture surfaces), and the surfaces of the mobile colloids (Haber and Brenner 1993). Transport of groundwater colloids is likely to be preferentially through fractures in the unsaturated zone (see Section 5) and in the fractured volcanics within the saturated zone (Section 6). The stability of a colloidal suspension increases with particle size because of the reduced number of particles per unit volume that can collide (i.e., the larger the colloids the more stable the suspension will be and the higher the probability that the colloids will remain entrained in the groundwater flow) (Haber and Brenner 1993). However, larger colloids are unlikely to be effectively transported through the unsaturated zone and the volcanic fractures in the saturated zone in the vicinity of Yucca Mountain due to simple geometric considerations (BSC 2003m, Section 6.18). Larger colloids are more likely to be physically trapped in the pore constrictions and do not penetrate the porous matrix. As noted in Radionuclide Transport Models Under Ambient Conditions (BSC 2003m), colloids are discharged to the unsaturated zone directly into fractures for the model elements corresponding to the repository. During transport through fractures as both dissolved species and in colloidal form, radionuclides undergo (1) sorption of dissolved radionuclides onto the fracture surfaces, (2) sorption of dissolved radionuclides onto the porous matrix following diffusion, (3) sorption of dissolved radionuclides onto the surface of colloids, (4) attachment of radioactive colloids onto the surface of fractures (filtration), (5) transport as dissolved species and as colloids, (6) diffusion of dissolved radionuclides from the fractures into the porous matrix, and (7) decay. The conservation equation for each radionuclide needs to account for all of these phenomena to adequately estimate the amount of radioactive species discharged to the biosphere. Diffusion of the larger, more stable colloids into the rock matrix is not considered an important phenomenon because of the very low probability of the larger colloids being able to physically penetrate into the pores (BSC 2003m, Section 6.18). September 2003 2-4 No. 8: Colloids Revision 2 Sorption of dissolved radionuclides onto the fracture surfaces and onto the rock matrix after diffusion has been traditionally treated using an equilibrium isotherm model and defined by a distribution coefficient. Diffusion of dissolved species from the fractures into the rock matrix has been treated using a flux boundary condition at the fracture–matrix interface based on Fick’s law of diffusion. These modeling approaches are well established and accepted (de Marsily 1986). Sorption of radionuclides onto the surface of colloids depends on a number of phenomena; namely: (1) electrostatic forces, (2) ion exchange, (3) surface reactions, and (4) co-precipitation (BSC 2003f). These are complex phenomena, and it has been assumed that the effect of these phenomena can be captured using the linear isotherm model for reversible sorption (BSC 2003f). However, for this assumption to be applicable, some important conditions must exist. First, the dissolved radionuclide–colloid system must be in thermodynamic equilibrium. Second, the rate of radionuclide sorption must be linearly correlated to the concentration of dissolved radionuclides. Third, the rate of sorption must be independent of other solutes present in the groundwater. Fourth, the entire colloid surface area must be available for sorption. The likelihood that these conditions are met under repository conditions is not particularly high (BSC 2003f). Haber and Brenner (1993) investigated the sorption of dissolved contaminants onto the surface of colloidal particles under both equilibrium and nonequilibrium conditions. They developed a mathematical expression for the rate of solute sorption from the solvent onto the surface of colloids for nonequilibrium conditions (Haber and Brenner 1993, Equation 2.9b). That expression demonstrates that, under nonequilibrium conditions, the rate of sorption increases linearly with the solute concentration and decreases linearly with the concentration of solute already adsorbed onto the surface of colloids. The key parameter in the Haber and Brenner expression for nonequilibrium sorption is a mass transfer coefficient. The asymptotic limit of the Haber and Brenner expression for high values of the mass transfer coefficient is the linear isotherm model. The Haber and Brenner nonequilibrium sorption expression indicates that under nonequilibrium conditions the use of the linear isotherm model to describe the sorption of solutes onto the surface of colloids will overestimate the mass of solutes adsorbed. In Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003f) it was concluded that equilibrium conditions are not expected at the Yucca Mountain repository. This conclusion, combined with the work of Haber and Brenner (1993), shows that the mass of radionuclides that can be transported as pseudocolloids will be overestimated by using a linear isotherm model to describe the partitioning of radionuclides between dissolved species and colloids. The mass of dissolved radionuclides that can be adsorbed onto the surface of colloids is a function of the colloids’ surface area per unit volume (BSC 2003f). Larger colloids, the ones that could transport farther in the fractures, have a smaller surface-area-to-volume ratio than the smaller colloids. The smaller colloids are more susceptible to attachment to the fracture walls. September 2003 2-5 No. 8: Colloids Revision 2 The mass of radionuclides that can adsorb onto colloids will also be a function of the colloid concentration. The higher the concentration of colloids, the higher the colloidal surface area available to compete with the immobile rock surfaces for sorption of radionuclides. Analyses of colloid concentrations and size distributions in Yucca Mountain groundwaters have found colloid concentrations to be too low for significant colloid-facilitated transport of radionuclides (DOE 2002, pp. 4-335 to 4-336). Moreover, Triay et al. (1996), in a compendium of colloid concentration as a function of water chemistry, found that under thermal and chemical conditions similar to those expected at Yucca Mountain, colloid concentrations are unlikely to be high enough for colloids to effectively compete with the immobile rock for the sorption of dissolved radionuclides, particularly when the immobile rock is coated with secondary minerals. As dissolved radionuclides travel from the repository towards the biosphere, their concentration is expected to decrease due to the various retardation processes that the radionuclides undergo, such as diffusion on the rock matrix and sorption onto the surfaces of the fractures and within the rock matrix. These processes act as effective mass sinks for dissolved radionuclides. This means that there will be lesser amounts of dissolved radionuclides available for sorption onto colloids and a lower likelihood for the formation of radioactive colloids. It is possible that far away from the repository desorption of radionuclides from the surface of colloids will take place, as discussed in the following paragraph. Based on conclusions reached by Mills et al. (1991), van der Lee et al. (1994) questioned whether colloid-facilitated transport of radionuclides would be significant at a nuclear waste disposal site. Mills et al. (1991) evaluated the transport of colloids and metals in porous media. They analyzed the transport of only dissolved species (solutes) under a variety of transport conditions and repeated the analyses under the same conditions including colloids. The basic system Mills et al. (1991) analyzed consisted of a contaminant source located in an unsaturated zone and the transport of the contaminants to a receptor downgradient in a saturated zone. The solute-colloid transport analyses considered uncertainty in several key parameters: source duration (1,000 and 100,000 years), natural colloid concentration (0, 10, and 100 mg/l), infiltration rate (0.007 and 0.35 m/year), partition coefficient for adsorption of solutes onto colloids (103, 104, and 105 l/kg) and partition coefficient for adsorption of solutes to the immobile soil matrix (0, 10, 102, 103, 104, and 105 l/kg). Mills et al. (1991, p. 206) concluded that, “As colloids move through subsurface media, and if adsorbed metal species are present, then the colloids reach a ‘clean’ portion of the aquifer, solutes are rapidly adsorbed to the aquifer-soil matrix and rapidly desorbed from the colloids to reestablish a new equilibrium between solute and colloid-adsorbed species. Thus, the colloids are rapidly stripped of adsorbed metal species and continue migrating with relatively little metal adsorbed.” According to Mills et al. (1991), this desorption phenomenon explains their results in which, for many of the cases they analyzed, the impact of colloid transport on the total mobile phase concentration (dissolved metal concentration plus metal concentration adsorbed to mobile colloids), in terms of either increased concentration or early arrival at the receptor location, was not “remarkable.” Mills et al. (1991, p. 206) further concluded that, “Typically, when travel times to a source are thousands of years in the absence of colloids, the travel times still remain on the order of thousands of years in the presence of colloids.” September 2003 2-6 No. 8: Colloids Revision 2 2.4 SUMMARY AND CONCLUSIONS While the potential exists for fast transport and early release of radioactive colloids to the biosphere at the Yucca Mountain repository, several conditions must simultaneously be met for transport of radioactive colloids to be a significant contributor to dose. Those conditions are: • Colloids must be present in high enough concentrations and provide high surface area per unit volume to carry a significant amount of radionuclides, either embedded in the colloids or attached on the surface of the colloids, relative to the concentration of dissolved radionuclides. • Colloidal suspensions must be stable for distances in the order of tens of kilometers and for time periods in the tens of thousands of years. • Radioactive colloids must not be appreciably filtered, reversibly or irreversibly, by the host rock. If any one of these conditions is not met, the likelihood that colloid transport provides a significant transport mechanism for the release of radionuclides to the biosphere is low. The higher temperatures and higher ionic strength within and in the vicinity of the repository during the first one-third of the postclosure regulatory period are expected to result in unstable colloidal suspensions. As radionuclides adsorb onto the surface of colloids, they may neutralize negative colloid surface charges, thus rendering the colloids more susceptible to filtration by attachment to the surface of fractures, particularly if the fractures are coated with secondary minerals. Therefore, radionuclide sorption may have a beneficial effect on decreasing the potential for fast transport of radioactive colloids. As radionuclides move farther away from the repository, their concentration as dissolved species decreases; therefore, there will be less mass of dissolved radionuclides available for sorption onto colloids. Diffusion into and sorption onto the rock matrix lead to the decrease in the concentration of dissolved radionuclides. It has been concluded (Mills et al. 1991) that over long distances and long travel times, a decrease in dissolved contaminants concentration far away from the source can result in desorption of contaminants from the surface of colloids. This potential desorption process has led some researchers to conclude that when contaminant travel times are thousands of years in the absence of colloids, they are also thousands of years in the presence of colloids. Most contaminant transport studies reported in the literature pertain to saturated conditions. There is little information regarding the transport of colloids under unsaturated conditions. This seeming lack of information for unsaturated systems notwithstanding, some intuitive arguments based on physics can be made regarding colloid transport in unsaturated conditions. Assuming that the flow in the unsaturated zone below the proposed repository horizon is primarily films of groundwater adjacent to the fracture walls, then only colloids of a size considerably smaller than the films could be transported with the groundwater flow. Also, as stated earlier, the smaller September 2003 2-7 No. 8: Colloids colloids are more susceptible to Brownian deposition and, hence, to attachment to the fracture walls. Finally, the colloids within the groundwater film could be physically closer to the fracture walls to enhance attachment to the latter by other mechanisms. 2-8 No. 8: Colloids Revision 2 September 2003 Revision 2 3. WASTE PACKAGE This section of the technical basis document summarizes the current understanding of the formation of radionuclide-laden colloids, prediction of their concentration within the waste package and their release into the invert. The waste package is the first of the repository system subfeatures examined to determine the importance of colloid-facilitated radionuclide transport with respect to the postclosure performance of the Yucca Mountain repository, as shown in Figure 1-1. This section provides a summary-level synthesis of the processes and phenomena governing the formation, prediction of their concentration, and transport of radionuclide-carrying colloids within the waste package. The modeling assumptions to be used in the TSPA for the LA (TSPA-LA) are discussed. In preparing this section, information has been summarized from the following pertinent analysis and model reports, as well as other technical products: • Multiscale Thermohydrologic Model (BSC 2003n) • Engineered Barrier System: Physical and Chemical Environment Model (BSC 2003k) • In-Package Chemistry Abstraction (BSC 2003h) • Dissolved Concentration Limits of Radioactive Elements (BSC 2003g) • Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003f). 3.1 DESCRIPTION OF RELEVANT PROCESSES AROUND WASTE PACKAGE The relevant physical/chemical processes for colloid generation and transport inside waste packages prior to any disruption by igneous intrusion are shown in Figure 3-1. 3.1.1 In-Drift and In-Package Chemistry The evolution of general physical and chemical environments is important for understanding colloid generations and stabilities inside the drift and the waste package. Detailed analyses of indrift thermal-hydrologic conditions are documented in the Multiscale Thermohydrologic Model (BSC 2003n, Figure 6.3-12). A typical evolution of temperature, relative humidity, and liquidphase saturation degree over time is illustrated in Figure 3-2. Relative humidity decreases to low values shortly after emplacement of the waste packages. As the waste packages cool over time, relative humidity begins to rise until it reaches 100 percent after a few thousand years. During the period with relative humidity less than 98 percent, any dilute species in groundwater flowing into the drift become concentrated by evaporation, which results in a high ionic strength solution, which, in turn, destabilizes colloid suspensions. The ionic strength of some representative seepage waters as a function of relative humidity is shown in Figure 3-3 (BSC 2003k). As long as the relative humidity is less than 98 percent, the representative solutions have an ionic strength greater than the upper limit of colloid stability (0.05 M) (BSC 2003f, Sections 6.3.2.2 and 6.3.2.3). Therefore, the formation of a stable colloid suspension during the first thousand years after closure is unlikely. Colloid transport can be possible only after the effects of evaporation cease to be significant after the thermal period when temperatures are below 90°C (Figure 3-2). September 2003 3-1 No. 8: Colloids Revision 2 Furthermore, the degradation of the waste form and the waste package can lead to the formation of concentrated solutions. In process modeling, in-package chemistry has been predicted for two scenarios—water vapor condensation and seepage water dripping—with an assumption of no water evaporation. For the scenario of seepage water, the reaction-path calculations show that the initial seepage-water compositions are quickly modified by water–package chemistry produced as components within the waste package degrade (BSC 2003h). Depending on the conditions, in most cases, the resulting solution can attain ionic strength higher than 0.05 M as the water–waste package reaction progresses (Figure 3-4). Under such conditions, a stable colloid suspension, if there is any, will become unstable. This prediction is consistent with the observation in the static-saturated tests that colloid concentrations increase with time, up to the point where the colloid concentrations reach maximum values and start to decrease (Figure 3-5). In summary, the formation of a stable colloid suspension during the first few thousand years of the regulatory period is unlikely because of evaporation during the thermal period. Colloids could be generated at a late stage of the regulatory period, when evaporation becomes less effective. However, these colloids can be destabilized by high ionic strength environments that can be potentially created during waste degradation. Therefore, the general physical/chemical environments have imposed restrictive constraints on colloid stability inside the drift. It is expected that in-drift colloid transport can be possible only for limited time periods and limited combinations of physical and chemical conditions. September 2003 3-2 No. 8: Colloids Figure 3-1. Relevant Physical and Chemical Processes for Colloidal Generation and Transport from Waste Forms and Waste Packages Revision 2 September 2003 3-3 No. 8: Colloids Source: BSC 2003n, Figure 6.3-12. NOTE: These waste packages bracket the entire range of temperature at this location. Figure 3-2. No. 8: Colloids Thermal-Hydrologic Conditions for the Mean Infiltration-Flux Case for a Range of Waste Packages at the P2WR8C8 Location in the Tptpmn (tsw34) Unit Revision 2 September 2003 3-4 Source: BSC 2003k, DTN: MO0308SPAPCESA.001. Figure 3-3. Source: BSC 2003h, Figure 11. Figure 3-4. No. 8: Colloids Revision 2 Ionic Strength (in Molality) of Some Representative Seepage Waters as a Function of Relative Humidity An Example of Ionic Strength Evolution during the Degradation of Commercial Spent Nuclear Fuels under the Assumption of No Water Evaporation September 2003 3-5 Revision 2 Source: BSC 2003f, Figure 3. DTN: LL991109751021.094. Figure 3-5. No. 8: Colloids Concentrations of Plutonium and Colloids as a Function of Defense High-Level Radioactive Waste Glass Corrosion Test Duration 3.1.2 Waste Types and Potential Colloid Types As explained later in Section 3.4, three types of colloids are modeled in the current waste package model abstraction: • Natural colloids in seepage water/groundwater include mineral fragments, humic substances, and microbes. Humic substances are not sufficiently abundant in Yucca Mountain groundwater to impact transport (Minai et al. 1992). Microbes are susceptible to filtration due to their large sizes (~1 to 10 µm). (See Section 3.2.6.). The mineral colloids are represented as smectite colloids (BSC 2003f). • Corrosion product colloids are derived from the corrosion of waste package and metallic invert materials. These colloids are primarily composed of iron oxyhydroxides. The concentration range of the colloids is estimated based on experiments performed at the University of Nevada, Las Vegas on scaled-down miniature waste packages (DTN: MO0212UCC034JC.002; BSC 2003f). • Waste form colloids are formed from the corrosion of defense high-level radioactive waste glass. Glass waste forms tested include Savannah River Laboratory and West Valley Demonstration Project glasses (CRWMS M&O 2001a; BSC 2003f). (Glass is not available from the yet-to-be-built Hanford vitrification plant.) Colloids produced from both waste forms are primarily smectite clays containing measurable plutonium, as well as discrete radionuclide-bearing phases including brockite (thorium-calcium orthophosphate) and an amorphous thorium-titanium-iron silicate, similar to thoriutite. September 2003 3-6 Revision 2 Long-term corrosion testing of CSNF and DOE spent nuclear fuel (DSNF) under hydrologically unsaturated, oxidizing conditions has been conducted. Results from this testing show both very low colloid concentrations and low fractions of uranium in the colloid mass (Mertz et al. 2003). Thus, colloids from both CSNF and DSNF are excluded in the colloidal modeling. Note that CSNF accounts for the major fraction of the wastes by metric tons of heavy metal to be emplaced in the Yucca Mountain repository. The formation of true or intrinsic colloids depends on the degree of saturation of the solution (i.e., the solution must be supersaturated) with respect to the corresponding radionuclide-bearing mineral phase. There has been no evidence of their formation in the tests performed at Argonne National Laboratory for degradation of defense high-level radioactive waste glass. Degradation of CSNF may form true or intrinsic colloids (e.g., schoepite), but they are likely to dissolve in groundwater in the unsaturated zone. In the Argonne National Laboratory tests (Mertz et al. 2003), it was observed that these metastable colloids dissolved upon the introduction of J-13 groundwater. J-13 groundwater was assumed to be a lower bound with respect to the temperature and chemistry of water inside the drift. Typical water inside the drift is expected to be at a higher temperature and have higher ionic strength than J-13 water. Consequently, colloids will dissolve more readily in the water inside the drift than in J-13 groundwater. Therefore, the possibility of formation of intrinsic colloids is eliminated from further consideration in the colloid modeling (BSC 2003f, Section 6.3.1). 3.1.3 Radionuclide Sorption onto Colloids The effectiveness of colloid-facilitated transport depends on the sorption capability of colloids for radionuclides of interest. For reversible sorption processes, a linear isotherm model is used: (Eq. 3-1) K m m = d d c d is the distribution coefficient. The ranges of Kd values are where mc is mass of radionuclide adsorbed on a unit of mass of solid; md is the concentration of dissolved radionuclide; and K derived from literature data, which are obtained mostly from noncolloidal systems. The results of experiments with plutonium and americium with colloidal hematite and goethite show that the rates of desorption of these radionuclides are significantly lower than the rates of sorption. Over a time period up to 150 days, the extent of desorption is considerably less than that of sorption. Based on these data (Lu et al. 1998) and observations from field studies (Brady et al. 2002), 90 to 99 percent of sorbed plutonium and americium onto iron oxyhydroxides are modeled as irreversibly adsorbed (BSC 2003f). Colloid Filtration–Colloids generated within the defense high-level radioactive waste glass and at its outer surfaces can be filtered. Removal of suspended colloids within fractured and granular material occurs because of two types of phenomena: (1) physical effects, and (2) surface chemical effects. Physical filtration of colloids generally means the retention of colloids moving with the suspending fluid in pores, channels, and fracture apertures that are too small or dry to allow passage of the colloids. Two types of physical filtration are recognized in the unsaturated areas (Wan and Tokunaga 1997): conventional straining and film straining. Conventional straining, sieving, and pore-clogging will filter colloids if they are between 5 and 10 percent of September 2003 3-7 No. 8: Colloids Revision 2 the size of grains in the media and thus likely larger than a pore throat diameter or fracture aperture. Where water saturation is low, colloids may be filtered by film straining if their size is greater than the thickness of the adsorbed water film coating the grains of the rock. The rate of colloid transport through thin water films depends upon the colloid size relative to the film thickness. Therefore, physical straining because of low saturation and surface chemical effects such as adhesion and flocculation through chance interception and diffusion will dominate. However, these processes are more important in the extensive unsaturated and saturated zones. Within the waste package, the filtration process is implicitly included in the colloid generation source term. Colloid Sorption at the Air–Water Interface–Both hydrophilic and hydrophobic colloids may be sorbed at the gas–water interface for partially saturated conditions (Wan and Wilson 1994). Similar to physical straining at low saturation, the concentration of colloids sorbed depends on the saturation degree. However, other factors include the affinity of colloids for the gas–water interface, the electrostatic charge, and the salinity of the aqueous phase (BSC 2003f, Section 5.7). Hydrophobic colloids have higher affinities for the air–water interface. Colloids with low negative charge exhibit a stronger affinity. Also, high salinity of the aqueous phase generally promotes sorption. Buck et al. (2003) found that hydrophobic colloids of meta-studtite and meta-schoepite generated from degradation of CSNF tend to irreversibly attach to the air– water interface. These colloids contain moderate levels of neptunium and plutonium as well as high levels of strontium, cesium, and technetium. Colloid sorption to stationary air–water interfaces generally retard colloid transport, except in cases of relatively high saturation where bubbles with attached colloids could be transported. However, high-saturation conditions are expected to be unlikely in the drift and the unsaturated zone. Therefore, within the waste package, the colloid sorption at the air–water interface is not considered in the colloid generated source term. 3.2 MODELING ASSUMPTIONS 3.2.1 Colloids from the Corrosion of Commercial and DOE Spent Nuclear Fuel Long-term corrosion testing of CSNF and DSNF under unsaturated, oxidizing conditions has been performed to examine the release of dissolved radionuclides, as well as radionuclides associated with colloids (BSC 2003f). Testing was designed to simulate a variety of Yucca Mountain repository relevant water-exposure conditions for several spent nuclear fuels with a range of fuel burnup and composition. Results from the unsaturated testing of CSNF and DSNF at Argonne National Laboratory indicated formation of alteration products containing very low concentrations of uranium-based colloids and dissolution of the uranium-based colloids in less than several months (BSC 2003f, Section 6.3.1.2). Uranium-based spent nuclear fuels will be prevalent in the repository, and the colloidal properties of a mixture of two uranium minerals, meta-schoepite [(UO2)4(OH)6.5H2O] and UO2+x, have been examined (BSC 2003f, Section 6.3.1.2.3). These colloids are stable under short duration tests with respect to dissolution and interparticle interactions at near neutral and higher pH values in solutions saturated with the respective mineral phases. Furthermore, these colloids dissolve after introduction to J-13 groundwater in short duration tests. Natural analog studies also suggest that colloidal uranium will not be a significant contributor to radionuclide transport under oxic pH-neutral environments September 2003 3-8 No. 8: Colloids Revision 2 in uranium deposits (BSC 2003f, Section 6.3.1.2.4). Thus, colloids from CSNF and DSNF were eliminated from further consideration in the colloid modeling and abstraction. Commercial Spent Nuclear Fuel 3.2.1.1 Assessment of the importance of potential colloid formation from CSNF is based on four major observations: (1) very low colloid concentrations were observed in the CSNF degradation tests, at least an order of magnitude less than concentrations observed in the defense high-level radioactive waste glass degradation tests (based on dynamic light-scattering measurements) (Mertz et al. 2003); (2) the fraction of uranium in the colloid mass was uniformly low in the CSNF tests, the only deviation from this occurring immediately following one of two changes in vessel configuration in which the uranium fraction increased but rapidly decreased to the approximate level of earlier values (Mertz et al. 2003); (3) suspensions of meta-schoepite and UO2+x colloids in J-13 groundwater appear to dissolve in short-term saturated tests (their stability in unsaturated solutions has not been tested (Mertz et al. 2003)); and (4) field studies at uranium-bearing deposits indicate generally that under oxidizing conditions at near-neutral pH, colloid particles contain little uranium, and there is little sorption of uranium complexes to colloids (BSC 2003f). One of the reasons hypothesized for the low colloid release in the CSNF tests was the test configuration in which the Zircaloy-4 support for the fuel fragments had 7-micron holes. However, the results from the unirradiated UO2 tests also show few colloids after the formation of alteration products (Wronkiewicz et al. 1997). The unsaturated tests on unirradiated UO2 had a test configuration with large 2 to 3 mm holes at the holder base allowing for the spallation of UO2+x particulate during initial corrosion. However, the formation of a dense mat of alteration products during the UO2 corrosion apparently reduced particulate release by trapping particulate in the altered products (Wronkiewicz et al. 1997). A similar mechanism whereby the alteration products minimize particulate release may be applicable to the CSNF unsaturated tests. The concentration of released particulates or colloids from the CSNF tests is very low except during movement of the fuel samples from one setup to another (Mertz et al. 2003). In that case, colloid and particulate concentrations increased temporarily but returned to very low concentrations after the disruption (Mertz et al. 2003). While this indicates that disruptive events may contribute to the release of particulates and colloids from CSNF, it also indicates that the longevity of the colloids in the leachate is very short relative to the regulatory period. DOE Spent Nuclear Fuel 3.2.1.2 Of the approximately 250 different types of fuel in the DSNF inventory, metallic uranium fuel comprises approximately 85 percent (by weight of heavy metal) of that inventory and, as the only waste form in significant quantities distinct from the other spent fuel types, was selected for corrosion testing. An irradiated uranium metal fuel from the N-Reactor at Hanford was tested in an experimental setup similar to that used at Argonne National Laboratory for testing CSNF. Additional details on the testing can be found elsewhere (DTN: MO0306ANLSF001.459). Corrosion testing of metallic uranium samples resulted in rapid oxidization (within a few months) of the uranium primarily to an oxide sludge consisting of UO2 and higher oxides of uranium (DTN: MO0306ANLSF001.459). Although the uranium fuel disintegrated rapidly, September 2003 3-9 No. 8: Colloids Revision 2 corrosion testing was continued to determine the effect of groundwater leaching on the fuel sludge. Results from the corrosion tests showed that the composition of the DSNF colloids evolve over time from an initially UO2-rich population, to a mixed colloid population containing UO2 and higher oxides of uranium as well as smectite clays, to a population that appears to be dominated by uranium-containing smectite clays. After approximately one year of testing, the total quantity of uranium in the sludge represented approximately all the original uranium fuel sample. The uranium associated with the colloids corresponds to 0.002 to 0.006 weight percent of the original uranium fuel sample. The quantity of uranium in the fraction attached to the stainless steel vessel was 0.1 to 0.3 weight percent of the original fuel sample (DTN: MO0306ANLSF001.459). The attached material is measured by washing the stainless steel vessel in HNO3; the attached material includes sorbed solutes, sorbed colloids, and precipitates (DTN: MO0306ANLSF001.459). During DSNF corrosion, plutonium is predominantly associated with the colloidal, particulate, and sorbed size fractions. The 239Pu/238U ratios in the colloid fraction are significantly larger than those in the other fractions (sorbed, particulate, and dissolved) and is the only fraction that showed enrichment of plutonium in comparison to that in the fuel prior to corrosion (DTN: MO0306ANLSF001.459). Results from the testing suggest that plutonium is significantly adsorbed to the surface of colloids (such as corrosion products of the waste package or groundwater clays). However, it does not form as an embedded radionuclide in waste form colloids (DTN: MO0306ANLSF001.459). 3.2.2 Filtration of Colloids Near the waste, colloids may form at corroded waste fuel pellets and at its outer surfaces. They could be filtered within fractures in fuel pellets or trapped at grain boundaries. Colloids forming within fuel rods whose cladding has been breached could be filtered at perforations in the cladding. Colloids formed and spalled from the defense high-level radioactive waste glass could be filtered at perforations in the stainless steel high-level radioactive waste canister. No differentiation was made between colloid filtration and generation in the waste degradation experiments. Physical and chemical filtration of colloids is thus implicitly part of the generation process of colloids for radioactive waste. This filtration is a possible reason for the observed lack of waste form colloids in leachate leaving the CSNF and DSNF degradation experiments at Argonne National Laboratory noted above in Sections 3.2.1.1 and 3.2.1.2. Colloids reaching the interior of the waste package (after escaping from fuel-rod cladding and high-level radioactive waste containers) could be filtered at perforations in the skin of the waste package. However, further filtration within the waste package and the drift is excluded from modeling. There have been no comprehensive studies of colloid filtration within the defense high-level radioactive waste glass. Hence, meaningful analysis of colloid filtration separate from colloid generation within the waste package is currently not feasible. Therefore, a conservative assumption is made: all colloids formed within the waste (the calculated colloid source term) are assumed to exit the waste package and invert (see Section 4 for a discussion of the invert) and enter the unsaturated zone without filtration (BSC 2003f). September 2003 3-10 No. 8: Colloids Revision 2 3.2.3 Microbes and Colloidal Organic Components 2, The occurrence of microbes in the repository has been evaluated in In-Drift Microbial Communities (CRWMS M&O 2000a). In assessing the potential effects on microbial populations within the engineered barrier system, In-Drift Microbial Communities (CRWMS M&O 2000a) considered the drift mineralogy; drift physical parameters; metals, alloys, and cement used in engineered barrier system components; waste dissolution rates and quantities; groundwater compositions and infiltration rates; and compositions and fluxes of gases (e.g., CO water vapor). Environmental limits on microbial activity considered include redox conditions, temperature, radiation, hydrostatic pressure, water activity, pH, salinity, available nutrients, and others. The abundance of water and phosphorous were found to be the two environmental components which, could limit the development of microbial communities in the Yucca Mountain repository under the probable physicochemical conditions likely to be present in the proposed repository configuration. However, water and phosphorous in the repository would potentially be available in sufficient quantities to allow for the growth of microbial communities at certain stages of the postclosure period (CRWMS M&O 2000a). Microbes can accelerate or retard the transport of radionuclides in a number of ways. First, in some systems, microbes can passively or actively bioaccumulate radionuclides across the cell membrane. If the microbes are mobile, they can facilitate transport of the radionuclides. Alternatively, the microbes may be readily filtered by the rock or may form biofilms, a part of biomass attached to rocks, in which case they retard transport. Second, microbes may produce exudates, for example organic complexants, which may enhance the solubility of radionuclides and affect their sorption characteristics if complexes are formed. Third, in the course of extracting energy or nutrients, microbes may generate colloids by degrading materials or may actually destroy colloids by consuming them or by facilitating agglomeration. For example, microbial oxidation of metallic iron can produce iron-oxide colloids and aggregates. Conversely, microorganisms can decrease the concentration of stable colloids by aggregating colloidal material that they use as a food source. This has been shown to result in a significant decrease in colloid concentrations (Hersman 1995). Fourth, in some systems, microbes are able to reduce the oxidation states of some multivalent radioelements (e.g., uranium, neptunium, and plutonium). Typically, reduced forms of radionuclides are less soluble and more strongly sorptive. They may also impact local groundwater chemistry in and around the waste package. Microbes and inorganic colloids in close proximity to one another may result in collisions, which allow mutual adhesion or adsorption and result in particle size growth (agglomeration) (Buffle et al. 1998; Hersman 1995). This increase in particle size can have several effects on the potential for colloids to facilitate radionuclide transport within the repository: (1) for a given range of pore sizes in the transport medium, the larger composite particles may become filtered more readily than the individual microbe or colloid; (2) the larger particles will tend to diffuse more slowly and/or may precipitate through gravitational settling; and (3) the interactions of relatively large populations of colloids and microbes can result in agglomerated particles sufficiently large that the suspension becomes unstable and the microbe and colloid particles flocculate. Reduced transport from increased particle size is even more significant in the situation where water is present as thin films within the pore spaces and on the engineered barrier system components. In all of these processes, the net result is particle size increase and reduced transport of colloids and associated radionuclides. Hersman (1995) conducted experiments with September 2003 3-11 No. 8: Colloids Revision 2 Yucca Mountain-native bacteria and bentonite clay. In one test, agglomeration of clay colloids in a sterile microbial growth medium was compared to agglomeration in a medium into which a bacterium was introduced. The test results showed greater agglomeration in the growth medium inoculated with bacteria. In a second test, the bacteria were cultured in the medium and the test repeated with those bacteria, and similar results were noted. The development of biofilms on a substrate has been shown to result in an increased tendency to attract suspended inorganic colloids. Sprouse and Rittmann (1990) and Rittmann and Wirtel (1991) demonstrated that biofilm colonization increased the colloid cohesion efficiency, á, in a system where the collector was granular activated carbon in a methanogenic fluidized bed and the colloids were milk solids approximately one micron in diameter. Lo et al. (1996) concluded from an experiment with Fe(III) oxide colloids in a biofilm reactor that the deposition of Fe(III) oxide colloids increased slightly with biofilms present. Further iron deposition on surfaces increased with increasing particle size, suggesting interception of the colloids and/or sedimentation. Although uncertainty surrounds the microbial effect, the experiments with bacteria suggest that microbial action will tend to increase the sizes of inorganic colloids, and promote gravitational settling and filtration. Thus, not including the effects of microbes in the colloid source term and transport analysis is considered conservative with respect to total system performance. 3.2.4 Intrinsic Colloids Intrinsic colloids are colloidal-sized assemblages (between approximately 1 nm and 1 µm in longest dimension) formed from the hydrolysis and polymerization of actinide ions dissolved in solution (BSC 2003f). They may form in the waste package and engineered barrier system during waste-form degradation and radionuclide transport. Intrinsic colloids are also called primary colloids, Type I colloids, Eigenkolloide, real colloids, and true colloids. The formation of intrinsic colloids is solubility limited and, thus, based on the solution chemistry. This fact prevents significant introduction of intrinsic colloids to the environment (CRWMS M&O 2001a, Section 6.1). There has been no evidence of their formation in Argonne National Laboratory defense high-level radioactive waste glass degradation tests (Ebert 1995, Sections 6.2.1 and 6.2.2). Degradation of CSNF may form intrinsic colloids (e.g., schoepite-like uranium hydroxides) close to the fuel surface where the fluid may be saturated with respect to uranium, but they are likely to dissolve in unsaturated fluid. Because experimental data supports the conclusion that intrinsic colloids are negligible, intrinsic colloids are not considered in the TSPA-LA. 3.2.5 Effect of Temperature on Colloids Coupled thermal, hydrologic, and chemical effects are likely to result in unstable colloid suspension and, therefore, reduce the number concentration of colloids in suspension. With increasing temperature and increasing ionic strength, both conditions expected as a result of thermal, hydrologic, and chemical effects, colloid suspensions become less stable. Specifically, the coupled thermal-chemical perturbation will take place in the first few thousand years after repository closure (Figure 3-2). The thermal event produced by radiation heat will create a September 2003 3-12 No. 8: Colloids Revision 2 drying-out zone and induce water evaporation within the drift and the surrounding area, resulting in the increase in the ionic strength in the percolating solution. The chemical degradation of waste package will further concentrate the solution (Figure 3-4). The increase in ionic strength will reduce the stability of both smectite and iron oxyhydroxide colloids inside the drift. For example, the rate of particle coagulation is described by the stability relationship, W, defined as (BSC 2003f): (Eq. 3-2) W = exp(Vmax/kT) where, Vmax is the height of the energy barrier preventing particle coagulation; k is the Boltzman constant; and T is temperature. Thus, increasing the temperature will cause a colloid suspension to become less stable. This is because the elevated temperature will enhance Brownian motion of the particles and therefore increase the probability of interparticle collisions. In addition, the thermal event and its induced chemical perturbations are limited in space and transient in time. Because of the drying-out effect, a significant release of radionuclides from waste packages can be possible only after the thermal event, when the coupled thermal-chemical perturbations become completely attenuated and enough seepage water becomes available for leaching radionuclides. It is possible that some chemical perturbations may result in an increase in colloid concentration. However, such perturbations are expected to be transient and will attenuate rapidly as the colloid suspension moves away from the disposal room. This conclusion is consistent with the results reported by Triay et al. (1996) who found that, under the perturbed conditions expected at Yucca Mountain, colloid concentrations are unlikely to be high enough to effectively compete with the immobile rock for the sorption of radionuclides. High temperatures and high ionic strengths are not favorable to colloid transport. Therefore, the screening out of coupled thermal-hydrologic-chemical effects on the transport of radioactive colloids in the TSPA is a reasonable assumption. 3.2.6 Colloid Sorption at the Air–Water Interface The concentration of colloids sorbed at the gas–water interface is a function of the following conditions (BSC 2003f, Section 5.7): • The interface surface area available for colloid uptake, which is a function of the total gas saturation • The affinity of colloids for the gas–water interface (hydrophobic colloids have higher affinities than hydrophilic colloids) • The electrostatic charge on the colloid—colloids with lower negative charge exhibit a stronger affinity • The salinity of the aqueous phase with higher salinity promoting sorption. Empirical evidence suggests that the sorption affinity of colloids at the gas–water interface may be stronger than to the rock matrix (Wan and Wilson 1994). September 2003 3-13 No. 8: Colloids Revision 2 Partially saturated conditions may be classified by considering degrees of saturation. At low water saturations, the surface area of the gas–water interface approximates that of the rock matrix. Overall, colloid migration is retarded, although colloids may still move through the adsorbed water films. At intermediate water saturations, there is still an interconnected gas phase, although gas flux may be lower. The interface may act as a static sorbing surface, but estimating the geometry and surface area is complicated. At high water saturations, the majority of the gas is present as small gas bubbles that may migrate, transporting sorbed colloids. Colloid migration rates depend more strongly on colloid size as lower saturation states are considered (CRWMS M&O 2001b; McGraw 1996). To examine the influence of colloid size on transport, McGraw (1996) investigated transport of monodisperse colloids (five different sizes, between 20 nm and 1,900 nm) under both saturated and unsaturated conditions in a quartz sand. The results indicated that under saturated conditions the time required for breakthrough of 50 percent of the original colloid concentration was the same as that of the breakthrough of a nonreactive tracer, indicating no relationship of colloid size to migration under saturated conditions. However, the times required for breakthroughs of the colloids under highly unsaturated conditions exhibited a strong relationship between the colloid breakthrough and the colloid size with fairly complete breakthrough of the 20-nm colloid and little or no breakthrough of the 1,900-nm colloid. Another set of experiments (McGraw 1996) compared four sets of hydrophobic and hydrophilic colloids (modified latex microspheres). The results indicated that transport of hydrophobic colloids depends on colloid size, water film thickness, and colloid charge density. In contrast, hydrophilic colloids were not affected by these variables and were rapidly transported through the system even under very low moisture contents. It was concluded that for hydrophobic colloids, the cumulative mass of colloids recovered relative to column input was logarithmically dependent upon the ratio of the water film thickness to colloid diameter. In contrast, for hydrophilic colloids, the cumulative mass of colloids recovered relative to the column input was linearly dependent upon the ratio of the water film thickness to colloid diameter; similar, but more pronounced than, the effect with the nonreactive tracer. These findings suggest that unsaturated porous media may not completely impede colloid migration when a water film is present, even for relatively large colloids, however, larger colloids will tend to be retarded more than smaller ones. Although the potential effects of degree of saturation on colloid transport are varied and complex, on balance colloids would be somewhat retarded under low-saturation both inside and outside the waste package. Because of this conclusion, sorption at the air–water interface is conservatively omitted within the waste package. 3.2.7 Selection of Radioisotopes Transport of radioisotopes on colloids is potentially important for radioisotopes that (1) have long half-life and low solubility; (2) can be entrained in, or sorbed onto, waste forms, engineered barrier materials, or geologic barrier materials that generate colloidal particles; (3) represent a major portion of the inventory; and (4) have large dose conversion factors. Considering these four criteria as part of radionuclide screening, Waste Form and In-Drift Colloids-Associated Radionuclides Concentrations: Abstract and Summary (BSC 2003f) evaluated eight September 2003 3-14 No. 8: Colloids Revision 2 radionuclides for sorption onto colloids: plutonium, americium, thorium, cesium, protactinium, neptunium, uranium, and strontium. Considering these four criteria as part of radioisotope screening, the colloidal concentration abstraction concluded that five radioisotopes attached to colloids should be evaluated: plutonium, americium, thorium, cesium, and protactinium. Four of these were analyzed in TSPA for site recommendation (CRWMS M&O 2000b; Leigh and Rechard 2001). Cesium has been added for TSPA-LA based on an updated screening analysis (BSC 2003f). Plutonium, americium, thorium, cesium, protactinium are assumed to be attached to colloids reversibly using a linear isotherm model. Plutonium and americium are assumed to be predominantly attached to iron oxyhydroxide colloids and smectite waste form colloids irreversibly. The following discussion summarizes the rationale used to include or exclude these radionuclides from the model for reversible attachment to colloids (BSC 2003f, Section 6.3.3.1). Rationale for Including Five Radioisotopes 3.2.7.1 Plutonium–Plutonium meets all four criteria. A large quantity of plutonium will exist in the repository (criterion 3). Plutonium is sparingly soluble (criterion 1) but sorbs strongly to oxide mineral surfaces (generally less strongly to silicates). Plutonium is observed to sorb strongly to soil mineral, and laboratory investigations have shown that it sorbs readily to colloids as well (criterion 2). Finally, plutonium has a large dose conversion factor. Americium–Americium also meets all four criteria. Americium will be a significant contributor to radioactivity during the first 10,000 years. Like plutonium, americium is sparingly soluble but strongly sorbs to mineral surfaces, including colloids. Laboratory investigations have shown that it sorbs strongly to colloids. Protactinium–Protactinium will be a significant contributor to radioactivity during the first 10,000 years. Because of this, and the fact that relatively little is known of the colloid behavior of protactinium, it was included in this analysis. Thorium–Thorium will be a significant contributor to radioactivity during the first 10,000 years. Because of this, and the fact that there is evidence that thorium sorbs strongly to oxides, it was included in this analysis. There is relatively little known of the colloid-related behavior of thorium. Cesium–135Cs has a long half-life and can attach strongly to certain sheet silicates (including clays) by means of ion exchange. For this reason, cesium has been observed to sorb to soil minerals, and it could potentially form pseudocolloids particularly with groundwater and defense high-level radioactive waste glass-derived clay colloids. Rationale for Excluding Neptunium, Uranium, and Strontium 3.2.7.2 Neptunium–Because neptunium will be the most significant contributor to radioactivity beyond the first 10,000 years, it was considered for inclusion in this analysis. Neptunium is more soluble under anticipated repository conditions than many of the other important radionuclides, and it sorbs considerably less strongly than, for example, plutonium and americium (see Table 3-1). The typical Kd values for neptunium sorption on Yucca Mountain-vicinity colloids are less than 100 mL/g. As demonstrated in Section 3.5, as long as Kd is less than 5000 mL/g, colloid-facilitated transport is not likely to be significant relative to transport of dissolved September 2003 3-15 No. 8: Colloids species. It would appear then that the mobility of neptunium is influenced mostly by its solubility. For these reasons, and to simplify the modeling, neptunium was not included in the reversible-sorption portion of the colloid-associated radionuclide transport analysis. Uranium–Uranium will be by far the most abundant radioactive element in the repository and primarily for this reason was considered for the analysis. Uranium is more soluble under anticipated repository conditions than many of the other important radionuclides, and it sorbs considerably less strongly than, for example, plutonium and americium. Kd values for uranium sorption on Yucca Mountain-vicinity colloids typically are less than about 1,000 mL/g (Table 3-1). As demonstrated in Section 3.5, as long as Kd is less than 5,000 mL/g, colloidfacilitated transport should not be significant relative to transport of dissolved species. As with neptunium, the mobility of uranium is thus influenced mostly by its solubility. Field observations at uranium deposits and mine sites have indicated that little or no colloid uranium transport occurs. For these reasons, and to simplify modeling, uranium was not included in the reversible-sorption of the colloid-associated radionuclide transport analysis.” Strontium–Because of its very short half-life, strontium was not considered important in the groundwater pathway either as a dissolved species or attached to colloids. Modeled Kd for Plutonium, Americium, Thorium, Neptunium, and Uranium Sorption onto Kd Values, mL/g 1 to 6 × 102 1 × 101 to 1 × 102 1 × 103 to 1 × 104 2 × 103 to 9 × 104 1 × 104 to 1 × 107 September 2003 Table 3-1. Yucca Mountain-Vicinity Colloids 3.2.7.3 Radionuclide Sorbate and Oxidation States at YMP U(VI) Np(V) Pu(V) 3-16 Th(IV) Am(III) Source: BSC 2003f, Table 9. Rationale for Irreversible Adsorption Defense high-level radioactive waste glass degradation experiments show that plutonium is probably irreversibly attached to smectite colloids generated during the experiments. Further, evidence from sorption experiments with plutonium and americium (Lu et al. 2000) with colloidal hematite and goethite show that the rates of desorption (backward rate) of plutonium and americium are significantly slower than the rates of sorption (forward rate). More importantly, over a significant time period (up to 150 days in some experiments), the extent of desorption is considerably less than the extent of sorption. Plutonium and americium are considered so strongly sorbed to colloids that, in essence, they can be considered irreversibly sorbed and are modeled in this manner within the engineered barrier system. Plutonium transport velocities in soils reflect the fact that plutonium binds strongly to soils, leaving very little, if any, soluble plutonium available for groundwater transport or plant uptake. Coughtrey et al. (1985) estimate exchangeable plutonium to be less than one percent. At Rocky Flats, plutonium in soil is largely bound to soil metal hydroxides. Litaor and Ibrahim (1996) used 0.01M CaCl2 as an extractant and measured plutonium in Rocky Flats soil to be 0.04 to 0.08 percent exchangeable. Bunzl et al. (1995) measured exchangeable 239Pu and 240Pu (0.5 to No. 8: Colloids Revision 2 Revision 2 1 percent) and 241Am (1.5 to 15 percent) from fallout-contaminated soils in Germany using 1M C2H7NO2 (ammonium acetate NH4C2H3O2) as the extractant. Laboratory experiments of plutonium sorption onto iron oxides have shown that only approximately one percent of the initially sorbed plutonium can be desorbed into solution, even after months of time have elapsed (Lu et al. 2000), which is broadly consistent with field observations. For these reasons, plutonium and americium are modeled as irreversibly attaching to corrosion (iron oxyhydroxide) colloids. No other radionuclides are considered to be irreversibly attached to colloids. 3.3 SOURCE OF DATA AND TESTING 3.3.1 Tests on Defense High-Level Radioactive Waste Glass Tests on defense high-level radioactive waste in borosilicate glass have been conducted at Argonne National Laboratory using two modes of water contacting the waste (CRWMS M&O 2001a). In the static-saturated test, glass samples were immersed in fluid for more than four years. In the dripping tests, fluid was dripped at specified rates onto glass samples. Fluid used was J-13 water and deionized water. It was observed in the static-saturated tests that colloids developed and increased in concentration with time, up to the point where the colloid concentration reached a maximum value and then decreased (Figure 3-5). From Figure 3-5, it is estimated that 1 × 10-7 M plutonium is equivalent to 5 ppm of total radionuclide-embedded colloids when both concentrations reach their maximum values. 3.3.2 Tests of Waste Package Corrosion Maximum colloid concentration values are estimated based on experiments performed at University of Nevada, Las Vegas. In these experiments, scaled-down miniature waste packages were exposed to J-13 groundwater in either a bathtub mode or a flow-through mode. The experimental results obtained indicate that the iron corrosion products are mainly composed of magnetite (Fe3O4), lepidocrocite (FeOOH), and goethite (FeOOH) (DTN: MO0302UCC034JC.003). The cumulative results have yielded average concentrations of colloidal size materials in the range of 20 mg/L within the initial four weeks of the experiments (DTN: MO0212UCC034JC.002). 3.3.3 Iron Oxyhydroxide Colloids in Nature Few data for iron oxyhydroxide colloids concentrations in nature have been found in the scientific literature, and the colloid concentrations reported vary greatly. At an iron rich ore body in South America, the Morro de Ferro natural analog site, the concentration was near 1 mg/L. Values ranging from 0.6 mg/L to 260 mg/L were measured in the vicinity of a mined uranium ore formation at Cigar Lake in Northern Saskatchewan (Vilks et al. 1993). Two Swedish groundwater colloid concentrations were measured to be 0.02 mg/L and 0.043 mg/L for saline and nonsaline groundwater samples, respectively (Laaksoharju et al. 1995). The experiments performed at University of Nevada, Las Vegas on miniature waste packages produced colloids ranging in concentration from near 0 mg/L to approximately 50 mg/L. As mentioned in Section 3.4, this latter information was used to define a uniform distribution between 0.05 and 50 mg/L. September 2003 3-17 No. 8: Colloids Revision 2 3.3.4 Stability of Smectite and Iron Oxyhydroxide Colloids The zero-point of charge of smectite is about pH 2 (Figure 3-6), below possible pH values anticipated within waste packages (BSC 2003h). Therefore, smectite colloids would remain stable if the ionic strength of the solution is low (less than 0.05 M). In the defense high-level radioactive waste glass tests conducted at Argonne National Laboratory, which were run at pH 9 and 11.5, smectite colloids were detected (Buck and Bates 1999). Tombacz et al. (1990) investigated the stability of smectite (also referred to as montmorillonite) suspensions as a function of pH and ionic strength in an NaCl solution. The results are summarized in Figure 3-6. At a neutral pH, iron oxyhydroxide colloids tend to be unstable and agglomerate. The related stability diagram is displayed in Figure 3-7. Source: BSC 2003f, Figure 4. NOTE: zpc = zero-point of charge. Figure 3-6. No. 8: Colloids Experimental Determination of Montmorillonite (a Variety of Smectite) Stability as a Function of pH and Ionic Strength (M) September 2003 3-18 Revision 2 Source: BSC 2003f, Figure 7. NOTE: zpc = zero-point of charge. Figure 3-7. No. 8: Colloids Schematic Representation of Iron Oxyhydroxide Colloid Stability as a Function of pH and Ionic Strength (M) Note that the pH of groundwater at Yucca Mountain is generally close to neutral. Within this pH range, iron oxyhydroxide colloids would have a minimal surface charge, thus reducing mutual repulsive forces and resulting in flocculation, even at a low ionic strength. This fact explains why iron oxyhydroxide colloids may occur in low concentrations in groundwater and is one of the reasons why smectite colloids are used to represent seepage/groundwater colloids. This fact also implies that any iron oxyhydroxide colloids released from waste packages into groundwater may become unstable and flocculate as they move through the disposal system. 3.3.5 Colloid Concentrations in Groundwater The range of colloid concentration in seepage/groundwater was derived based on groundwater sampling from the Yucca Mountain area. Literature data (Degueldre et al. 2000) and groundwater sampling at the Idaho National Engineering and Environmental Laboratory (BSC 2003f) was used as additional technical information that corroborated the site-specific data. The results are summarized in Figures 3-8 and 3-9. Practically no colloids were detected for the ionic strength above 0.05 M. The upper limit of colloid concentration is about 200 ppm, with a median value around 0.1 ppm (Figure 3-9). September 2003 3-19 Source: BSC 2003f, Figure 11. NOTE: The ordinate values are in particles per milliliter (pt/mL). Figure 3-8. Groundwater Colloid Concentration Data Collected in the Vicinity of Yucca Mountain Compared with Data Collected from Groundwaters around the World Source: BSC 2003f, Figure 12. Figure 3-9. Cumulative Distribution Function Showing the Probability of Occurrence of Colloid Concentration Levels (ppm or mg/L) in Groundwater Samples in the Yucca Mountain Area and Idaho National Engineering and Environmental Laboratory No. 8: Colloids Revision 2 September 2003 3-20 Revision 2 3.3.6 Radionuclide Distribution Coefficients (K ds) The effectiveness of colloid-facilitated transport depends on the sorptive affinity of radionuclides to a substrate, which can be described by the distribution coefficient (Kd) (Eq. 3-1). The Kd values used in the colloid transport modeling were obtained from various data sources and measurements including those from National Cooperative for the Disposal of Radioactive Waste (Switzerland), U.S. Environmental Protection Agency, and Yucca Mountain-specific projects (BSC 2003f). Based on these sources, professional judgment was used to develop uncertainty distributions (Table 3-2). The work of Lu et al. (1998) needs to be mentioned specifically, because it provides the best direct evidence suggesting the irreversible sorption of plutonium onto colloidal particles. The work shows that Pu(IV) and Pu(V) were rapidly adsorbed by colloids of hematite, goethite, smectite, and silica in natural and synthetic groundwater. After five days, hematite colloids sorbed all Pu(IV) and Pu(V) present in the solution, goethite sorbed 97 to 100 percent of plutonium, smectite sorbed 94 to 100 percent plutonium, and silica sorbed 46 to 86 percent of plutonium. Desorption of plutonium from colloids of hematite, goethite, and smectite was much slower than the sorption process. After 150 days, less than 0.02 percent Pu(V) was desorbed from hematite colloids. Little colloidal Pu(IV) was desorbed from hematite colloids during 150 days, even using sequential extraction under vigorous shaking conditions. Although desorption of Pu(V), as well as Pu(IV), from goethite and smectite colloids is relatively faster, only less than 1 percent of plutonium was desorbed from goethite, and 1.5 percent Pu(V) and 2.5 percent to 11 percent of Pu(IV) were desorbed from smectite after 150 days. 3.4 MODEL FOR COLLOID-FACILITATED TRANSPORT IN WASTE PACKAGE This section summarizes the algorithm used to incorporate the colloids source term abstraction in the TSPA-LA model, using a simplified model intended to retain the important principles and processes of the analyses. The logic implemented in the TSPA-LA model is provided, although specific programming details are not. September 2003 3-21 No. 8: Colloids Table 3-2 Kd Values Used for Reversible Radionuclide Sorption on Colloids in Calculations for the Total System Performance Assessment for the License Application Radionuclide Pu Am, Th, Pa Cs Source: BSC 2003f, Table 10. NOTE: The Kd values for Tc and I are very low and not listed here. No. 8: Colloids Colloid Iron Oxyhydroxide Smectite Iron Oxyhydroxide Smectite Iron Oxyhydroxide Smectite Kd Value Range (mL/g) 104 to 106 103 to 106 105 to 107 104 to 107 101 to 103 102 to 104 3-22 Kd Value Intervals (mL/g) <1 × 104 1 × 104 to 5 × 104 5 × 104 to 1 × 105 1 × 105 to 5 × 105 5 × 105 to 1 × 106 >1 × 106 <1 × 103 1 × 103 to 5 × 103 5 × 103 to 1 × 104 1 × 104 to 5 × 104 5 × 104 to 1 × 105 1 × 105 to 5 × 105 5 × 105 to 1 × 106 > 1 × 106 <1x 105 1 × 105 to 5 × 105 5 × 105 to 1 × 106 1 × 106 to 5 × 106 5 × 106 to 1 × 107 >1 × 107 <1 × 104 1 × 104 to 5 × 104 5 × 104 to 1 × 105 1 × 105 to 5 × 105 5 × 105 to 1 × 106 1 × 106 to 5 × 106 5 × 106 to 1 × 107 >1 × 107 <1 × 101 1 × 101 to 5 × 101 5 × 101 to 1 × 102 1 × 102 to 5 × 102 5 × 102 to 1 × 103 >1 × 103 <1 × 102 1 × 102 to 5 × 102 5 × 102 to 1 × 103 1 × 103 to 5 × 103 5 × 103 to 1 × 104 >1 × 104 Revision 2 Kd Value Interval Probabilities 0 0.15 0.2 0.5 0.15 0 0 0.04 0.08 0.25 0.2 0.35 0.08 0 0 0.15 0.2 0.55 0.1 0 0 0.07 0.1 0.23 0.2 0.32 0.08 0 0 0.13 0.22 0.55 0.1 0 0 0.2 0.25 0.5 0.05 0 September 2003 Revision 2 3.4.1 Sources of Colloids As already noted in Section 3.1, three sources of colloids are considered (Figure 3-10): (1) natural colloids in the seeping groundwater, (2) colloids generated from degradation of the waste package and other metallic materials in the repository, and (3) colloids generated from degradation of high-level radioactive waste. Colloids formed from the defense high-level radioactive waste glass and the naturally occurring groundwater colloids are represented by smectite colloids as explained in Sections 3.3.1 and 3.3.5. The three colloid types can transport reversibly sorbed radioisotopes, but plutonium and americium radioisotopes attached on iron oxyhydroxides colloids can also transport as “irreversibly” attached radioisotopes (Figure 3-10). Furthermore, colloids generated from spallation of the high-level radioactive waste glass have plutonium and americium radioisotopes engulfed during formation and are thus irreversibly attached as well. The colloidal concentration model uses concepts developed for the Compliance Certification Application for the Waste Isolation Pilot Plant (DOE 1996; Larson 2000; Stockman et al. 2000), in particular, categories of colloids tracked, mode of radioisotope uptake, and colloid stability. Figure 3-10. Embedded, Reversibly, and Irreversibly Attached Radioisotopes on Colloids September 2003 3-23 No. 8: Colloids Revision 2 3.4.2 Algorithm Overview The algorithm in the colloidal concentration abstraction for TSPA-LA consists of three steps (BSC 2003f) that are executed at each time step in the calculations (Figure 3-11). First, the component determines the potential colloidal mass available, usually as a function of ionic strength. Second, the component determines the stability of the colloidal mass as a function of ionic strength and pH, where ionic strength and pH are determined by the in-package chemistry (BSC 2003h) of the waste form model (CRWMS M&O 2000c; Rechard and Stockman 2001). When the colloids are unstable, a low (nonzero) limit of colloid concentration is used. Third, the component determines the fraction of the available mass of the dissolved inventory of cesium, americium, plutonium, protactinium, and thorium to be adsorbed onto the colloids. The dissolved inventory of the five radioisotopes is determined by the dissolved concentration component model (BSC 2003g) of the waste form model (CRWMS M&O 2000c; Rechard and Stockman 2001). The colloidal mass generated in Step 1 and its stability as a function of ionic strength and pH in Step 2 are generally bounding, but some uncertainty is included in parameter values for sorption in Step 3. The specific methods employed for these three steps depend on the source of the colloids. The description follows. The abstraction calculates the concentration of dissolved radionuclides, the concentration of plutonium and americium embedded in defense high-level radioactive waste glass smectite colloids, the concentration of radionuclides sorbed reversibly onto the three colloid types, the concentration of radionuclides sorbed irreversibly onto iron-hydroxide colloids, and the mass concentration of each colloid type (Figure 3-11). The colloidal concentration abstraction assumes that colloids are in the groundwater that enters a breached waste package and adsorb radioisotopes. Groundwater colloids potentially present in the repository include (1) microbes, (2) organic macromolecules (humic and fulvic acids), and (3) mineral colloids (primarily clay minerals, silica, and iron oxyhydroxide minerals). Microbial colloids were disregarded because the relatively large size of microbes makes them susceptible to filtration by the geologic media as already described in Section 3.2.3. Organic macromolecules were also disregarded, based on their low capacity to form chemical complexes at Yucca Mountain. Specifically, Minai et al. (1992) studied humic substances from J-13 well water and found that the complexation capacity was only about 2.3 × 10-10 eq/L (equivalents per liter) at pH 6.9 and 2.7 × 10-11 eq/L at pH 8.2, because of the presence of calcium in J-13 well water. September 2003 3-24 No. 8: Colloids Revision 2 September 2003 Source: BSC 2003f, Figure 17a. Figure 3-11. Algorithm for Determining the Availability and Stability of Colloids within the Waste Package 3-25 No. 8: Colloids Revision 2 Mineral colloids were considered because quartz, feldspars, silica, cristobalite, fused silica, amorphous silica, aluminosilicates, layer silicates, zeolites, plagioclase, carbonate, smectite clay, hematite, and goethite colloids occur in groundwater in the vicinity of Yucca Mountain (Kingston and Whitbeck 1991). The mineral colloids were represented by smectite clays because smectite was the dominant colloid type observed and strongly adsorbs radioisotopes. As discussed in Section 3.3, the YMP has compiled the colloid concentration in groundwater from igneous rock in saturated and unsaturated regimes from the vicinity of Yucca Mountain. This relationship was used for Step 1 of the procedure to estimate the groundwater colloidal mass concentration (Figures 3-9 and 3-11). For Step 2, the colloidal concentration determined in Step 1 was maintained provided the pH and ionic strength were below the approximately linear stability function measured by Tombacz et al. (1990), based on known properties of smectite colloidal suspensions (Section 3.3.4, Figure 3-6). Otherwise, the colloidal concentration was reduced to a minimum value of 10-6 mg/L if ionic strength is less than 0.05 M. The threshold value of the ionic strength of the 0.05 M in the waste package used to calculate the concentration of groundwater colloids is identical to the upper threshold used for determining stability and concentration of waste form colloids, since both waste form and groundwater colloids are modeled as smectite. To determine the mass of radioisotopes (plutonium, americium, thorium, protactinium, cesium) reversibly sorbed by the groundwater colloids for Step 3, a linear Freundlich model (Eq. 3-1) (Langmuir 1997) was used, using the dissolved concentration of radionuclides within the waste package as calculated by the solubility component of TSPA-LA model. The sorption coefficient (Kd) values for cesium, plutonium, and americium (and, by analogy, protactinium and thorium) were assigned piecewise uniform distributions, based on sorption experiments on smectite (Table 3-2). For cesium, the values range from 102 to 104 mL/g; plutonium ranged from 103 to 106 mL/g; for americium, the values range from 104 to 107 mL/g (Table 3-2). 3.4.3 Colloids from Iron Oxyhydroxide The colloidal concentration component assumes that iron oxyhydroxide (rust) colloids (primarily goethite, ferrihydrite, and hematite) form during corrosion of waste packages. For Step 1, a uniform distribution was assigned with a range of 50 mg/L and 0.05 mg/L, based on the miniature colloid experiments conducted at University of Nevada, Las Vegas (Section 3.3.2). For Step 2, the stability of iron oxyhydroxide colloids has been measured (Liang and Morgan 1990) and was used for developing the stability region as a function of pH and ionic strength. At and near the zero-point of charge, colloids are unstable, even at low ionic strengths. For iron oxyhydroxide colloids, the zero-point of charge ranges from about pH 5.5 to about pH 8.5 depending on the mineralogy and water chemistry. Colloids are stable at higher and lower pH values, similar to other mineral colloids, provided the ionic strength is less than 0.05 M. The result is a typical “U-shaped” stability curve (Figure 3-7). The minimum concentration of rust colloids in the unstable region was 10-3 mg/L. For Step 3, two types of sorption are modeled, irreversible sorption of a large fraction of plutonium and americium onto iron oxyhydroxide corrosion colloids and fixed corrosion September 2003 3-26 No. 8: Colloids Revision 2 products; and reversible sorption of a small fraction of plutonium and americium and reversible sorption of protactinium, thorium, and cesium onto iron oxyhydroxide colloids. Plutonium and americium are considered so strongly sorbed to colloids that, in essence, can be considered irreversibly sorbed and are modeled in this manner within the engineered barrier system (BSC 2003o, Attachment II). Plutonium transport velocities in soils reflect the fact that plutonium binds strongly to soils, leaving very little, if any, soluble plutonium available for groundwater transport or plant uptake. Coughtrey et al. (1985) estimate exchangeable plutonium (“soil available,” best estimate, Table 2a, p. 119) to be less than one percent. At Rocky Flats, soil plutonium is largely bound to soil metal hydroxides. Litaor and Ibrahim (1996) used 0.01M CaCl2 as an extractant and measured plutonium in Rocky Flats soil to be 0.04 to 0.08 percent exchangeable. Bunzl et al. (1995) measured exchangeable 239+240Pu (0.5 to 1 percent) and 241Am (1.5 to 15 percent) from fallout-contaminated soils in Germany using 1M C2H7NO2 (ammonium acetate NH4C2H3O2) as the extractant. Laboratory experiments of plutonium sorption onto iron oxides have shown that only about 1 percent of the initially sorbed plutonium can be desorbed into solution, even after five months of time have elapsed as already mentioned in Section 3.3.6 (Lu et al. 2000), which is broadly consistent with field observations. In the colloid abstraction, the difference in specific surface areas between the colloids and the stationary corrosion products is taken into account as well as the total surface areas (based on total masses). For example, plutonium will tend to attach to the colloids with higher specific surface areas, while the much larger mass of stationary corrosion products relative to total colloid mass results in most of the dissolved plutonium attaching to the stationary corrosion products. Honeyman and Ranville (2002) incorporated a stationary phase in their analysis of the effects of colloids on radionuclide retardation. The stationary phase competes strongly with the colloids for radionuclide sorption. By far most of the americium mass was calculated as attached to the stationary phase; at most, 0.001 percent of the total americium was associated with colloids (Honeyman and Ranville 2002, p. 140). In order to accommodate the field and laboratory observations, distribution of plutonium between colloids and the stationary corrosion products was implemented such that a large fraction of total plutonium is sorbed to corrosion products, a small fraction to colloids, and a very small fraction remains dissolved in the fluid. The distribution of irreversibly and reversibly sorbed plutonium and americium onto fixed and colloidal substrates is performed using a kinetic model that is described in EBS Radionuclide Transport Abstraction (BSC 2003o). Figure 3-12 shows the two domains of the conceptual model of radionuclide sorption onto the iron oxyhydroxide colloidal and stationary phases. The upstream domain is assumed to be degraded fuel rods, including secondary phases, in equilibrium with the aqueous phase at the radionuclide solubility limit (CRNdiss) predicted by the solubility component (BSC 2003g). The radionuclides of concern are the plutonium and americium isotopes, but since the material balance equations are written as a mass balance, the equations are valid for any solute species. There is no sorption considered in the upstream September 2003 3-27 No. 8: Colloids Revision 2 domain. Plutonium and americium are transported by both advection and diffusion downstream into the reaction mixing cell domain, where they can be involved in six1 separate reactions: 1. Reversible plutonium and americium sorption onto iron oxyhydroxide colloidal particles 2. Reversible plutonium and americium sorption onto the stationary phase iron oxyhydroxide corrosion products 3. Irreversible plutonium and americium sorption onto iron oxyhydroxide colloidal particles 4. Irreversible plutonium and americium sorption onto the stationary phase iron oxyhydroxide corrosion products 5. Reversible plutonium and americium sorption onto waste form colloids 6. Reversible plutonium and americium sorption onto groundwater colloids. Note: SPu refers to dissolved plutonium from spent fuel, other parameters as defined in figure. §Ů ˇÂ ˇÂ 1 Because plutonium and americium are also embedded into waste spalling from the high-level radioactive waste, they actually are involved in a total of eight separate reactions, but the embedded plutonium and americium are = determined separately and not included in the kinetic model. Figure 3-12. First Two Computational Cells of Conceptual Model of Reversible and Irreversible Sorption (Eq. 3-3) . September 2003 No. 8: Colloids on Mobile Iron Oxyhydroxide Colloids and Immobile Rust The mass in the fluid exiting the reaction mixing cell is constrained such that the mass of plutonium sorbed both reversibly and irreversibly onto colloids is between 90 percent and 99 percent of the total mass of plutonium exiting the system in all forms.aqueous, reversibly sorbed, and irreversibly sorbed. This conforms to observations in nature, such as the transport of plutonium from the Benham test site (Kersting et al. 1999). This flux ratio is expressed as: _mass 0.90 0.99 _ flux_out _ flux_out colloid total _mass 3-28 Revision 2 Also of interest is the ratio of the mass flux leaving the mixing cell to the mass flux entering the mixing cell. This ratio of mass out to mass in is given by ¦· (Eq. 3-4) flux _ out flux _ in mass _ mass _ = and is a measure of the retardation due to the corrosion products. It is expected that most of the plutonium mass entering the mixing cell is sorbed to the stationary iron oxyhydroxide phase and only a small fraction of it flows downstream to the unsaturated zone. Reversible sorption onto the colloids is calculated from the sampled mass concentration of corrosion colloids, Kd values between the fluid and iron oxyhydroxide colloids, and the dissolved concentration of radionuclides. The Kd values are defined with piecewise-uniform distribution describing the distribution of radionuclides (BSC 2003f, Table 10): the Kd for plutonium ranges between 104 and 106 mL/g. The Kd for americium, thorium, and protactinium ranges between 105 and 107 mL/g. The Kd for cesium ranges between 101 and 103 mL/g. 3.4.4 Colloids from High-Level Radioactive Waste The laboratory evidence suggests that as high-level radioactive waste glass degrades, colloids are generated and often contain ˇ°embeddedˇ± or irreversibly attached plutonium and americium (CRWMS M&O 2001a). The embedded plutonium and americium are likely isolated from the aqueous system and thus not in equilibrium with the surrounding aqueous environment. Therefore, some plutonium or americium in the high-level radioactive waste is assumed to be embedded in the high-level radioactive waste colloids. On the other hand, few colloids have been observed in laboratory tests of CSNF, and those colloids observed did not have radioisotopes irreversibly attached (CRWMS M&O 2001a). Furthermore, DSNF is assumed to behave similarly to CSNF (Section 3.2.). Step 1¨CFor Step 1, colloidal concentration was set to a bounding value above the maximum value observed in the high-level radioactive waste experiments (1 ˇÁ 10-7 M) at low ionic strength (CRWMS M&O 2001a). At ionic strength greater than 0.05 M, colloidal concentration was set at a low value (10-11 M). At intermediate ionic strength (between 0.01 and 0.05 M), irreversible colloidal concentration was calculated by interpolating between concentrations of 10-11 and 8 ˇÁ 10-8 M. Step 1a¨CThe radionuclides plutonium and americium are modeled in Step 1a as embedded within waste form colloids, which are produced from corrosion of defense high-level radioactive waste glass. The concentrations are defined by ionic strength, based on experimental observations. In TSPA-LA, Ihi-thresh,coll,wf and Ilo-thresh,coll,wf are threshold values of the ionic strength of the fluid in the waste package used to calculate the concentration of embedded plutonium used in the abstraction. Americium is calculated as the product of the plutonium concentration times the Am/Pu ratio in the inventory, which is determined at each time step of the TSPA-LA model calculations. Stability of the defense high-level radioactive waste glass-derived colloids is September 2003 3-29 No. 8: Colloids Revision 2 determined in Step 1b. Colloid mass concentrations are calculated from the amount of plutonium embedded in smectite waste form colloids in Step 1c. Step 1b–The stability of waste form colloids from defense high-level radioactive waste glass is determined on the basis of the fluid ionic strength and pH, based on known properties of smectite colloidal suspensions. The Ihi-thresh,coll,wf and Ilo-thresh,coll,wf parameters are the same threshold values of the ionic strength of the fluid in the waste package used to determine plutonium concentration (Step 1a). They are used here to determine, along with pH, whether or not the colloids are stable. These threshold values may vary due to specific chemical and environmental conditions and are therefore sampled over a range. Step 1c–Calculation of the mass concentration of waste form colloids from defense high-level radioactive waste glass is done on the basis of experimental observations. RNcoll,wf,embed,max. It is an The colloid mass concentration is related to the embedded plutonium concentration (Equation 3-3). With reference to Equation 3-3, Mcoll,wf,both,max is the maximum colloid mass concentration corresponding to the maximum plutonium concentration, C intermediate result based on the experimental data showing the relationship between embedded plutonium concentration and colloid mass concentration. Mcoll,wf,both is a parameter that represents the total mass concentration of defense high-level radioactive waste glass colloids with both embedded and reversibly sorbed radionuclides. Step 1d–The calculation of the concentration of radionuclides (plutonium, americium, thorium, protactinium, cesium) reversibly sorbed on waste form colloids from defense high-level radioactive waste glass is based on the mass concentration of waste form colloids, Kd values describing the distribution of radionuclides between the fluid and smectite colloids, and the dissolved concentration of radionuclides as calculated by the TSPA-LA model. The CRNdiss is the concentration of radionuclide “RN”, determined by the TSPA-LA model as output from the solubility concentration model, which is used as an input to the colloid abstraction. Kd,RN,wf is a parameter derived from several sources and is an equilibrium sorption coefficient used to approximate the partitioning of dissolved radionuclide “RN” between colloids and fluid. Step 2–For Step 2, the simple linear stability relationship that was used for groundwater colloids (primarily smectite) was also used to determine the stability of the irreversible high-level radioactive waste colloid concentration as a function of pH because waste form colloids are composed primarily of smectite clay minerals (CRWMS M&O 2001a). When in the unstable region, the colloid concentration was set to 10-11 M. Step 3–For Step 3, the algorithm is more complicated for high-level radioactive waste because the radioisotopes can be both irreversibly and reversibly attached to the high-level radioactive waste colloids. The algorithm makes several assumptions: (1) only americium and plutonium are irreversibly attached; (2) the irreversibly bound isotopes of plutonium and americium are partitioned according to the isotopic mass fraction calculated at the time the colloids are formed; (3) no initial mass of decay daughters are assumed irreversibly bound; and (4) in any one time September 2003 3-30 No. 8: Colloids Revision 2 step, americium and plutonium do not attach reversibly until the maximum available mass of americium and plutonium in the high-level radioactive waste is irreversibly attached. That is, the concentration of irreversible americium and plutonium is first determined; then, the reversible available mass (including any available radioisotopes from DSNF) is calculated as the total mass available times the ratio of the irreversible concentration and the maximum irreversible concentration (Figure 3-11). The reversible concentration is determined with the linear Freundlich model. Both plutonium and americium used Kd values defined with piecewiseuniform distributions (Section 3.4.3). 3.5 UNCERTAINTY AND SENSITIVITY 3.5.1 Uncertainty of Colloid Concentration The parameters developed for the colloid transport model have various degrees of uncertainty. The sources of uncertainty arise due to (1) uncertainty associated with the use of non-sitespecific data in some cases to develop parameter values; (2) uncertainty associated with the prediction of actual physical and chemical conditions in the repository; (3) uncertainty associated with site- and environment-specific testing of colloid formation, stability, and transport; and (4) uncertainty associated with the use of supporting technical and corroborative information that may not have been developed specifically for these physical and chemical conditions. These uncertainties must be appropriately captured in the TSPA calculations by choosing conservative model formulations or appropriate upper/lower limits for parameter values. In the implementation of a TSPA, it is often necessary to use experimental data gathered over a few days to a few years and field observations recorded over a few days to a few decades to arrive at conclusions on what is anticipated to occur over the 10,000-year regulatory compliance period The parameters relating to irreversible association of radionuclides with colloids, which is influenced by kinetics, must be used cautiously when speculating about future conditions. Similarly, the extrapolation of laboratory-scale results to the scale of the repository setting also needs to be cautious. Corroboration with field data is generally required. Kersting et al. (1999) measured the total plutonium concentration at the ER-20 site, which is 1.3 km from the Nevada Test Site. The maximum plutonium concentration measured was about 10-14 M. Given the strong affinity of plutonium for solid surfaces, Kersting et al. (1999) argued that this low concentration indicated that a small fraction of plutonium could be potentially transported by colloids over a long distance. Whether this mechanism is plausible or relevant to the repository conditions (note that the physical/chemical conditions at Nevada Test Site can be very different from repository conditions), the low concentrations from Kersting et al. (1999) indicate that plutonium transported by groundwater can be attenuated very quickly over a short distance (~1.3 Km). The plutonium concentration was about six orders of magnitude lower than the solubility limits of 10-8 M experimentally determined as likely to be present in Nevada Test Site groundwater. Therefore, these observations indicate that there would be a potential for longdistance colloid-associated plutonium transport in the Yucca Mountain environment (given the fact that plutonium tends to be strongly sorbed on solid surfaces), but the contribution of such transport to system performance is unlikely to be significant. Uncertainties associated with the determination of colloid concentration in seepage/groundwater derive from (1) field sampling techniques, including differences in pumping rates at each well September 2003 3-31 No. 8: Colloids Revision 2 during extraction of the water samples, (2) other unknown factors affecting the quantities of particles suspended in the water samples, including the types of additives introduced in the wells during the drilling process itself, and (3) errors inherent to the laboratory methods used to measure the quantities of colloids suspended in the water samples (e.g., filter ripening, interference and detection limitations for dynamic light scattering measurement techniques). The sampling perturbation may result in over-estimating colloid concentrations. With respect to the spatial scale, there is the issue of the appropriateness of extrapolation of specific water sample measurements to represent the colloid concentrations in waters over a wider region or area. Temporal scaling issues could include potential seasonal variations in the quantities of colloids suspended in water samples extracted, or even shorter time-scale changes in colloid concentrations in water samples during the sampling of wells. To define this uncertainty, a cumulative distribution function was developed based on groundwater samples extracted in the vicinity of Yucca Mountain (Section 3.3.5). To evaluate the reasonableness of this distribution for the site-specific data, the distribution function compared with colloid mass concentrations reported in the literature and at Idaho National Engineering and Environmental Laboratory (Figure 3-9). As shown in Figure 3-9, the probabilistic distribution of colloid concentrations developed for YMP is reasonable. 3.5.2 Uncertainty in Stability of Colloids Uncertainty in the stability of colloids (smectite and iron oxyhydroxide) as a function of pH and ionic strength is associated mostly with the extrapolation of laboratory data reported in the literature or project-supported experimental work to actual conditions (i.e., solution chemistry) that would be present in the repository environment over the regulatory compliance period. Much of this uncertainty is accommodated by establishing conservative, but reasonable, bounding values and ranges of parameter values. This uncertainty is propagated through the TSPA-LA model by stochastic sampling of these distributions during Monte Carlo simulations employed in the model calculations. 3.5.3 Uncertainty in Partition Coefficients Uncertainty is associated with the development of sorption partition coefficients (Kd values) to describe the degree of sorption of specific radionuclides to colloids. Values reported in the literature are primarily the result of experimental work designed to establish Kd values for contaminants sorbed onto rocks, soils and other minerals, but literature specific to colloid-size minerals is not readily available. For this reason, the Kd value parameters established for smectite and iron oxyhydroxide colloids rely on limited experimental work conducted at Los Alamos National Laboratory (Lu et al. 1998). Corroborative data reported in the literature were evaluated to augment the Los Alamos National Laboratory data (Table 3-2). The transport of a radionuclide in the presence of a constant colloid concentration can be described by: ˇÓ ˇÓ . ˇÓ ˇÓ Ąń ) ( ) ( ) 1 [( ) ] ( ) ( ( m m V ) m cV c d c d s s c d + + = . Ąő Ąő D D ˇÓ ˇÓ ˇÓ ˇÓx Ąő ˇÓ Ąő ˇÓ ˇÓ Ąő ˇÓ cm t m x c m x m x ˇÓ t x .. . . ˙. . .. . + ˙. . x (Eq. 3-5) ˇÓ ˇÓ 3-32 Ąő ˇÓt No. 8: Colloids ˇÓ Ąő ˇÓ September 2003 Revision 2 where Ąő is the porosity; md is the concentration of dissolved radionuclide (M); c is the colloid concentration (g/L); mc is the moles of radionuclide sorbed on colloids (mol/g); Ąńs is the density of the immobilized adsorbing phase (g/dm3); ms is the moles of radionuclide sorbed on the immobilized adsorbing phase (mol/g); V is the flow velocity (dm/y); D is the dispersion coefficient of the dissolved species (dm2/y); t is the time (y); and x is the spatial coordinate (dm). Substituting Equation 3-1 into Equation 3-5 for mc, one can obtain: )V cK )D cK 2m d d d d d = (Eq. 3-6) 2 m x ˇÓm t ˇÓ Ąń Ąő ˇÓ ˇÓ ) 1 ( + 1 ( . ) 1 ( ) 1 ( cK K cK K x d d d s d . d Ąő cK ) ' (Eq. 3-7) ˇÓ ˇÓ d s by: Ąő Q . Ąńs V T ˇĂ ' (Eq. 3-8) L V T < ' Ąő1 ( + cK ) d Ąő1 ( + + d include technetium, iodine, and cesium. For 1 cK >> , Equation 3-8 is reduced to: d Ąő1 ( + + V d VT . L for .. . V ˇĂ 'T L for . Ąő L (Eq. 3-9) Q Ąő1 ( + ) + Therefore, the actual velocity of the advancement of the adsorption front ( ' V ) can be calculated by: V = Assuming that a fluid percolates through a porous medium of a length of L (dm), the total radionuclide release from the system over a time period T (y) can be approximately calculated AF V where A is the cross-section area of the system; and F is the incoming flux (mol/dm2/y). For L , Q = 0, i.e., no radionuclide will be released from the system. For the colloid-facilitated radionuclide transport to be negligible compared to the dissolved species transport, it is required that cK << 1. Using the upper colloid concentration limit of 200 ppm in the Yucca Mountain groundwater, it can be calculated that any radionuclide with a d K less than 5,000 mL/g can be ignored for colloid-facilitated transport. These radionuclides .. . . AF V Ąő1 ( + .. . colloids to the immobilized adsorbing phase, + .. . Therefore, as long as Kd is large enough (greater than 5000 mg/L), the total radionuclide release from the system becomes insensitive to Kd and only depends on the concentration ratio of Ąőc /(1. Ąő) Ąń , which is typically very small. s Ąő1 ( + cK ) + (1. Ąő) ĄńK cK ) d K 1 ( . d s Ąő) Ąń VT . Ąőc Ąőc + (1. Ąő) Ąńs 3-33 September 2003 No. 8: Colloids 3.6 MODEL CONFIDENCE BUILDING Sensitivity studies conducted to prioritize work for the LA (BSC 2002a) showed that the colloids had a small effect on the dose. Therefore, a relatively low level of confidence is required for the colloid model. The types and characteristics (including stability and concentration) of colloids formed from the degradation of the waste forms as used in the model abstraction is based on extensive observations of colloids from testing programs and natural groundwater. The validation activities for this model analysis and abstraction took into account the criteria established in the Yucca Mountain Review Plan, Final Report (NRC 2003). Post-modeldevelopment validation is accomplished through corroboration of model predictions and data used with data published in referred journals or literature and through corroboration by comparison to data from natural analog sites. Detailed validation arguments are documented in Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003f). Corroborating/supporting data and information used to develop and validate the parameters are listed in Table 3-3. It is concluded that the validation activities performed for building confidence in the model elements and data developed for YMP colloidal transport have sufficient scientific basis and that the acceptance criteria documented in the Yucca Mountain Review Plan, Final Report (NRC 2003) will be met satisfactorily. Supporting (Corroborating) Information Used to Build Confidence in the Colloid Model Table 3-3. Supporting (Corroborating) Information Source Tombacz et al. 1990 Mertz et al. 2003 Short et al. 1988 Payne et al. 1992 Zänker et al. 2000 Vilks et al. 1993 Brady et al. 2002 DTN LA0002SK831352.003 DTN LA0002SK831352.004 Coughtrey et al. 1985 Litaor and Ibrahim 1996 Bunzl et al. 1995 No. 8: Colloids Revision 2 Data/Information Stability of smectite (which is used as a surrogate mineralogy for defense high-level radioactive waste glass colloids) at full range of conditions anticipated in TSPA-LA calculations Corroborative data supporting conclusion of no significant colloids generated from CSNF degradation Corroborative information and data regarding low colloid-associated uranium concentrations in the vicinity of mines Corroborative information and data regarding low colloid-associated uranium concentrations in the vicinity of mines Corroborative information and data regarding low colloid-associated uranium concentrations in the vicinity of mines Corroborative information and data regarding low colloid-associated uranium concentrations in the vicinity of mines Corroborative information and data regarding limited extent of dissolved uranium plumes Corroborative (non-Q) data for qualified groundwater data Corroborative information regarding irreversibility of plutonium on mineral particles Corroborative information regarding irreversibility of plutonium on mineral particles Corroborative information regarding irreversibility of plutonium on mineral particles September 2003 3-34 Revision 2 3.7 SUMMARY Three different types of colloids are considered in the modeling of the radionuclide source term: • Waste-form colloids resulting from corrosion of defense high-level radioactive waste glass with embedded discrete phases of plutonium and americium and represented by smectite colloids • Iron oxyhydroxide colloids resulting from the corrosion of waste package and metallic invert materials • Natural colloids in seeping groundwater, including mineral fragments, humic substances and microbes and represented by smectite colloids. Humic substances are not considered to be sufficiently abundant in Yucca Mountain groundwater to be of significant impact to radionuclide transport. Also, microbes, due to their relatively large size (about 1 µm) within the colloid-size range, are highly susceptible to filtration. Smectite colloids are used to represent natural colloids at the Yucca Mountain site, because of their abundance and relatively high affinity for radionuclides. True or intrinsic colloids are eliminated from consideration in TSPA analyses for several reasons: (1) long-term corrosion testing with CSNF and DSNF under unsaturated, oxidizing conditions have shown very low colloid concentrations and small amounts of uranium within the colloid mass; and (2) the same test indicates that meta-schoepite and UO2+x colloids readily dissolve upon the introduction of J-13 groundwater. Defense high-level radioactive waste glass degradation experiments show that plutonium can be embedded in smectite colloids. Further, evidence from sorption experiments show that plutonium and americium are strongly adsorbed onto iron oxyhydroxide colloids and the rates of desorption of the two radionuclides are significantly slower than the rate of sorption. In the model it is assumed that over 90 percent of the adsorbed plutonium and americium is irreversibly adsorbed to both corrosion colloids and stationary corrosion products. The sorption of all other radionuclides is assumed to be reversible. Colloid suspensions are likely to be unstable partly because of evaporation during the thermal period and waste degradation reactions may lead to ionic strengths above 0.05 M. For the seepage-water dripping scenario, the most relevant in-package chemistry scenario with regard to colloid transport, reaction-path calculations indicate that the initial seepage water composition is rapidly overtaken by water-package interactions (BSC 2003h). Depending on the seepage flux, as the chemical reaction progresses, the water can reach an ionic strength more than 0.05 M, the upper ionic-strength limit for stable colloid suspension. Therefore, under such conditions, colloid suspensions tend to be unstable. Once formed and stably suspended, colloids could be filtered out. Notwithstanding the potential colloid filtration processes that would reduce the amount of radioactive colloids released from the waste package, it has been assumed that the waste package does not provide a barrier to the release of radioactive colloids. Therefore, the dominant mechanism leading to low September 2003 3-35 No. 8: Colloids Revision 2 concentrations of colloids in suspension is the expected lack of stability of the colloids due to chemical conditions that lead to high ionic strengths. September 2003 3-36 No. 8: Colloids Revision 2 4. INVERT This section of the technical basis document summarizes the current understanding of the formation of radionuclide-carrying colloids within the invert and their release into the unsaturated zone. The invert is the second of the repository system subfeatures examined to determine the importance of colloids with respect to the postclosure performance of the Yucca Mountain repository system, as shown in Figure 1-1. This section provides a summary-level synthesis of the processes and phenomena governing the formation, prediction of their concentration, and transport of pseudocolloids within the invert. Modeling assumptions to be used in the TSPA-LA are discussed. In preparing this section, information has been extracted and summarized in pertinent analysis and model reports as well as other technical products that are used: • Multiscale Thermohydrologic Model (BSC 2003n) • Advection Versus Diffusion in the Invert (BSC 2003j) • EBS Radionuclide Transport Abstraction (BSC 2003o) • Engineered Barrier System: Physical and Chemical Environment Model (BSC 2003k). 4.1 DESCRIPTION OF RELEVANT PROCESSES AND MODELING ASSUMPTIONS FOR INVERT 4.1.1 Collapse of Pallet Supporting Waste Package The invert, which is primarily composed of crushed tuff rocks, is 0.8 m thick. The relevant physical/chemical processes for colloid generation and transport in the invert are summarized in Figure 4-1. The waste package sits on a pallet composed of Alloy 22. The pallet, in turn, sits on an invert. Degradation of the pallet is not considered since it has been assumed to have collapsed by the time the waste package has breached. Hence, the waste package is assumed to be in intimate contact with the invert below. When releases of colloidal material are primarily through diffusion, as in the nominal case without any dripping seepage water, neglecting the pallet shortens the radionuclide transport path and, thus, is a conservative assumption as discussed in the feature, event, or process (FEP) screening-out argument for FEP 2.1.06.05.0A (BSC 2003o). In the following discussion, the invert also provides a large diffusion path for colloids in this situation as well. Either diffusion barrier by itself is probably sufficient to remove all colloids. Thus, it is necessary only to model one barrier. Because the pallet is reasonably omitted in the other two scenarios considered in the TSPA (the scenario where an igneous dike intrudes into the repository and the scenario where a seismic event severely shakes the repository), the pallet was not considered. September 2003 4-1 No. 8: Colloids Revision 2 Figure 4-1. Relevant Physical/Chemical Processes for Colloid Generation and Transport 4.1.2 Water Flow All flux that may originate from the waste package flows to the invert, independent of patch/pit location on the waste package. The flux that is diverted around the waste package and the drip shield also flows into the invert. In addition, seepage water may also enter the invert by a wicking process. Finally, evaporation may take place in the upper portion of the invert during the thermal pulse over the first few thousand years after repository closure (BSC 2003o). Evaporated water may condense on the inside of the drip shield, and drip onto the waste package. All mass flux into the invert, except for the evaporative flux, is released into the unsaturated zone. During the regulatory period, the type of water flow through invert depends on the scenario under consideration (nominal, seismic, or volcanic), as explained further in Section 4.3.3. For the most part, the crushed tuff invert will maintain unsaturated conditions under the waste package and thereby reduce advective transport of colloids out of the engineered barrier (see Figure 3-2). 4.1.3 Invert Chemistry Because of the generally slow seepage of water into the invert previously described, any seepage water entering the invert will have time to react with the crushed tuff. The resulting solution will tend toward neutral to mildly basic pH. Any seepage water entering the invert will combine with the flux from the waste package, and likely alter the chemistry of the flux from the waste package, as well. As a result, the iron oxyhydroxide colloids may become unstable (as shown in Figure 3-7). Also, the evaporation process may concentrate the solution, which may further September 2003 4-2 No. 8: Colloids Revision 2 increase the instability of colloids in the invert. As explained further in Section 4.3.3, the chemistry of the water used in the invert was dependent on the scenario. 4.1.4 Generation of Additional Colloids In addition to those colloids directly input from the waste package and the seepage water, similar colloids could be present in the invert and generated from the corrosion of pallet and steel supports within the invert. The corrosion of invert support material and electrical conductors will generate iron and copper oxides, which are generally strong adsorbents for various radionuclides. Although the mass of material in the drift including the steel mesh, ribs, invert support, and drip shield (~1.8 Mg/m) is about the same as in a CSNF waste package (~1.6 Mg/m), additional colloids present in the drift and the sorption effect were not included as important phenomena. First, the waste package material is in closer proximity to the actinides. Second, the spatial distribution of the corrosion products in the invert will tend to be localized and relatively widely spaced. Hence, seepage water from the waste package could completely miss corrosion products in the invert. Third, the iron oxyhydroxide colloids, representing the corrosion products, may be unstable in the invert water chemistry, as noted above. Finally, the additional sources are not expected to change the upper limits of colloid concentrations estimated in Section 3 from systems that have achieved metastable equilibrium states. 4.1.5 Colloidal Filtration The extent of colloidal filtration cannot be expressed with certainty since physical properties of the crushed tuff have not been specified. Because of the large difference in colloid particle size and crushed tuff in the invert, physical filtration such as straining will likely be unimportant in the invert. Within the invert, the filtration process is not included because of the small dimension of the invert (0.8 m maximum) and, thus, limited opportunity for sufficient interaction of particles with the media. As an example of a model applied to nuclear waste disposal, Aguilar et al. (1999) modeled the filtration potential in an analysis for the performance assessment of the Waste Isolation Pilot Plant in Carlsbad, New Mexico. Their model predicted substantial filtration of colloids suspended in Waste Isolation Pilot Plant brines, often in distances of several meters. Based on typical parameter ranges and using a similar approach, Rechard et al. (2003) have shown that on average only minor filtration would occur in the invert due to the short filtration path of 0.8 m (maximum) and it has been ignored in the TSPA. However, filtration is modeled in the unsaturated and saturated zones in Sections 5 and 6, because of its greater extent. 4.2 SOURCE OF DATA AND TESTING Diffusion coefficients used by the YMP for the invert material are tabulated in Table 4-1. Corroborative information is also available since the diffusion coefficient of ions has been determined specifically for crushed tuff invert materials as a function of volumetric moisture content (m3 water/m3 bulk rock), using electric conductivity measurements (BSC 2003o, Table 41). The data are tabulated in Table 4-2. The diffusion coefficient for colloids is estimated from the Stokes-Einstein relationship (see Equation 4-1 in Section 4.3). September 2003 4-3 No. 8: Colloids Table 4-1. Diffusion Coefficient for Granular Materials at Various Volumetric Moisture Contents Volumetric Moisture Content (%) 66.3 49.0 40.0 32.3 30.9 20.4 17.5 10.1 9.24 5.40 3.70 Source: BSC 2003o, Table 6. DTN: MO0007MWDIDD31.001. Diffusion Coefficient of Crushed Tuff Invert Materials Table 4-2. Volumetric Moisture Content (%) 32.13 18.15 9.26 5.41 3.64 0.20 1.25 × 10-10 Source: BSC 2003o, Table 7. DTN: MO0002EBSDDC02.003. No other specific tests have been performed simulating the configuration of the invert; however, tests on the invert material, iron and tuff, have been performed. The results of these tests, as discussed in Section 3.3, are used. For example, for the radionuclide partitioning, the same distribution coefficients tabulated in Table 3-2 are applied. 4.3 MODEL FOR COLLOID-FACILITATED TRANSPORT IN INVERT This section summarizes the algorithm used to reevaluate the colloid stability. The logic implemented in the TSPA-LA model is provided, although specific programming details are not. No. 8: Colloids Diffusion Coefficient (cm2/s) 1.8 × 10-5 6.09 × 10-6 6.90 × 10-6 4.60 × 10-6 1.51 × 10-6 4.19 × 10-6 1.10 × 10-5 3.51 × 10-7 2.55 × 10-7 7.60 × 10-8 3.70 × 10-8 Diffusion Coefficient (cm2/s) 2.02 × 10-6 5.40 × 10-7 4.05 × 10-8 9.97 × 10-10 5.00 × 10-10 4-4 Revision 2 September 2003 Revision 2 4.3.1 Two Flow Regimes in Invert The crushed tuff in the invert in the current LA design is characterized by intergranular pore space (pore space in between the aggregate) and intragranular pore space (pore space in the matrix aggregate) (BSC 2003j; BSC 2003k; BSC 2003o). The intragranular pore space has a relatively low intrinsic permeability and porosity whereas the intergranular pore space has a relatively high permeability and porosity. After the thermal period the invert should achieve hydrologic equilibrium with the host rock through capillary action or imbibition, that is, water saturations in the intragranular pore space will reach approximately 90 percent (same as host rock matrix) and water saturations in the intergranular pore space will remain near zero. During the thermal period the pore space in the invert will be dry or near dry and water will gradually enter the intragranular pore space as the invert cools. 4.3.2 Algorithm Overview The algorithm for the invert cell (BSC 2003k) divides the invert into these two pore spaces and implements the chemistry that corresponds to each pore space. That is, first, waste-package releases are partitioned into two fractions (Figure 4-2): (1) the intergranular pore space release fraction and (2) the intragranular pore space fraction. An important simplification is made by assuming no mass transfer occurs between the two pore spaces. The partition of the flow in the two flow regimes in the algorithm differs for the three main scenarios evaluated: nominal scenario, seismic scenario, and igneous scenario. The flow fraction entering the intragranular pore space is transported by diffusion using chemistry (i.e., ionic strength ionic strength and pH) appropriate for the tuff matrix, as evaluated by the drift-scale thermal-hydrologic-chemical model (BSC 2003p) and then evolved by the physical and chemical environment model (BSC 2003k). The flow fraction entering the intergranular pore space is transported by advection and diffusion using chemistry appropriate for the fractures using the tables developed by the physical and chemical environmental model (BSC 2003k). Second, the abstraction determines the stability of the colloidal mass as a function of ionic strength and pH, using the same stability curves as developed for the colloidal source-term discussed in Section 3. Third, the amount of radioisotopes reversibly sorbed on colloids is recalculated based on the radioisotope solubility for cesium, americium, plutonium, protactinium, and thorium adjusted for the appropriate invert chemistry. Finally, the colloids are transported via diffusion or advection through the invert. 4.3.3 Partition of Contaminated Water Between Inter- and Intraporosity of Invert The distribution of water contaminated with dissolved radioisotopes and colloids with radioisotopes attached to the inter- and intraporosity of the invert depends on whether the nominal, seismic, or igneous scenario is being evaluated. In turn, the appropriate water chemistry to evaluate the stability of colloids is dependent upon whether the colloids enter the inter- or intraporosity of the invert. September 2003 4-5 No. 8: Colloids Source: BSC 2003k. 4.3.3.1 No. 8: Colloids Figure 4-2. Colloidal Transport Algorithm within the Invert Nominal Scenario The nominal scenario represents the collection of FEPs most likely to occur. Based on corrosion analysis, the drip shield will remain intact for the regulatory period and, thus, any seepage water that enters the drifts will be diverted away from the waste package. Hence, the only source of water is condensation or dripping of condensation onto the waste package. With this limited amount of water, in the nominal scenario the waste packages do not fail through generalized or localized corrosion or stress corrosion cracking within the 10,000-year regulatory period. This is 4-6 Revision 2 September 2003 Revision 2 similar to the model treatment in the TSPA for the site recommendation (DOE 2002). Hence, for the nominal scenario, releases can only occur because a small fraction of the waste packages have small manufacturing defects. In this situation, all the releases occur via diffusive pathways through the manufacturing defects in the waste packages. Within the shadow of the drip shield, most radionuclides will be transported via diffusion through the intragranular pore space. Diffusive transport through the intergranular pore space will be insignificant since water saturations in the upper portion of the invert will be near zero. Diffusive transport of colloidal material through the intragranular pore space of the invert will likely be small, since the diffusion coefficient for colloids is at least 100 times smaller than for dissolved species based on the relative size of colloids to dissolved species. The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein relationship (BSC 2003o). ion (Eq. 4-1) ion Dcoll = D r rcoll ˙ ˙. . 100 times smaller than that of a dissolved ion. (Eq. 4-2) T = Seismic Scenario 4.3.3.2 . .. . where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the diffusion coefficient of an ion of radius rion. Given a typical ion radius and colloidal particle radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally Furthermore, the characteristic time (T) for dissolved radionuclides to diffuse through the invert of thickness L can be calculated by: D L / 2 where D is the diffusion coefficient. For an invert transport thickness of 0.8 m, the characteristic time T for colloid diffusion is ~3000 years at 90 percent saturation (i.e., the diffusion coefficient is 6.09 ˇż 10.8 cm2/s for colloids for 90 percent saturation, which corresponds to a moisture content of 49 percent in Table 4-1, assuming porosity equal to 0.54 for the tuff). Any mixing that occurs between waters in the shadow of the drip shield will be by the slow process of diffusion of anions and cations. The water chemistry in this shadow will remain similar to matrix pore water as ions migrate and react with the water constituents and minerals in the matrix. Thus, the chemistry for evaluating the stability of the colloids corresponds to the matrix pore water chemistry as determined by the physical and chemical environment model (BSC 2003k). In the seismic scenario evaluated out to 10,000 years, an earthquake damages both the drip shield and the waste package. Hence, some advective transport of colloids can occur for those fractions of packages that encounter dripping water. Colloidal releases from diffusion for those packages that either do not have damaged drip shields or do not encounter dripping water will likely be negligible as described for the nominal scenario. In those packages encountering dripping water, colloidal releases can also occur through diffusion into the inter- and intragranular porosity of the invert. Unlike the nominal scenario, intergranular diffusion could occur since the invert saturation will not be zero for waste packages with dripping water. Consequently, the stability of September 2003 4-7 No. 8: Colloids Revision 2 colloids that are advected through the invert or diffuse via the intergranular porosity must be evaluated using the fracture water chemistry as estimated by the physical and chemical environment model (BSC 2003k). Igneous Scenario 4.3.3.3 In the igneous scenario, an igneous dike is envisioned to intersect the repository. Two distinct zones are formed. In Zone 1 where the disposal drifts are intersected by the dike, the drip shields and packages are assumed to be disrupted and breached from either the shock wave or heat from the igneous intrusion. In Zone 2, no damage is assumed to occur, and, thus, the same conditions used in the nominal scenario are assumed. Degradation of the waste and formation of colloids in both Zones 1 and 2 is assumed to occur once the drift cools. In Zone 1, the drift is assumed to be completely filled and the host rock and waste package disrupted enough that the drift no longer influences water percolating through Yucca Mountain. Hence, a uniform percolation flux through drifts and waste is hypothesized. Consequently, advective transport of colloids from all the packages through the invert will occur in Zone 1. The water chemistry for evaluating the stability of the colloids is modeled as that formed from the pore water equilibrated with basalt. 4.3.4 Re-evaluation of Colloidal Stability and Solubility Based on the appropriate ionic strength and pH of the seeping water carrying the colloids in the invert, the colloid stability is reevaluated. Because the material is the same, the stability functions developed and used for the source-term (Section 3) are used in the invert as well. Irreversibly attached plutonium and americium are modeled as a separate dissolved species. If the water chemistry is such that the colloids are unstable, colloids would flocculate and settle out. This situation is modeled as if the “species” precipitated. If at a later time step, the water chemistry is such that the colloids are stable, then the irreversibly attached and embedded colloids are assumed to be resuspended and modeled as if the species redissolved. The situation is different for colloids with reversibly attached radioisotopes. If the colloids are unstable, then they would flocculate and settle out. However, the radioisotopes are still assumed to be reversibly attached and able to desorb if the solution is below the solubility limit. Then, the solubility of the radioisotopes is reevaluated based on the invert water chemistry. 4.3.5 Re-evaluation of Adsorption In the final step, the dissolved radioisotope mass is redistributed between the solution and total number of colloids that are available based on the colloid stability functions and the reversible adsorption coefficients (Kds). The total number of colloids is used since the conceptual model does not exclude iron oxyhydroxide (rust) colloids with irreversible attached radioisotopes from also reversibly adsorbing radioisotopes. A distinct difference, however, is that irreversible adsorption on iron rust colloids is no longer active in the invert. Adsorption on immobile tuff material can occur in the invert, but is probably only minor. The adsorption coefficients for tuff are small (three orders of magnitude less than iron rust) and so adsorption on the 0.8 m maximum thickness of the invert will be small compared to potential adsorption in about 300 m of unsaturated tuff. Steel support structures will be placed throughout the invert to support the steel rails on which the waste packages and September 2003 4-8 No. 8: Colloids Revision 2 drip shields will be transported, and on which the emplacement pallets will be placed. When this support corrodes, much of the iron rust will remain in the invert. In addition, communication and electrical cables will eventually corrode, leaving copper oxides in the invert. Although these oxides are capable of sorbing substantial amounts of radioisotopes, the oxides will be highly localized and widely separated in relation to the 5-m-long waste packages. For example, the transverse support beams in the invert are spaced 1.5 m apart (BSC 2003o). Thus, the oxides will reside in a strip several centimeters wide separated by 1.5 m of crushed tuff containing little corrosion products. In short, these materials are out of the general flow paths under the waste package, and they are of small spatial extent. Thus, the chance of radioisotopes being released from the waste package and passing through the adsorbing oxides is not great. 4.3.6 Advective and Diffusive Transport The final step is the evaluation of the advective transport of radioisotopes attached to colloids out of the invert cell based on the fluid velocity through the cell, and the diffusive transport of colloids based on the colloid concentration gradient and the colloid diffusion coefficient (Dcoll). The colloid diffusion coefficient is set 100 times smaller than the diffusion coefficient of dissolved radioisotopes based on the 100 times difference between the ionic radius of a radioisotope and the radius of a colloidal particle. The actual calculation involves the solution of a similar one-dimensional differential equation used when evaluating transport within the waste package mentioned in Section 3. Intergranular pore space water saturation during the postclosure period is calculated based on an analysis that defines intergranular pore space saturation as a function of dripping flux. The saturation of the intragranular porosity is assumed equal to the tuff matrix saturation, based on the Multiscale Thermohydrologic Model (BSC 2003n). 4.4 UNCERTAINTY AND SENSITIVITY 4.4.1 Uncertainty in Chemical Environment Within the invert, the stability of colloids and the solubility of radioisotopes is a function of the chemical environment. For the nominal and seismic scenarios, the uncertainty in composition of seepage water entering the emplacement drifts and the invert during the postclosure period is represented by five different seepage water groups. The time-dependent compositions of these five seepage groups are provided by the drift-scale thermal-hydrologic-chemical model (BSC 2003p) and the in-drift physical and chemical environment model (BSC 2003k). Each seepage water group includes two water types, matrix water and fracture water. Fracture water is assumed to enter the emplacement drift by dripping at the crown of the drift. Matrix water enters the drift by imbibition from the host rock matrix into the intragranular pore space occupying the invert ballast material. The capillary pressures in the relatively coarse intergranular pore space are too small to produce imbibition. When crown seepage drips onto the invert, some of it may imbibe into the intragranular matrix and some of it will flow downward through the intergranular pore space. The dripping water will tend to travel through the intergranular pores since the permeability of the matrix is much lower than the intergranular permeability. Because of the low matrix permeability, advection September 2003 4-9 No. 8: Colloids Revision 2 through the matrix will be minimal even when seepage is occurring. Water that imbibes into the matrix will react with the matrix water and the minerals in the matrix. Hence, the mixed water in the matrix over time will tend to become similar to matrix water. The time-dependent fracture and matrix water compositions for each of the five different seepage water groups are used as input to the physical and chemical environment model. The physical and chemical environment model evaporates these different waters over a range of representative environment conditions for the engineered barrier system (e.g., temperature, partial pressure of CO2, and relative humidity) and the results for both water types—matrix water and fracture water—are abstracted into the form of lookup tables for use by the TSPA-LA model. Each realization selects a water and the corresponding lookup tables for invert and crown seepage chemistry are accessed. The five uncertain water groups are given equal probability weighting. 4.4.2 Uncertainty in Adsorption The reversible coefficients for adsorption on colloids in the invert are the same coefficients as used in the waste package. Hence, the uncertainty discussed in Section 3.5 is applicable here as well. 4.5 SUMMARY The distribution of water contaminated with colloids with radioisotopes attached to the inter- and intraporosity of the invert depends on whether the nominal, seismic, or igneous scenario is being evaluated. For the nominal scenario, only diffusive releases can occur and thus the potential for colloidal transport is negligible. For the seismic and igneous scenarios, some advective transport can occur. In these scenarios, the appropriate water chemistry to evaluate the stability of colloids is dependent upon whether the colloids enter the inter- or intraporosity of the invert. Based on the appropriate ionic strength and pH of the seeping water carrying the colloids in the invert, the colloid stability is reevaluated. Because the material is the same, the stability functions developed and used for the source-term (Section 3) are used in the invert as well. In the final step, the dissolved radioisotope mass is redistributed between the solution and total number of colloids that are available, based on the colloid stability functions, and the reversible adsorption coefficients (Kds). Irreversible adsorption on iron rust colloids is no longer active in the invert. September 2003 4-10 No. 8: Colloids Revision 2 5. TRANSPORT OF COLLOIDS IN UNSATURATED ZONE This section of the report summarizes the current state of knowledge concerning the transport of radionuclide-carrying colloids in the unsaturated zone, and their release into the saturated zone. The unsaturated zone is the third of the repository system subfeatures examined to determine the importance of colloid transport with respect to the postclosure performance of the Yucca Mountain repository, as shown in Figure 1-1. Modeling assumptions to be used in the TSPA-LA are discussed below. The information presented in this section primarily draws on the discussions contained in the following documented sources: 1. Radionuclide Transport Models Under Ambient Conditions (BSC 2003m). 2. Unsaturated Zone Colloid Transport Model (CRWMS M&O 2000d), which is used for reference only in the discussion of the alternate conceptual model of colloid transport, Section 5.5. This chapter is organized in the following way: (1) Section 5.1 describes the processes that could occur during the transport of colloids; (2) Sections 5.2 through 5.6 describes the unsaturated zone process model, which includes a subset of the processes described in Section 5.1 along with input data and results; and (3) Section 5.7 describes the TSPA model abstraction. 5.1 DESCRIPTION OF RELEVANT PROCESSES This section describes processes relevant to the transport of colloids through the unsaturated zone at Yucca Mountain. The discussion focuses on a realistic view of the transport and does not necessarily reflect the conceptual model or numerical models used to predict the transport of colloids through the unsaturated zone. Often in a model conservative assumptions are made when there is a lack of data to support what is believed to be more realistic. Assumptions made in moving to the conceptual model and numerical models used are discussed in Sections 5.2 through 5.6. Many of the processes relevant to the transport of dissolved species are also relevant to the transport of colloids. However, there are processes unique to colloid transport. Figure 5-1 presents a schematic illustration of colloidal transport processes in the relevant geologic units of the unsaturated zone. Transport below the repository may proceed by two very different means depending on whether devitrified, vitric or zeolitic tuff is encountered. When devitrified or zeolitic rock is encountered, fracture flow will be dominant due to the low permeability of the rock matrix, and colloids will tend to remain in the fractures. When vitric rock is encountered, matrix flow dominates leading to colloid transport mainly through the rock matrix, where colloids are more likely to be filtered or trapped. The common transport processes are summarized in Section 5.1.1. Transport processes specifically relevant to colloids are summarized in Section 5.1.2. Finally, current information pertaining to colloids at the Yucca Mountain site is summarized in Section 5.1.3. September 2003 5-1 No. 8: Colloids Revision 2 Schematic Illustration of Colloidal Transport Processes in the Relevant Geologic Units of the Unsaturated Zone Figure 5-1. 5.1.1 Common Transport Processes The following transport processes are relevant to both dissolved species and colloids throughout the unsaturated zone. Advection–Dissolved species and colloids will move by advective transport in groundwater. Advection in fractures is expected to be the dominant transport mechanism in many layers of the various hydrogeologic units. In a few hydrogeologic units, such as the CHv (Calico Hills vitric unit), pp2 (Prow Pass Unit 2), and pp3 (Prow Pass Unit 3) (BSC 2003m, Section 6.6.6), matrix flow is dominant, resulting in much slower transport velocities (compared to those in the fractures of other units) and longer radionuclide-matrix contact times (BSC 2003m, Section 6). No. 8: Colloids September 2003 5-2 Revision 2 Colloids are much more likely to be filtered in matrix flow than fracture flow because of lower velocities and higher rock surface area to water volume ratios. Hydrodynamic Dispersion–Hydrodynamic dispersion includes both mechanical dispersion arising from local velocity variations and molecular diffusion driven by concentration gradients. Dispersion of the radionuclides occurs both along (longitudinal) and transverse to the average flow direction. Hydrodynamic dispersion dilutes and smears sharp concentration gradients and reduces the breakthrough time of radionuclides to the water table. The dispersion coefficient is a function of dispersivity and flow velocity. Hydrodynamic dispersion is not expected to play an important role in unsaturated zone transport. First, significant dispersion effects are implicitly accounted for by the fracture-matrix dual-continuum approach, which explicitly models velocity variations between these continua. Second, the potential repository emplacement area is very broad (relative to the distance to the water table), which tends to make lateral dispersion effects less important (CRWMS M&O 2000e, Section 6.2). Sorption–Sorption accounts for a combination of chemical interactions between the dissolved radionuclides and the solid phases. The solid phases may be the immobile rock matrix or colloids, which are mobile or immobile depending on their transport characteristics. By removing a portion of the dissolved species from the mobile liquid phase and transferring it to the immobile solid phase, sorption reduces the rate of advance of the concentration front (i.e., retards a dissolved or suspended species). However, the sorption of a dissolved species onto a mobile colloid can enhance the rate of advance of the species relative to its rate of advance as a solute. This process is called colloid-facilitated transport. Matrix Diffusion–Matrix diffusion plays an important role in radionuclide exchange between the fractures and the rock matrix. Radionuclide diffusion into the rock matrix and away from the fracture surface is driven by a concentration gradient, and it will slow the advance of radionuclides by removing them from the faster flowing fractures. Matrix diffusion of colloids is modeled in the radionuclide transport process model using the Stokes-Einstein equation to represent colloid diffusion rates in water and a tortuosity factor to represent the geometrical effects of the pore structure (BSC 2003m, Section 6.2.6.2). Radioactive Decay–Radioactive decay reduces the concentration of most radionuclides over time. However, the production of radioactive daughter products adds complexity because transport simulations must compute the total radioactivity distribution (i.e., the sum of the concentrations of all the members of the radioactive decay chain). This is especially significant if the daughters have long half-lives. The daughter products may have significantly different transport behavior than the parent radionuclide. These processes affect both dissolved radionuclides and those attached to colloids. 5.1.2 Transport Processes Relevant to Colloids The following processes are relevant to colloid transport in the unsaturated zone. Colloid Deposition onto Rock Surfaces–Colloid deposition (physico-chemical filtration) during flow through a porous medium is commonly assumed to occur in two steps: (1) transport of September 2003 5-3 No. 8: Colloids Revision 2 colloids to matrix surfaces by Brownian diffusion, interception, or gravitational sedimentation and (2) attachment of colloids to matrix surfaces. The attachment efficiency (i.e., the fraction of collisions resulting in attachment) is strongly influenced by short-range interactions between colloids and matrix surfaces, such as van der Waals and electric double-layer interactions, steric stabilization, and hydrodynamic forces (Kretzschmar et al. 1995, p. 435). The magnitude of these interactions is dependent on water chemistry, colloid and substrate composition and surface chemistry, and water flow rates. Kretzschmar et al. (1997, p. 1,129) demonstrated that colloid deposition generally follows a first-order kinetic rate law and experimentally determined the corresponding collision efficiencies. Colloid Interactions with Rock and Radionuclides–Colloid attachment to the host rock is strongly dependent on electrostatic interactions. Once attached, colloid detachment (declogging) is generally slow to irreversible. Sorption of radionuclides on colloids is controlled by a number of chemical processes such as ion exchange, surface complexation, and organic complexation (EPRI 1999, p. 4-9). Colloid Filtration–Wan and Tokunaga (1997, p. 2,413) distinguish two types of straining: conventional straining (if the colloid is larger than the pore throat diameter or the fracture aperture) and film straining (if the colloid is larger than the thickness of the adsorbed water film coating the grains of the rock). Wan and Tokunaga (1997, pp. 2,413 and 2,419) developed a conceptual model to describe colloid transport in unsaturated media as a function of water saturation, Sw. If the rock Sw exceeds a critical saturation value, Sc, colloids move through the system within locally saturated pore spaces. For Sw 1 x 106 < 1 x 103 1 x 103 to 5 x 103 5 x 103 to 1 x 104 1 x 104 to 5 x 104 5 x 104 to 1 x 105 1 x 105 to 5 x 105 5 x 105 to 1 x 106 > 1 x 106 < 1 x 105 1 x 105 to 5 x 105 5 x 105 to 1 x 106 1 x 106 to 5 x 106 5 x 106 to 1 x 107 > 1 x 107 < 1 x 104 1 x 104 to 5 x 104 5 x 104 to 1 x 105 1 x 105 to 5 x 105 5 x 105 to 1 x 106 1 x 106 to 5 x 106 5 x 106 to 1 x 107 > 1 x 107 < 1 x 101 1 x 101 to 5 x 101 5 x 101 to 1 x 102 1 x 102 to 5 x 102 5 x 102 to 1 x 103 > 1 x 103 < 1 x 102 1 x 102 to 5 x 102 5 x 102 to 1 x 103 1 x 103 to 5 x 103 5 x 103 to 1 x 104 > 1 x 104 Revision 2 Kd Value Interval Probabilities 0 0.15 0.2 0.5 0.15 0 0 0.04 0.08 0.25 0.2 0.35 0.08 0 0 0.15 0.2 0.55 0.1 0 0 0.07 0.1 0.23 0.2 0.32 0.08 0 0 0.13 0.22 0.55 0.1 0 0 0.2 0.25 0.5 0.05 0 September 2003 Revision 2 Source: BSC 2003e. Figure B-5. Cumulative Distribution Function of Uncertainty in Groundwater Colloid Concentrations Source: BSC 2003b, Figure 7. NOTE: The dashed lines correspond to two standard deviations above and below the statistical fit to the data. Figure B-6. Uncertainty in the Statistical Fit for the Diffusion Coefficient, D, of Solute as a Function of Log Percent Moisture, č, where D0 is the Solute Diffusion Coefficient in Bulk Water September 2003 B-12 No. 8: Colloids Revision 2 The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein relationship (BSC 2003b): ion (Eq. B-1) D D ion coll r rcoll B.5 REFERENCES MOL.20020506.0917. = Company. ACC: DOC.20030626.0006. . .. . where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the diffusion coefficient of a ion of radius rion. Given a typical ion radius and colloidal particle radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 100 times smaller than that of a dissolved ion. BSC (Bechtel SAIC Company) 2001. Uncertainty Distribution for Stochastic Parameters. ANLNBS- MD-000011 REV 00 ICN 01. Las Vegas, Nevada: Bechtel SAIC Company. ACC: BSC 2003a. Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary. MDL-EBS-PA-000004 REV 00. Las Vegas, Nevada: Bechtel SAIC BSC 2003b. EBS Radionuclide Transport Abstraction. ANL-WIS-PA-000001 REV 01F. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0199. BSC 2003c. Features, Events and Processes in UZ Flow and Transport. ANL-NBS-MD- 000001 REV 02B. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0198. BSC 2003d. Drift-Scale Coupled Processes (DST and THC Seepage) Models. MDL-NBS-HS- 000001 REV 02. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030804.0004. BSC 2003e. SZ Flow and Transport Model Abstraction. MDL-NBS-HS-000021 REV 00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030612.0138. CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating Contractor) 2000. Total System Performance Assessment for the Site Recommendation. TDR-WIS-PA-000001 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20001220.0045. Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy Technical Exchange and Management Meeting on Evolution of the Near-Field Environment (January 9.12, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), January 26, 2001, with enclosure. ACC: MOL.20010810.0033. Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. ˙ ˙. . B-13 September 2003 No. 8: Colloids Schlueter, J. 2002. “Evolution of the Near-Field Environment Key Technical Issue Agreements” Letter from J. Schlueter (NRC) to S. Brocoum (DOE/YMSCO), February 14, 2002, 0225021620, with enclosure. ACC: MOL.20020607.0086. No. 8: Colloids Revision 2 September 2003 B-14 SCREENING OUT COUPLED THERMAL-HYDROLOGIC-CHEMICAL EFFECTS (RESPONSE TO ENFE 4.03 AND GEN 1.01 (COMMENTS 35 AND 37)) No. 8: Colloids APPENDIX C Revision 2 September 2003 Revision 2 Note Regarding the Status of Supporting Technical Information This document was prepared using the most current information available at the time of its development. This Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were prepared using preliminary or draft information reflect the status of the Yucca Mountain Project’s scientific and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the AMRs and other references will be reflected in the License Application (LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to reflect changes in the supporting references prior to submittal of the LA. September 2003 No. 8: Colloids Revision 2 APPENDIX C SCREENING OUT COUPLED THERMAL-HYDROLOGIC-CHEMICAL EFFECTS (RESPONSE TO ENFE 4.03 AND GEN 1.01 (COMMENTS 35 AND 37)) This appendix provides a response for Key Technical Issue (KTI) agreement Evolution of the Near-Field Environment (ENFE) 4.03 and general agreement (GEN) 1.01, comments 35 and 37. These KTI agreements relate to providing the technical basis for the screening of thermal-hydrologic-chemical effects on colloids and colloid transport. C.1 KEY TECHNICAL ISSUE AGREEMENTS C.1.1 ENFE 4.03 and GEN 1.01 (Comment 35 and 37) Agreement ENFE 4.03 was reached during the U.S. Nuclear Regulatory Commission (NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management Meeting on Evolution of the Near-Field Environment held January 9 through 12, 2001, in Pleasanton, California. ENFE KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). Agreement GEN 1.01 was reached during the NRC/DOE Technical Exchange and Management Meeting on Range of Thermal Operating Temperatures held September 18 through 19, 2001. At that meeting, NRC provided additional comments relating to ENFE 4.03 (GEN 1.01, comments 35 and 37), and DOE provided an initial response (Reamer and Gil 2001). The wording of these agreements and of DOE’s initial response to the general agreement comments is as follows: ENFE 4.03 Provide the technical basis for screening out coupled THC effects on radionuclide transport properties and colloids. The DOE will provide the technical basis for screening out coupled THC effects on radionuclide transport properties and colloids in a new AMR or in a revision to an existing AMR, expected to be available in FY 02. GEN 1.01 (Comment 35) The SSPA recommends new values for EBS colloid transport parameters. If these are adopted by TSPA in the future, the technical basis for the new distributions will require close scrutiny. Relevant KTI agreements are RT 3.07, ENFE 4.03, ENFE 4.04, and ENFE 4.06. DOE Initial Response to GEN 1.01 (Comment 35) The new values for EBS colloidal transport parameters were designed to evaluate unquantified uncertainty for the SSPA. DOE understands that prior to any potential LA, a stronger technical basis must be provided for EBS colloidal transport parameter values carried forward to the base case analysis. September 2003 C-1 No. 8: Colloids Revision 2 GEN 1.01 (Comment 37) The discussion of THC effects on UZ transport does not address chemical effects of the repository. This concern is related to KTI agreements ENFE 4.03 and ENFE 4.06, and TSPAI FEPs item J-8. DOE Initial Response to GEN 1.01 (Comment 37) DOE acknowledges this comment and notes that some limited studies were documented in Section 11.3.5.4.2 of SSPA Volume 1. Work is underway, consistent with the cited agreements, to study the effects of alkaline plumes generated by the cement-seepage interactions on rock properties (such as porosity and permeability) and thereby effects on radionuclide transport from the waste placement drifts, with preliminary results expected in FY03. C.1.2 Related Key Technical Issues GENERAL 1.01: “For NRC comments 3, 5, 8, 9, 10, 12, 13, 15, 16, 18, 21, 24, 27, 36, 37, 41, 42, 45, 46, 50, 56, 64, 69, 75, 78, 81, 82, 83, 93, 95, 96, 97, 98, 102, 103, 104, 106, 109, 110, 111, 113, 116, 118, 119, 120, 122, 123, 124, and 126, DOE will address the concern in the documentation for the specific KTI agreement identified in the DOE response (Attachment 2). The schedule and document source will be the same as the specific KTI agreement.” TSPAI 2.02: “Provide the technical basis for the screening argument, as summarized in Attachment 2. See Comment #3, 4, 11, 12, 19 (Parts 1, 2, and 6), 25, 26, 29, 34, 35, 36, 37, 38, 39, 42, 43, 44, 48, 49, 51, 54, 55, 56, 57, 59, 60, 61, 62, 63, 64, 65, 66, 68, 69, 70, 78, 79, J-1, J-2, J-3, J-4, J-7, J-8, J-9, J-10, J-11, J-12, J-13, J-14, J-15, J-17, J-20, J-21, J-22, J-23, J-24, J-25, J-26, and J-27. NRC Comment J-7 on FEP #2.2.08.01.00 (Groundwater chemistry/composition in UZ and SZ). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, RT Subissue 1 Agreement 5, and RT subissue 2 Agreement 10). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work.” NRC Comment J-8 on FEP #2.2.08.02.00 (Radionuclide transport occurs in a carrier plume in geosphere). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work.” NRC Comment J-10 on FEP #2.2.08.06.00 (Complexation in geosphere). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD- 000001 will be revised upon completion of this work.” September 2003 C-2 No. 8: Colloids Revision 2 NRC Comment J-11 on FEP #2.2.08.07.00 (Radionuclide solubility limits in the geosphere). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 4 Agreement 3). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work.” NRC Comment J-12 on FEP #2.2.10.01.00 (Repository-induced thermal effects in geosphere). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work.” NRC Comment J-14 on FEP #2.2.10.07.00 (Thermo-chemical alteration of the Calico Hills unit). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work. DOE also stated that alteration of vitric rock has not been addressed and will need to be included in the overall thermal-hydrological-chemical analyses.” NRC Comment J-15 on FEP #2.2.10.09.00 (Thermal-chemical alteration of the Topopah basal vitrophyre). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work.” NRC Comment J-21 on FEP #2.2.11.02.00 (Gas Pressure Effects). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreements 5 and 7, and ENFE Subissue 4 Agreement 3). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work.” NRC Comment J-22 on FEP #1.2.04.02.00 (Igneous activity causes changes to rock properties). NRC/DOE Agreed Path Forward: “This issue is addressed by existing agreements between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work.” C.2 RELEVANCE TO REPOSITORY PERFORMANCE Radionuclide transport as either dissolved species or as colloids represents the means for the release of radionuclides from the repository system to the biosphere. The time and amount of radionuclide release to the biosphere will depend on the transport properties of the dissolved radionuclides and of radionuclide-laden colloids. The transport properties may be dependent on coupled thermal-hydrologic-chemical effects. The technical basis for the response for this KTI is presented in Sections 3, 4, and 5 of this technical basis document. This KTI is related to the engineered and near-field environment as shown in Figure C-1. September 2003 C-3 No. 8: Colloids Revision 2 Figure C-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System Components September 2003 C-4 No. 8: Colloids Revision 2 C.3 RESPONSE Thermal effects may affect radionuclide transport directly by causing changes in radionuclide speciation and solubility in the unsaturated zone or indirectly by causing changes in the host rock mineralogy that affect the flow path. Relevant processes include volume effects associated with silica phase changes, precipitation and dissolution of fracture-filling minerals (including silica and calcite), and alteration of zeolites and other minerals to clays. The effects of colloid formation are accounted for in the colloid source term. Colloids are expected to be formed from the degradation of the high-level radioactive waste and spent nuclear fuel waste forms, engineered barrier system materials, and rock. Radionuclides associated with colloids are assumed to be either irreversibly or reversibly attached to colloids (CRWMS M&O 2000a, Section 6; CRWMS M&O 2000b, Section 6). The near-field thermal-chemical analysis indicates only small changes in hydrologic properties and mineralogy as a result of these coupled processes (BSC 2003a, Section 6). Therefore, far-field changes are likewise expected to be small, including mineral precipitation/dissolution and alteration of minerals such as zeolites and clays. Therefore, coupled thermal-hydrologic-chemical effects on radionuclide transport properties and colloids are excluded from TSPA on the basis of low consequence. Cementitious materials will not be used in the emplacement drift ground support (BSC 2003b). Therefore, the effects of alkaline plumes generated by cement-seepage interactions on rock properties and radionuclide transport in the vicinity of the emplacement drift are no longer an issue that needs to be considered. Coupled thermal-hydrologic-chemical effects are likely to result in unstable colloid suspensions and, therefore, reduce the concentration of colloids in suspension. With increasing temperature and increasing ionic strength, both conditions expected as a result of thermal-hydrologicchemical effects, colloid suspensions become less stable. High temperatures and high ionic strengths are not favorable to colloid transport. Therefore, the screening out of coupled thermal-hydrologic-chemical effects on the transport of radioactive colloids in the TSPA is justified. New values for the diffusivity of dissolved species through the invert are presented in EBS Radionuclide Transport Abstraction (BSC 2003c, Section 6.3.4.1). From the dissolved species diffusivity values, diffusivities for colloids are estimated. The information in this report is responsive to agreements ENFE 4.03 and GEN 1.01 comments 35 and 37 made between the DOE and NRC. The report contains the information that DOE considers necessary for NRC review for closure of these agreements. C.4 BASIS FOR THE RESPONSE Features, Events and Processes in UZ Flow and Transport (BSC 2003d, Section 6.8.7) presents the screening argument for excluding coupled thermal-hydrologic-chemical effects on radionuclide transport properties and colloids (FEP 2.2.10.06.0A; FEP 2.2.08.03.0B) in the unsaturated zone (solubility, speciation, phase changes, and precipitation/dissolution). This FEP is conservatively ignored with respect to solubility reduction in the far field, and the effects of radionuclide precipitation are ignored in the unsaturated zone transport for TSPA. If solubility September 2003 C-5 No. 8: Colloids Revision 2 limits increase in the geosphere compared with the waste emplacement drift, there is no effect on transport because all available radionuclides from the source at the waste emplacement drift are already aqueous species. The thermal-chemical effects on colloid formation have already been accounted for in the colloid source term (BSC 2003e). Therefore, any impact from thermal-chemical alteration on colloid entrainment will be insignificant with respect to total repository system performance. This screening argument is reasonable because in the rock mass around the repository the temperature may be relatively high even at 10,000 years. For example, in Section 6.5.2 in Drift-Scale Coupled Processes (DST and THC Seepage) Models (BSC 2003a) it is shown that after 10,000 years the temperature is 45° to 50°C, which is significantly higher than the initial temperature of around 25°C. This higher temperature tends to decrease the stability of a colloidal suspension, as discussed in Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003e). The diffusion coefficient for dissolved species has been determined specifically for crushed tuff invert materials as a function of volumetric moisture content (m3 water/m3 bulk rock), using electric conductivity measurements (BSC 2003c). The thermal-hydrologic-chemical model simulations have been performed for both the base case and the extended case and for both Tptpmn and Tptpll geologic formations (BSC 2003a). The extended case includes the major solid phases (minerals and glass) encountered in geologic units at Yucca Mountain, together with a range of possible reaction product minerals, CO2 gas, and the aqueous species necessary to include these solid phases and the pore-water composition within the thermal-hydrologic-chemical model. The base case is a subset of the extended case excluding aluminum silicate minerals, which form or dissolve much less easily than minerals such as calcite or gypsum. The results for the Tptpmn rock unit are shown in Figures C-2 and C-3. The pH in fracture waters varies within 7.4 to 8.6 but still remain within a neutral to mildly basic range over the 10,000-year regulatory time period (Figure C-2), and thus its effect on colloid stability is expected to be negligible. The concentrations of soluble components (e.g., Na and Cl) and therefore the ionic strength in the waters are predicted to increase over the time period of thermal event (Figure C-4), which will reduce colloid stability. The THC effect may also change the transport properties of rock. However, Figure C-3 shows that this change is small and can be negligible for colloid transport. Figure C-5 presents the uncertainty in the statistical fit for the diffusion coefficient. September 2003 C-6 No. 8: Colloids Source: BSC 2003a. Figure C-2. Thermal-Hydrologic-Chemical Simulation (Tptpmn Model): Time Profiles of the Modeled pH of Fracture Water at Three Drift-Wall Locations under Heating (Heat) and Nonheating (Ambient) Conditions for the Extended (E) and Base-Case (B) Geochemical Systems No. 8: Colloids Revision 2 September 2003 C-7 Revision 2 Source: BSC 2003a. NOTE: Decrease in porosity is primarily due to the precipitation of calcite and amorphous silica. Figure C-3. Thermal-Hydrological-Chemical Model Simulation (Tptpmn Model): Contour Plot of Modeled Fracture Porosity Change at 10,000 Years for (a) Base-Case and (b) Extended Geochemical Systems September 2003 C-8 No. 8: Colloids Revision 2 Source: BSC 2003a. Figure C-4. Thermal-Hydrologic-Chemical Simulation (Tptpmn Model): Time Profiles of Modeled Total Aqueous Sodium Concentrations in Fracture Water at Three Drift-Wall Locations under Heating (Heat) and Nonheating (Ambient) Conditions for Extended (E) and Base-Case (B) Geochemical Systems September 2003 C-9 No. 8: Colloids Source: BSC 2003c, Figure 7. NOTE: The dashed lines correspond to two standard deviations above and below the statistical fit to the data. D is . .. . where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the diffusion coefficient of a ion of radius rion. Given a typical ion radius and colloidal particle radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 100 times smaller than that of a dissolved ion. BSC (Bechtel SAIC Company) 2003a. Drift-Scale Coupled Processes (DST and THC Seepage) Models. MDL-NBS-HS-000001 REV 02. Las Vegas, Nevada: Bechtel SAIC Company. BSC 2003b. Repository Design Project, Repository/PA IED Emplacement Drift Committed Materials. 800-IED-EBS0-0030-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. the diffusion coefficient of the crushed invert materials, D0 is the diffusion coefficient of the fully saturated crushed invert materials, and Ąč is the percentage of saturation. No. 8: Colloids (Eq. C-1) September 2003 Figure C-5. Uncertainty in the Statistical Fit for the Diffusion Coefficient The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein relationship (BSC 2003c): ion ion Dcoll = D r rcoll C.5 REFERENCES ACC: DOC.20030804.0004. ACC: ENG.20030311.0021. C-10 Revision 2 ˙ ˙. . Revision 2 BSC 2003c. EBS Radionuclide Transport Abstraction. ANL-WIS-PA-000001 REV 01F. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0199. BSC 2003d. Features, Events and Processes in UZ Flow and Transport. ANL-NBS-MD- 000001 REV 02B. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0198. BSC 2003e. Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary. MDL-EBS-PA-000004 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030626.0006. CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating Contractor) 2000a. Particle Tracking Model and Abstraction of Transport Processes. ANL-NBS-HS-000026 REV 00. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20000502.0237. CRWMS M&O 2000b. UZ Colloid Transport Model. ANL-NBS-HS-000028 REV 00. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20000822.0005. Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy Technical Exchange and Management Meeting on Evolution of the Near-Field Environment (January 9–12, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), January 26, 2001, with enclosure. ACC: MOL.20010810.0033. Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. September 2003 C-11 No. 8: Colloids INTENTIONALLY LEFT BLANK C-12 No. 8: Colloids Revision 2 September 2003 APPENDIX D CONTRASTING COLLOID CONCENTRATIONS IN THE ENGINEERED BARRIER SYSTEM AND SATURATED ZONE (RESPONSE TO TSPAI 3.30 AND GEN 1.01 (COMMENTS 43 AND 46)) No. 8: Colloids Revision 2 September 2003 Revision 2 Note Regarding the Status of Supporting Technical Information This document was prepared using the most current information available at the time of its development. This Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were prepared using preliminary or draft information reflect the status of the Yucca Mountain Project’s scientific and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the AMRs and other references will be reflected in the License Application (LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to reflect changes in the supporting references prior to submittal of the LA. September 2003 No. 8: Colloids Revision 2 APPENDIX D CONTRASTING COLLOID CONCENTRATIONS IN THE ENGINEERED BARRIER SYSTEM AND SATURATED ZONE (RESPONSE TO TSPAI 3.30 AND GEN 1.01 (COMMENTS 43 AND 46)) This appendix provides a response for Key Technical Issue (KTI) agreement Total System Performance Assessment and Integration (TSPAI) 3.30 and general agreement (GEN) 1.01, comments 43 and 46. These agreements relate to providing the technical basis for concentrations of colloids available for reversible attachment. D.1 KEY TECHNICAL ISSUE AGREEMENTS D.1.1 TSPAI 3.30 and GEN 1.01 (Comments 43 and 46) Agreement TSPAI 3.30 was reached during the NRC/DOE technical exchange and management meeting on total system performance assessment and integration held August 6 through 10, 2001, in Las Vegas, Nevada. TSPAI KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). Agreement GEN 1.01 was reached during the NRC/DOE technical exchange and management meeting on range of thermal operating temperatures held September 18 through 19, 2001. At that meeting, NRC provided additional comments (GEN 1.01, comments 43 and 46) relating to TSPAI 3.30, and DOE provided an initial response to those comments (Reamer and Gil 2001). The wording of these agreements and of DOE’s initial response to the general agreement comments is as follows: TSPAI 3.30 Provide the technical basis for the contrasting concentrations of colloids available for reversible attachment in the engineered barrier system and the saturated zone. Sensitivity analyses planned in response to RT Agreement 3.07 should address the effect of colloid concentration on Kc. Update, as necessary, the Kc parameter as new data become available from the Yucca Mountain region … c DOE will provide the technical basis for the contrasting concentrations of colloids available for reversible attachment in the engineered barrier system and the saturated zone. The sensitivity analyses planned in response to RT Agreement 3.07 will address the effect of colloid concentration on the K parameter. The technical basis will be documented in the Waste Form Colloid Associated Concentration Limits: Abstractions and Summary (ANL-WIS-MD- 000012) in FY 2003. The Kc parameter will be updated as new data become available from the Yucca Mountain region in the Uncertainty Distribution for Stochastic Parameters AMR (ANL-NBS-MD-000011) in FY 2003. September 2003 D-1 No. 8: Colloids Revision 2 GEN 1.01 (Comment 43) The SSPA presents a new distribution for retardation of colloids with irreversibly-attached radionuclides. The distribution takes into account new site-specific alluvium data. However, any future use of this distribution in TSPA will require comparison with results of field and laboratory tests. This concern is indirectly related to agreement TSPAI 3.30. DOE Initial Response to GEN 1.01 (Comment 43)1 DOE acknowledges that any future use of this distribution in TSPA will require comparison with results of field and laboratory tests 1,2. This concern is indirectly related to KTI agreements RT 3.07 and RT 3.08. Laboratory testing of microsphere and silica colloid retardation in alluvium-packed columns is in progress. Microspheres will be used as colloid tracers in ATC cross-hole tracer testing. GEN 1.01 (Comment 46) The analysis of sensitivity to increased uncertainty in the reversible colloid parameter Kc (Section 12.5.2.4) yielded “somewhat longer transport times” in the saturated zone. This analysis does not illustrate the effect of possibly underestimating Kc, because it is not clear that the mean value of Kc is significantly different from the base case. This concern is related to agreements RT 3.07 and TSPAI 3.30. DOE Initial Response to GEN 1.01 (Comment 46) This issue will be handled as part of agreements RT 3.07 and TSPAI 3.30. D.1.2 Related Key Technical Issue Agreements None. D.2 RELEVANCE TO REPOSITORY PERFORMANCE Radionuclide transport as either dissolved species or colloids represents the means for the release of radionuclides from the repository to the biosphere. The concentration of colloids released from the engineered barrier system provides a source term for colloid transport within the unsaturated zone. In turn, the colloid concentration reaching the water table provides the source term for colloid transport in the saturated zone. The colloid concentration (mc) and the distribution coefficient (Kd) are two important parameters controlling colloid-facilitated radionuclide transport because Kc, the ratio of radionuclide mass in colloids to the concentration in solution, is defined as the product of Kd and mc. 1 “[F]ield and laboratory tests 1, 2” refers to test conducted using CML microspheres as colloid surrogates. These tests will be discussed in detail in Appendix M (Response to KTI Agreement RT 3.08) of the saturated zone technical basis document. September 2003 D-2 No. 8: Colloids Revision 2 The technical basis for the response for this KTI is presented in Sections 3, 4, 5 and 6 of this technical basis document. This KTI is related to the engineered and near-field environment, the unsaturated and the saturated zones as shown in Figure D-1. D.3 RESPONSE D.3.1 Response to Key Technical Issue Agreement TSPAI 3.30 c In contrast to the constraints on colloid concentrations used in the total system performance assessment (TSPA) for the site recommendation to represent reversible radionuclide attachment, the engineered barrier system and the saturated zone colloid models use the same distribution of colloidal concentration for natural system colloids. This distribution has been revised since the TSPA for the site recommendation with additional data that raise the upper limits of colloid concentration by about two orders of magnitude over the previous values considered. This translates to about a four-order-of-magnitude increase for the saturated zone colloid treatment from the TSPA for the site recommendation that used only the mean value of colloid concentration. This change in colloid concentration is reflected directly in changes to the K parameter used in the saturated zone colloid treatment. Such increased concentration limits for colloids that can reversibly sorb radionuclides in the saturated zone provides a higher level of confidence that the evaluation of saturated zone radionuclide transport by reversible colloids is bounding. This increased level of consistency for colloid concentrations in the saturated zone does not mean that the treatment is identical to that for the engineered barrier system. The differences and the respective technical bases for the colloid concentrations in the engineered barrier system and saturated zone are discussed below. Within the engineered barrier system, the concentration of colloids available for reversible sorption, mc, is evaluated for the effects of temperature and the chemical environment. Such effects are not expected within the saturated zone, and therefore the colloid concentration is unaffected by them. In the saturated zone, colloid concentration is affected by attachment/detachment of the colloids to the immobile rock surfaces. It is expected that under the high-temperature and high ionic strengths conditions within the engineered barrier system, particularly in the first 2,000 to 3,000 years after closure, the concentration of colloids will not be sufficiently high to effectively compete with the immobile rock surfaces for the reversible attachment of dissolved radionuclides. Colloids leaving the engineered barrier system will travel through the unsaturated zone to reach the saturated zone. Within the unsaturated zone, colloid transport is expected to be primarily through fractures, and only those colloids with characteristic dimension considerably smaller than the film coating the fracture surfaces are expected to be transported. Colloids will be subjected to retardation effects, thus increasing their transport time through the unsaturated zone. Larger colloids are expected to be subjected to filtration by film straining. As colloids travel through the unsaturated zone within the Topopah Spring Tuff, they will reach the Calico Hills Formation, which has a significantly different porosity. The interface between the Topopah Spring and the Calico Hills units presents a barrier to colloid transport. At that interface colloids may move laterally until they find another fracture system to continue traveling vertically towards the saturated zone. Therefore, the concentration of colloids reaching the saturated zone is expected to be significantly lower than that within the engineered barrier system. Because the only direct September 2003 D-3 No. 8: Colloids Revision 2 source of colloids in the saturated zone is the natural system materials, they are the focus of the reversibly sorbing colloids in the treatment for this subsystem. Figure D-1. Mapping of Colloid-Related Key Technical Issues Agreement to Repository System Components No. 8: Colloids September 2003 D-4 Revision 2 D.3.2 Response to Agreement GEN 1.01 (Comments 43 and 46) c), a product of sorption coefficient Kd and colloid concentration, is sampled based The saturated zone transport simulations of radionuclides that are irreversibly attached to colloids are conducted for radioisotopes of plutonium and americium. The retardation of colloids with irreversibly attached radionuclides is a kinetically controlled process, which approaches equilibrium behavior for long transport times. For transport of colloids through the saturated zone, equilibrium behavior is nearly achieved. However, nonequilibrium behavior results in unimpeded migration of some of the colloids. Consequently, a small fraction of these colloids are transported through the saturated zone with no retardation; whereas the larger fraction is delayed by a retardation factor. The saturated zone transport simulations of radionuclides that are reversibly attached to colloids are conducted for radioisotopes of plutonium, americium, thorium, protactinium, and cesium. In the simulations, the distribution coefficient (K on the distributions of both Kd values and colloid concentrations. The simulations show a significant retardation of radionuclide transport in the saturated zone as compared to the case where radionuclides irreversibly sorbed on colloids. The information in this report is responsive to agreements TSPAI 3.30 and GEN 1.01 comments 43 and 46 made between the DOE and NRC. The report contains the information that DOE considers necessary for the NRC to review for closure of these agreements. D.4 BASIS FOR THE RESPONSE D.4.1 Response to Key Technical Issue Agreement TSPAI 3.30 Reversible Colloid Concentrations in the Engineered Barrier System–The technical basis for selection of typical colloid concentrations in the engineered barrier system and the saturated zone are discussed in Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003a) and summarized below: • The static-saturated tests for Defense High-Level Radioactive Waste Glass at Argonne National Laboratory indicated that colloids developed and increased in concentration with time, up to the point where the colloid concentration reached a maximum value and then the colloidal suspension became less stable because of high ionic strengths resulting in agglomeration and settling (BSC 2003a). The maximum colloid concentration from the experimental degradation of high-level radioactive waste glass is estimated to be 5 ppm (BSC 2003a, Figure 3). • The concentration range of iron oxyhydroxide colloids generated from the defense high-level radioactive waste glass was estimated based on experiments performed at the University of Nevada at Las Vegas and on professional judgment. In these experiments, scaled-down miniature waste packages were exposed to J-13 groundwater in either a bath-tub mode or a flow-through mode. The cumulative results have yielded average concentrations of colloidal size materials in the range of 20 mg/L within the initial four weeks of the experiments (BSC 2003a, p. 56). September 2003 D-5 No. 8: Colloids Revision 2 • The range of colloid concentration in seepage/groundwater was derived based on literature data and groundwater sampling from the Yucca Mountain area and Idaho National Engineering and Environmental Laboratory (BSC 2003a, Figures 11 and 12). Practically no colloids were detected for the ionic strength above 0.05 M. The upper limit of colloid concentration is about 200 ppm, with average values of approximately 0.1 ppm (Figure D-2). • Results from the unsaturated testing of commercial spent nuclear fuel and defense spent nuclear fuel at Argonne National Laboratory indicated the formation of alteration products containing very low concentrations of uranium-based colloids and dissolution of uranium-based colloids in a few months (BSC 2003a, Section 6.3.1.2). For purposes of this discussion, colloid concentration within the engineered barrier system will consist of colloid concentrations within the waste package (Section 3 of this technical basis document) and the invert (Section 4 of this technical basis document). Source: BSC 2003a, Figure 12. Figure D-2. No. 8: Colloids Cumulative Distribution Function Showing the Probability of Occurrence of Colloid Concentration Levels (ppm or mg/L) in Groundwater Samples in the Yucca Mountain September 2003 Area and Idaho National Engineering and Environmental Laboratory Within the waste package, the concentration of three different types of colloids available for reversible sorption are estimated (see Figure 3-11 of this technical basis document): (1) reversible defense high-level radioactive waste glass colloids (also referred to as waste form colloids, (2) reversible iron oxyhydroxide colloids, and (3) reversible groundwater colloids. In general, the concentration of each of these three types of colloids are calculated at each time step D-6 Revision 2 of the simulation in TSPA. At each time step, (1) the colloid mass available for reversible sorption is estimated as a function of ionic strength, (2) the stability of colloids as a function of ionic strength and pH is determined, and (3) the sorption of available mass of the dissolved inventory of cesium, americium, plutonium, protactinium, and thorium onto colloids is calculated. Colloids are assumed to move unretarded from the waste package to the invert. Colloids are not assumed to form within the invert. First, the invert is divided into two pore spaces (see Section 4.3.2 of this technical basis document). As colloids are released from the waste package into the invert, the released colloids are divided into two fractions (see Figure 4-2 of this technical basis document): (1) the inter-granular pore space fraction and (2) the intragranular pore space fraction. No mass transfer of colloids is assumed to occur within these two pore spaces. The distribution of colloids between the two pore spaces depends on the particular scenario (nominal scenario, seismic scenario, or igneous scenario) being simulated. Colloids entering the intra-granular pore space are transported by diffusion as defined by the chemistry (ionic strength and pH) of the tuff matrix. The colloids entering the inter-granular pore space are transported by advection and diffusion as defined by the chemistry of the fractures. The stability of the colloids is reevaluated using the same ionic strength and pH relationships for the three types of reversible colloids shown in Figure 3-11 of this technical basis document. The cumulative distribution function of colloid concentration for natural groundwater colloids is shown in Figure D-2. In the calculations summarized above, this cumulative distribution function is sampled and modified based on the chemical conditions within the waste package and the invert. d and the colloid concentration, values of Kc are calculated as In the TSPA, the transport model for the engineered barrier system explicitly accounts for the partitioning coefficient (Kd) and the colloid concentration. Updated Kd values are tabulated in Table D-1. From the values of K the product of Kd and colloid concentration. The Kc approach is used for both the unsaturated zone and saturated zone colloid transport. The same concentration distribution of smectite colloids in Figure D-2 is used for the engineered barrier system, unsaturated zone, and saturated zone colloid models. Note that in the engineered barrier system this distribution of smectite colloid concentration is reevaluated using the pH and ionic strength values, while for the unsaturated zone and saturated zone colloid transport models the distribution is assumed to be unaffected by temperature or chemical effects. As discussed in Section 3.2.5 of this technical basis document, the thermal and chemical perturbations within the engineered barrier system are likely to destabilize colloid suspensions by increasing Brownian motion and particle collision due to the elevated temperature and increasing ionic strength of the solution. Therefore, the bounding values summarized above are still suitable for the engineered barrier system. In addition, any perturbed colloid concentration inside the drift will ultimately attenuate to the background concentration in the Yucca Mountain groundwater, as the colloidal solution percolates through the invert and the unsaturated zone into the saturated zone, and the water chemistry converges to the ambient groundwater. As a result, the total system performance becomes insensitive to the near-field thermal and chemical perturbations. September 2003 D-7 No. 8: Colloids Table D-1. Kd Values (mL/g) Used for Reversible Radionuclide Sorption on Colloids in Total System Performance Assessment-License Application Calculations Radionuclide Pu Am, Th, Pa Cs Source: BSC 2003a, Table 10. NOTE: The Kd values for Tc and I are very low and not listed here. No. 8: Colloids Colloid Iron Oxyhydroxide Smectite Iron Oxyhydroxide Smectite Iron Oxyhydroxide Smectite Kd Value Intervals (mL/g) Kd Value Range (mL/g) 104 to 106 <1 x 104 1 x 104 to 5 x 104 5 x 104 to 1 x 105 1 x 105 to 5 x 105 5 x 105 to 1 x 106 > 1 x 106 103 to 106 < 1 x 103 1 x 103 to 5 x 103 5 x 103 to 1 x 104 1 x 104 to 5 x 104 5 x 104 to 1 x 105 1 x 105 to 5 x 105 5 x 105 to 1 x 106 > 1 x 106 105 to 107 < 1 x 105 1 x 105 to 5 x 105 5 x 105 to 1 x 106 1 x 106 to 5 x 106 5 x 106 to 1 x 107 > 1 x 107 104 to 107 < 1 x 104 1 x 104 to 5 x 104 5 x 104 to 1 x 105 1 x 105 to 5 x 105 5 x 105 to 1 x 106 1 x 106 to 5 x 106 5 x 106 to 1 x 107 > 1 x 107 101 to 103 < 1 x 101 1 x 101 to 5 x 101 5 x 101 to 1 x 102 1 x 102 to 5 x 102 5 x 102 to 1 x 103 > 1 x 103 102 to 104 < 1 x 102 1 x 102 to 5 x 102 5 x 102 to 1 x 103 1 x 103 to 5 x 103 5 x 103 to 1 x 104 > 1 x 104 D-8 Revision 2 Kd Value Interval Probabilities 0 0.15 0.2 0.5 0.15 0 0 0.04 0.08 0.25 0.2 0.35 0.08 0 0 0.15 0.2 0.55 0.1 0 0 0.07 0.1 0.23 0.2 0.32 0.08 0 0 0.13 0.22 0.55 0.1 0 0 0.2 0.25 0.5 0.05 0 September 2003 Revision 2 Concentration of Reversible Colloids in the Saturated Zone–Radionuclides that are reversibly sorbed onto colloids are modeled to be temporarily attached to the surface of colloids. Thus, these radionuclides are available for dissolution in the aqueous phase and their transport characteristics are a combination of the transport characteristics of solute and colloids. The saturated zone transport simulations of radionuclides that are reversibly attached to colloids are conducted for radioisotopes of plutonium, americium, thorium, protactinium, and cesium (BSC 2003b). (Note that a major fraction of plutonium and americium is transported as irreversibly sorbed onto colloids.) The distribution coefficient (Kc), a product of sorption coefficient Kd and colloid concentration, represents the equilibrium partitioning of radionuclides between the aqueous phase and the colloidal phase. The sorption coefficient distributions are listed in Table D-1. The distribution of colloid concentration is shown in Figure D-3. The same Kc applies to transport of a radionuclide in both the volcanic units and the alluvium. The simulation results are shown in Figure D-4. Source: BSC 2003b. Figure D-3. Cumulative Distribution Function of Uncertainty in Groundwater Colloid Concentrations September 2003 D-9 No. 8: Colloids Revision 2 Source: BSC 2003b. Figure D-4. Mass Breakthrough Curves (Upper) and Median Transport Times (Lower) for Plutonium on September 2003 Reversible Colloids at 18-km Distance D.4.2 Response to Agreement GEN 1.01 (Comments 43 and 46) Transport of Irreversibly Sorbed Radionuclides in Saturated Zone–For the TSPA for the license application, colloid-facilitated transport of radionuclides in the saturated zone is simulated to occur by two basic modes (BSC 2003b). In the first mode, radionuclides that are irreversibly attached to colloids are transported at the same rate as the colloids, which are themselves retarded by interaction with the aquifer material. In the second mode, radionuclides that are reversibly attached to colloids are in equilibrium with the aqueous phase and the aquifer D-10 No. 8: Colloids Revision 2 material. In this mode of transport, the effective retardation of these radionuclides during transport in the saturated zone is dependent on the sorption coefficient of the radionuclide onto colloids, the concentration of colloids, and the sorption coefficient of the radionuclide onto the aquifer material. The saturated zone transport simulations of radionuclides that are irreversibly attached to colloids are conducted for radioisotopes of plutonium and americium (BSC 2003b). The retardation of colloids with irreversibly attached radionuclides is a kinetically controlled process, which approaches equilibrium behavior for long transport times. For transport of colloids through the saturated zone, equilibrium behavior is nearly achieved. However, nonequilibrium behavior results in unimpeded migration of some of the colloids. Consequently, a small fraction of these colloids is transported through the saturated zone with no retardation; whereas the larger fraction is delayed by a retardation factor. The processes important to the transport of irreversible colloids in the volcanic units of the saturated zone are as follows: advection and dispersion of colloids in the fracture water, exclusion of the colloids from the matrix waters, and chemical filtration or adsorption of the colloids onto the fracture surfaces. The processes modeled for irreversible colloids in the alluvium are the same as those modeled for irreversible colloids in the volcanic units, with the exception of matrix exclusion, because the alluvium is modeled as a single porous medium. Figure D-5 shows the cumulative distribution function used for retardation factors for the saturated zone transport abstraction model. The simulation results for plutonium and americium are shown in Figure D-6. Source: BSC 2003b. Figure D-5. No. 8: Colloids Cumulative Distribution Function Used for Retardation Factors for the Saturated Zone Transport Abstraction Model September 2003 D-11 Revision 2 Source: BSC 2003b. Figure D-6. No. 8: Colloids Mass Breakthrough Curves (Upper) and Median Transport Times (Lower) for Plutonium and Americium on Irreversible Colloids at 18-km Distance D.5 REFERENCES BSC (Bechtel SAIC Company) 2003a. Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary. MDL-EBS-PA-000004 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030626.0006. BSC 2003b. Saturated Zone Flow and Transport Model Abstraction. MDL-NBS-HS-000021 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030612.0138. September 2003 D-12 Revision 2 Reamer, C.W. 2001. “U.S. Nuclear Regulatory Commission/U.S. Department of Energy Technical Exchange and Management Meeting on Total System Performance Assessment and Integration (August 6 through 10, 2001).” Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), August 23, 2001, with enclosure. ACC: MOL.20011029.0281. Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. September 2003 D-13 No. 8: Colloids INTENTIONALLY LEFT BLANK D-14 No. 8: Colloids Revision 2 September 2003 APPENDIX E SENSITIVITY STUDIES TO TEST IMPORTANCE OF COLLOID TRANSPORT PARAMETERS AND MODELS (RESPONSE TO RT 3.07 AND GEN 1.01 (COMMENTS 35, 43, AND 46)) No. 8: Colloids Revision 2 September 2003 Revision 2 Note Regarding the Status of Supporting Technical Information This document was prepared using the most current information available at the time of its development. This Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were prepared using preliminary or draft information reflect the status of the Yucca Mountain Project’s scientific and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the AMRs and other references will be reflected in the License Application (LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to reflect changes in the supporting references prior to submittal of the LA. September 2003 No. 8: Colloids Revision 2 APPENDIX E SENSITIVITY STUDIES TO TEST IMPORTANCE OF COLLOID TRANSPORT PARAMETERS AND MODELS (RESPONSE TO RT 3.07 AND GEN 1.01 (COMMENTS 35, 43, AND 46)) This appendix provides a response for Key Technical Issue (KTI) agreement Radionuclide Transport (RT) 3.07 and general agreement (GEN) 1.01, comments 35, 43 and 46. These KTI agreements relate to providing sensitivity studies for colloid transport parameters and models. E.1 KEY TECHNICAL ISSUE AGREEMENTS E.1.1 RT 3.07 and GEN 1.01 (Comments 35, 43, and 46) Agreement RT 3.07 was reached during the U.S. Nuclear Regulatory Commission (NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management Meeting on Radionuclide Transport held December 5 through 7, 2000, in Berkeley, California. RT KTI subissues 1, 2, and 3 were discussed at that meeting (Reamer and Williams 2000). Agreement GEN 1.01 was reached during the NRC/DOE Technical Exchange and Management Meeting on Range of Thermal Operating Temperatures, held September 18 through 19, 2001. At that meeting, NRC provided additional comments (GEN 1.01, comments 35, 43, and 46) relating to RT 3.07, and DOE provided an initial response to those comments (Reamer and Gil 2001). The wording of these agreements and of DOE’s initial response to the general agreement comments is as follows: RT 3.07 Provide sensitivity studies to test the importance of colloid transport parameters and models to performance for UZ and SZ. Consider techniques to test colloid transport in the Alcove 8/Niche 3 test (for example, microspheres). DOE will perform sensitivity studies as the basis for consideration of the importance of colloid transport parameters and models to performance for the unsaturated and saturated zones and will document the results in updates to appropriate AMRs, and in the TSPA-LA document, all to be available in FY 2003. DOE will evaluate techniques to test colloidal transport in Alcove 8/Niche 3 and provide a response to the NRC in February 2001. GEN 1.01 (Comment 35) The SSPA recommends new values for EBS colloid transport parameters. If these are adopted by TSPA in the future, the technical basis for the new distributions will require close scrutiny. Relevant KTI agreements are RT 3.07, ENFE 4.03, ENFE 4.04, and ENFE 4.06. September 2003 E-1 No. 8: Colloids Revision 2 DOE Initial Response to GEN 1.01 (Comment 35) The new values for EBS colloidal transport parameters were designed to evaluate unquantified uncertainty for the SSPA. DOE understands that prior to any potential LA, a stronger technical basis must be provided for EBS colloidal transport parameter values carried forward to the base case analysis. GEN 1.01 (Comment 43) The SSPA presents a new distribution for retardation of colloids with irreversibly-attached radionuclides. The distribution takes into account new site-specific alluvium data. However, any future use of this distribution in TSPA will require comparison with results of field and laboratory tests. This concern is indirectly related to agreement TSPAI 3.30. DOE Initial Response to GEN 1.01 (Comment 43)1 DOE acknowledges that any future use of this distribution in TSPA will require comparison with results of field and laboratory tests 1, 2. This concern is indirectly related to KTI agreements RT 3.07 and RT 3.08. Laboratory testing of microsphere and silica colloid retardation in alluvium-packed columns is in progress. Microspheres will be used as colloid tracers in ATC cross-hole tracer testing. GEN 1.01 (Comment 46) The analysis of sensitivity to increased uncertainty in the reversible colloid parameter Kc (Section 12.5.2.4) yielded “somewhat longer transport times” in the SZ. This analysis does not illustrate the effect of possibly underestimating Kc, because it is not clear that the mean value of Kc is significantly different from the base case. This concern is related to agreements RT 3.07 and TSPAI 3.30. DOE Initial Response to GEN 1.01 (Comment 46) This issue will be handled as part of agreements RT 3.07 and TSPAI 3.30. E.1.2 Related Key Technical Issue Agreements None. E.2 RELEVANCE TO REPOSITORY PERFORMANCE Transport parameters and models for colloid transport in the unsaturated zone and saturated zone will determine (1) the ability of colloids to move through the unsaturated zone and saturated zone and into the accessible environment, and (2) the ability of the unsaturated zone and 1 “[F]ield and laboratory tests 1, 2” refers to test conducted using CML microspheres as colloid surrogates. These tests will be discussed in detail in Appendix M (Response to KTI Agreement RT 3.08) of the saturated zone technical basis document. September 2003 E-2 No. 8: Colloids Revision 2 saturated zone to effectively filter colloids or retard colloid transport. The interplay among several key colloid transport parameters and mechanisms will determine the significance of colloid transport through the unsaturated zone and saturated zone. Based on several pertinent studies, it has been concluded that significant filtration and retardation of colloids within both the unsaturated zone and saturated zone will take place. The technical basis for the response for this KTI agreement is presented in Sections 3, 4, 5 and 6 of this technical basis document. This KTI agreement is related to the engineered and near-field environment, the unsaturated and the saturated zones as shown in Figure E-1. E.3 RESPONSE There are a number of factors associated with the transport of colloids through the unsaturated zone and saturated zone. These include: (1) where (into fractures or into the matrix) colloids are released from the repository, (2) the degree of colloid resuspension or declogging, (3) colloid retardation factors, (4) colloid reversible sorption, (5) the degree of matrix diffusion, (6) the application of extrapolating saturated zone determined colloid parameters to the unsaturated zone, (7) colloid particle size, (8) the specific discharge rate, (9) the use of linear equilibrium isotherms to describe the sorption of radionuclides onto colloids, and (10) the use of linear kinetics models to describe the retardation of colloids. Sensitivity analyses have been done that address many of these uncertainties. Details of the analyses can be found in Radionuclide Transport Models under Ambient Conditions (BSC 2003a, Section 6). The impact of these uncertainties on colloid transport varies from negligible (items 1 and 2) to important (items 7 and 8). The uncertainties are dealt with in two ways: 1) distributions describing the parameter uncertainty are established and propagated through the modeling systems leading to a quantified uncertainty in the results and 2) choosing a conservative conceptual model (one which results in a larger release) when two or more conceptual models are potentially possible. The information in this report is responsive to agreements RT 3.07 and GEN 1.01 comments 35, 43, and 46 made between the DOE and NRC. The report contains the information that DOE considers necessary for NRC review for closure of these agreements. E.4 BASIS FOR THE RESPONSE Sensitivity analyses for colloid transport in the unsaturated zone have been performed. Four colloid transport simulations using TOUGH2 V1.11MEOS9NTV1.0 (Module EOS9nTV1.0) were conducted, referred herein as Cases 1, 2, 3, and 4. Details and presentation of the results for these four cases appear in Radionuclide Transport Models under Ambient Conditions (BSC 2003a, Section 6.18, Attachment VI). It is useful to define two terms for the purpose of facilitating the analysis of the results. These are (1) the relative mass fraction of a species in the fractures or matrix, (XR), defined as the ratio of the mass fraction in the liquid in a fracture or matrix to the mass fraction of that species in liquid released from the repository, and (2) the relative filtered concentration, (FR), defined as the ratio of the concentration of species sorbed or filtered onto a fracture or the matrix to the concentration of that species released in the liquid from the repository. September 2003 E-3 No. 8: Colloids Revision 2 Figure E-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System Components September 2003 E-4 No. 8: Colloids Revision 2 The simulation for Case 1 assumes that no declogging occurs once colloids are attached to the matrix. Two observations appear particularly important. 1. The fast breakthrough of the larger colloids (characterized by a rapid rise of the breakthrough curve), the rapidity of which is about the same for all the larger colloids. 2. Contrary to expectations, the smallest colloid (6 nm) exhibits the slowest breakthrough. It is noteworthy that this colloid never reaches the 0.1 fraction level, indicating that it is sufficiently small to enter the matrix or become sorbed to the fracture walls. This behavior results from a combination of the following factors: a. The larger transport velocity of larger colloids, which, by virtue of their size, can only move in the center of pores/fractures where velocities are larger than the average water velocity. b. The inability of larger colloids to penetrate into the matrix from the fractures because of size exclusion. Thus, the colloid mass in the fractures is not reduced through colloidal diffusion and/or hydrodynamic dispersion, and practically all of it moves exclusively in the fractures. The simulation for Case 2 assumes that there is a strong declogging factor. The difference in breakthrough curves between Case 1 and Case 2 is very small for all colloids, and is most prominent for 6 nm colloids. The relative insensitivity to the clogging model in the matrix indicates the dominant role of the fractures in the three-dimensional site scale system, with the matrix appearing to have a negligible contribution. A comparative analysis based on Case 2 (Section 5.4.3) was conducted and reported in Radionuclide Transport Models Under Ambient Conditions (CRWMS M&O 2000, Section 6.17) using an earlier version of the unsaturated zone transport model. This analysis examined the transport of colloids with and without colloid matrix diffusion. The analysis concludes that diffusion is less significant in colloid transport than in solute transport, because (1) colloid diffusion is smaller than solute molecular diffusion because of the larger colloid size, and (2) size-exclusion effects at the interfaces of different geologic units further limit entry by diffusion into the matrix (especially for larger colloids). However, diffusion effects become increasingly important for a decreasing colloid size. Uncertainty in colloid matrix diffusion is treated by sampling the diffusion coefficient over a range of values in the total system performance assessment for the license application unsaturated zone transport model (BSC 2003b, Section 4.2). The simulation for Case 3 shows that the change in the magnitude of the reverse kinetic filtration coefficient ę- (as a fraction of ę+) has a very small effect on colloid transport, attesting to the fact that the role of advection through fracture flow (the same in Cases 1 through 3) is by far the dominant mechanism of colloid transport. The effect of a change in the reverse filtration coefficient on transport is apparent only in the case of the 6 nm colloid (which can enter the pores, and thus be subject to filtration). The insensitivity of transport of the larger colloids to the sorption model in the matrix indicates that the matrix has practically no participation in the September 2003 E-5 No. 8: Colloids Revision 2 retardation, which is attributed to straining at matrix/fracture interfaces. The pattern that emerges is the same as the one discussed in Case 1. In Case 4, assigning a porosity of 1 percent to the fractures, and making the fracture filtration properties equal to those of the matrix (assumed to be those of Case 2), is equivalent to creating a system of minor partial fracture filling. The effect of limited diffusion on transport (because of pore-size exclusion and filtration) becomes more obvious in this case. While the more-freely diffusing 6 nm colloids exhibit a behavior similar to the one in Cases 1 through 4, the effect on the larger colloids is more dramatic. The occurrence of even a minor fracture filling retards the colloid transport. The potential points of release from the repository are uncertain and may vary with time; however, this does not impact the transport of colloids. Because of the importance of transport and flow within faults on the migration of dissolved radionuclides and colloids, the impact of releasing radionuclides directly into faults was assessed (BSC 2003a, Section 6.20). Despite the importance of faults (particularly the Drill Hole Wash, the Pagany Wash, and the Sundance faults) identified in the analyses of Radionuclide Transport Models Under Ambient Conditions (BSC 2003a, Section 6), eliminating potential sources from the immediate vicinity of the faults has a small (even negligible) effect on transport and arrivals at the water table. This is not to suggest that faults are unimportant but only that they continue to dominate whether radionuclides are directly released into them or not. Sensitivity Analysis for Colloid Transport in the Saturated Zone–Uncertainties associated with model parameters are explicitly captured by using probability distributions of model parameter values. These uncertainties are propagated through the model to the output breakthrough curves. The results are presented in Saturated Zone Colloid Transport (BSC 2003c, Section 7.3). Radionuclide transit times are most sensitive to groundwater specific discharge. This is because increasing the specific discharge not only increases the advective velocity but also reduces the time available for matrix diffusion to be effective. In assessing the sensitivity of breakthrough times to the specific discharge through the model, permeabilities of the various units are scaled along with the specific discharge to preserve the model calibration. Other parameters of importance to the breakthrough times are the retardation factor for irreversible colloids in the alluvium, matrix diffusion, the sorption coefficient in the matrix as well as the alluvium, the effective fracture porosity in the volcanics, the effective porosity in the alluvium, and the sorption coefficients for reversible colloids. Quantification of the sensitivity of the model output breakthrough curves for various radionuclides of concern to parameter uncertainties are further evaluated in SZ Flow and Transport Model Abstraction (BSC 2003d). There are uncertainties associated with scaling parameter values from the scale of measurements to the scale of interest. Much of the data used for deriving parameter values in this report is from laboratory or field experiments conducted on spatial and temporal scales much smaller than those expected to occur in site-scale saturated zone model. The sensitivity of the output breakthrough curves to each of the uncertain input parameters is summarized as follows. September 2003 E-6 No. 8: Colloids Revision 2 Specific Discharge: The results show that the output is sensitive to this parameter. The upper limit of specific discharge, 6 m/year, with extremely fast fluid flow (such as would be expected for the unlikely case of a high permeability channel going continuously over the distance of 18 kilometers in a highly faulted region), results in breakthrough as fast as 50 years. The lower limit of specific discharge, 0.02 m/year, leads to breakthrough times greater than 10,000 years. Colloid Retardation Factor in Alluvium for Irreversible Colloids: Colloid retardation is not modeled in the base case. Compared to that, both the low and high limiting values of the colloid retardation factor for irreversibly sorbed radionuclides lead to significant retardation, resulting from retardation of the colloids themselves. The time for the 50 percent breakthrough for the lower limit of the retardation factor (R = 7.9) is 5,041 years, and that for the upper limit of the retardation factor (R = 5188) exceeds 10,000 years. Reversible Sorption onto Colloids in the Alluvium: The range of values for the modified sorption coefficient used in this case is the same range of values as the original sorption coefficient in the alluvium. Thus, the results for the lower limiting case are identical to the base case, and those for the upper limiting case show no breakthrough at all within 10,000 years. Colloid Retardation Factor in Volcanics for Irreversible Colloids: The colloids are modeled as having no diffusion into the volcanic matrix, and the retardation arises from the reversible filtration of the colloids by attachment onto the surfaces of the fractures. The range of uncertainty in the retardation factor of 6 to 794 translates into an output uncertainty range of 1,420 to more than 10,000 years for the breakthrough time for 50 percent of the colloids. Reversible Sorption onto Colloids in the Volcanics: The modified sorption coefficient for this case has the same range of values as the original sorption coefficient in alluvium. In this case, the diffusion coefficient is also modified for the upper limit. Thus, the results for the lower limiting case are identical to the base case, and those for the upper limiting case show no breakthrough at all within 10,000 years. A small fraction of colloids traveled unretarded. Alcove 8/Niche 3 Test–The matrix diffusion model is an important component of the unsaturated zone transport model. This model is corroborated with data from the Alcove 8/Niche 3 fault test (BSC 2003a, Section 7.4). The test was carried out in the upper lithophysal and middle nonlithophysal subunits in the Yucca Mountain unsaturated zone. These geological subunits correspond to model layers TSw33 and TSw34, respectively, in the site-scale model for the Yucca Mountain unsaturated zone. The TSw33 has some lithophysal cavities that may intersect fractures. Liquid water with and without tracers was released at the floor of an alcove along the fault (about 5 m long) within TSw33. Seepage from the fault into a niche and tracer concentrations of seeping liquid as a function of time were monitored. The niche is located within TSw34, about 20 m below the floor of the alcove; the interface between TSw33 and TSw34 is about 15 m below the floor of the alcove. A water-pressure head of 2 cm was applied at an infiltration plot along the fault at the alcove. The plot consists of four trenches that have different infiltration rates as a result of subsurface heterogeneity along the fault. Seepage from the fault into the niche was measured during the test, with a number of trays used to cover the areas where seepage might occur. Seepage was found to be highly spatially variable. Several boreholes were installed around the niche. Water arrival times at these boreholes were monitored through electrical resistivity probes. After 209 days, two tracers with different September 2003 E-7 No. 8: Colloids Revision 2 molecular diffusion coefficients (Br and pentafluorobenzoic acid, or PFBA) were introduced into infiltrating water at the infiltration plot. Tracer concentrations in three of the trays (at the niche) capturing seeping water from the fault were measured. For technical reasons, seepage rates corresponding to these three trays were not measured during the period of tracer concentration measurement. In this study, a flux-averaged breakthrough curve (concentration as a function of time) from these trays was used to represent the average breakthrough curve for all trays at the niche where seepage was captured. A constant flux value for each of the three trays was used for calculating the flux-averaged breakthrough curve. The constant flux values for the three trays were determined as the averaged value over 56 days before tracers were introduced. This flux-averaged breakthrough curve is comparable to the simulation results. Use of CML Microspheres–All of the field measurements have involved fluorescent carboxylate-modified polystyrene latex (CML) microspheres ranging in size from 250 to 500 nm diameter. Laboratory fracture experiments have been conducted using silica, montmorillonite, and clinoptilolite colloids in addition to CML microspheres. In one study, silica colloids (approximately 100 nm diameter) were compared directly with CML microspheres transport (330 nm diameter), and it was found that the microspheres transported conservatively relative to silica colloids. This result suggests that colloid filtration and retardation parameters derived from CML microsphere responses in field tracer tests should be conservative if used to predict natural inorganic colloid transport in fracture systems. See detailed discussions in Saturated Zone Colloid Transport (BSC 2003c). E.5 REFERENCES BSC (Bechtel SAIC Company) 2003a. Radionuclide Transport Models under Ambient Conditions. MDL-NBS-HS-000008 REV 01A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030828.0326. BSC 2003b. Particle Tracking Model and Abstraction of Transport Processes. MDL-NBS-HS- 000020 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030611.0040. BSC 2003c. Saturated Zone Colloid Transport. ANL-NBS-HS-000031 REV 01A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030602.0288. BSC 2003d. SZ Flow and Transport Model Abstraction. MDL-NBS-HS-000021 REV 01. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030916.0008. CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating Contractor) 2000. Radionuclide Transport Models Under Ambient Conditions. MDL-NBS-HS- 000008 REV 00. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.19990721.0529. Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. September 2003 E-8 No. 8: Colloids Revision 2 Reamer, C.W. and Williams, D.R. 2000. Summary Highlights of NRC/DOE Technical Exchange and Management Meeting on Radionuclide Transport. Meeting held December 5-7, 2000, Berkeley, California. Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20010117.0063. September 2003 E-9 No. 8: Colloids INTENTIONALLY LEFT BLANK E-10 No. 8: Colloids Revision 2 September 2003 APPENDIX F TRANSPORT OF DISSOLVED AND COLLOIDAL RADIONUCLIDES THROUGH INVERT (RESPONSE TO TSPAI 3.17 AND GEN 1.01 (COMMENTS 36 AND 38)) No. 8: Colloids Revision 2 September 2003 Revision 2 Note Regarding the Status of Supporting Technical Information This document was prepared using the most current information available at the time of its development. This Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were prepared using preliminary or draft information reflect the status of the Yucca Mountain Project’s scientific and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the AMRs and other references will be reflected in the License Application (LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to reflect changes in the supporting references prior to submittal of the LA. September 2003 No. 8: Colloids Revision 2 APPENDIX F TRANSPORT OF DISSOLVED AND COLLOIDAL RADIONUCLIDES THROUGH INVERT (RESPONSE TO TSPAI 3.17 AND GEN 1.01 (COMMENTS 36 AND 38)) This appendix provides a response for Key Technical Issue (KTI) agreement Total System Performance Assessment and Integration (TSPAI) 3.17 and general agreement (GEN) 1.01, comments 36 and 38. These KTI agreements relate to providing an uncertainty analysis of the colloid transport diffusion coefficient. F.1 KEY TECHNICAL ISSUE AGREEMENTS F.1.1 TSPAI 3.17 and GEN 1.01 (Comments 36 and 38) Agreement TSPAI 3.17 was reached during the U.S. Nuclear Regulatory Commission (NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management Meeting on Total System Performance Assessment and Integration held August 6 through 10, 2001, in Las Vegas, Nevada. TSPAI KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). Agreement GEN 1.01 was reached during the NRC/DOE Technical Exchange and Management Meeting on Range of Thermal Operating Temperatures held September 18 through 19, 2001. At that meeting, NRC provided additional comments (GEN 1.01, comments 36 and 38) relating to TSPAI 3.17, and DOE provided an initial response to those comments (Reamer and Gil 2001). The wording of these agreements and of DOE’s initial response to the general agreement comments is as follows: TSPAI 3.171 Provide an uncertainty analysis of the diffusion coefficient governing transport of dissolved and colloidal radionuclides through the invert. The analysis should include uncertainty in the modeled invert saturation (ENG4.4.1). DOE will provide an uncertainty analysis of the diffusion coefficient governing transport of dissolved and colloidal radionuclides through the invert. The analysis will include uncertainty in the modeled invert saturation. The uncertainty analysis will be documented in the EBS Radionuclide Transport Abstraction AMR (ANL-WISPA- 000001) expected to be available to NRC in FY 2003. GEN 1.01 (Comment 36) The discussion of uncertainty in the saturation level of the invert does not consider the possibility of higher saturation. This comment is related to KTI agreement TSPAI 3.17. 1 ENG4.4.1 in this agreement refers to NRC integrated subissue ENG 4 (NRC 2002, Table 1.1-2). September 2003 F-1 No. 8: Colloids Revision 2 DOE Initial Response to GEN 1.01 (Comment 36) Studies with the MSTH model, as reported in Chapter 5 of the SSPA Volume 1, investigated the sensitivity of invert liquid saturation to a variety of repository parameters. These parameters included bulk permeability, host-rock thermal conductivity, lithophysal porosity, and invert thermal conductivity. Predicted liquid saturation remained within a narrow range, between four percent and 10 percent, for all parameter variations. In addition, the diffusive breakthrough time for the invert is already relatively rapid, so any increase in saturation levels is expected to have a negligible impact. DOE will provide an uncertainty analysis of diffusion in the invert. This analysis will include uncertainty in invert saturation per KTI agreements TEF 2.05 and TSPAI 3.17. GEN 1.01 (Comment 38) The effect of the drift shadow assumption on invert transport needs to be evaluated. Also, as mentioned in the chapter, any adoption of a drift shadow model will require additional justification. This concern may be related to agreement TSPAI 3.17. DOE Initial Response to GEN 1.01 (Comment 38) The invert transport abstraction does not incorporate any direct assumptions related to a drift shadow effect. The hydrologic inputs to the invert transport calculation come primarily from the MSTH model that tracks water and gas within the near-field rock and the drift. The specific inputs from the MSTH model to invert transport are the temperature of the invert and liquid saturation of the invert. DOE acknowledges that models carried forward to support a potential license application will be qualified and documented, and may require supplemental justification or analysis. Also, see response to comment 29. F.1.2 Related Key Technical Issue Agreements GEN 1.01 (comment 36), TSPAI 3.30, and RT 3.07. F.2 RELEVANCE TO REPOSITORY PERFORMANCE Radionuclide transport through the invert is expected to be primarily dominated by diffusion of both dissolved and colloidal species. One of the key parameters is the diffusion coefficient for both types of species. The diffusion coefficient for dissolved species is a function of degree of saturation, with higher values at the higher moisture contents. The diffusion coefficient for colloids is estimated from diffusion coefficients for dissolved species multiplied by the ratio of ionic radius to colloid radius. September 2003 F-2 No. 8: Colloids Revision 2 The technical basis for the response for this KTI agreement is presented in Section 4 of this technical basis document. This KTI agreement is related to the invert as shown in Figure F-1. Figure F-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System Components No. 8: Colloids September 2003 F-3 Revision 2 F.3 RESPONSE Using electric conductivity measurements, the diffusion coefficient for dissolved species has been measured for crushed tuff invert material as a function of volumetric moisture content and presented in EBS Radionuclide Transport Abstraction (BSC 2003). From these values, diffusion coefficients for colloids can be estimated using the ratio of ionic size (molecular size) to colloid size. These measurements show that for typical moisture and saturation levels expected in the invert, diffusion of dissolved species would take more than 10,000 years, and much longer for colloids. Uncertainty of the dissolved species diffusivity is discussed in EBS Radionuclide Transport Abstraction (BSC 2003, Section 6.3.4.1). As discussed in EBS Radionuclide Transport Abstraction (BSC 2003, Section 7.1.2), the invert diffusion coefficient has a low impact on performance because the response of the unsaturated zone generally dominates the response of the invert. The unsaturated zone is dominant because the invert is filled with crushed tuff that is similar to the host rock in the unsaturated zone and because the thickness of the invert is quite small in relation to the travel distance in the unsaturated zone. The information in this report is responsive to agreements TSPAI 3.17 and GEN 1.01 comments 36 and 38 made between the DOE and NRC. The report contains the information that DOE considers necessary for the NRC to review for closure of these agreements. F.4 BASIS FOR THE RESPONSE The diffusion coefficient for dissolved species has been determined specifically for crushed tuff invert materials as a function of volumetric moisture content (m3 water/m3 bulk rock), using electric conductivity measurements (BSC 2003). Figure F-2 presents the uncertainty in the statistical fit for the diffusion coefficient. For the nominal scenario evaluated out to 10,000 years, releases only occur because a small fraction of the waste packages have small manufacturing defects. Hence, all the releases occur via diffusive pathways through the waste package and invert. In this scenario, the only source of water is vapor from evaporation and condensation. Based on corrosion analysis, the drip shield will remain intact for the regulatory period and, thus, any seepage water that enters the drifts will contact the drip shield and be diverted away from the package. This diversion creates a drip shadow zone underneath the waste package. Therefore, no advective transport will occur through the waste package or invert even in seepage environments. Within this drip shadow, all radionuclides will be transported via diffusion through the intragranular pore space. Diffusive transport through the intergranular pore space will be insignificant since water saturations in the intergranular pore space will be near zero. Hence, transport of colloidal material through the invert will likely be negligible, since the diffusion coefficient for colloids is at least 100 times lower than for dissolved species based on the relative size of colloids and dissolved species. Any mixing that occurs between waters in this shadow will be by the slow process of diffusion of anions and cations. The water chemistry in this shadow will remain similar to matrix water as ions migrate and react with the water constituents and minerals in the matrix. Thus, the chemistry for evaluating the stability of the colloids corresponds to the matrix water chemistry. September 2003 F-4 No. 8: Colloids Revision 2 Source: BSC 2003, Figure 7. NOTE: The dashed lines correspond to two standard deviations above and below the statistical fit to the data. D is ˙ ˙. . . .. . where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the diffusion coefficient of a ion of radius rion. Given a typical ion radius and colloidal particle radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 100 times smaller than that of a dissolved ion. Thus, it takes at least 10,000 years for colloids to diffuse through the invert. the diffusion coefficient of the crushed invert materials, D0 is the diffusion coefficient of the fully saturated crushed invert materials, and Ąč is the percentage of saturation. (Eq. F-1) (Eq. F-2) September 2003 No. 8: Colloids Figure F-2. Uncertainty in the Statistical Fit for the Diffusion Coefficient The lack of diffusion can be shown as follows. The characteristic time (T) for dissolved radionuclides to diffuse through the invert of thickness L can be calculated by: D L / 2 T = where D is the diffusion coefficient. With L = 0.8 m, the calculated T values are listed in the fourth column of Table F-1. As shown in Table F-1, it will take 100 to 1.65 ˇż 106 years for a dissolved species to diffuse through the invert, depending on the saturation degree. The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein relationship (BSC 2003): ion ion Dcoll = D r rcoll F-5 As discussed in EBS Radionuclide Transport Abstraction (BSC 2003, Section 7.1.2), the invert diffusion coefficient has a low impact on performance because the response of the unsaturated zone generally dominates the response of the invert. The unsaturated zone is dominant because the invert is filled with crushed tuff that is similar to the host rock in the unsaturated zone and because the thickness of the invert is quite small in relation to the travel distance in the unsaturated zone. Diffusion Coefficient of and Time of Diffusion through Crushed Tuff Invert Materials Table F-1. Volumetric Moisture Content (%) 32.13 18.15 9.26 8.29 7.54 7.36 7.22 7.03 6.97 6.89 6.84 6.75 6.63 6.63 6.23 6.11 6.00 5.55 5.46 5.41 4.45 3.64 0.29 0.20 Source: BSC 2003, Table 43. DTN: MO0002EBSDDC02.003 NOTE: a Saturation degree = volumetric moisture/porosity. The porosity of crushed tuff invert material is assumed b to be 0.545 (BSC 2003, Table 1). Time for diffusion is calculated using Equation F-1 with L = 0.8 m. Diffusion Coefficient (cm2/s) 2.02E-06 5.40E-07 4.05E-08 2.24E-09 6.81E-09 6.21E-09 4.38E-09 6.75E-09 7.45E-09 6.73E-09 2.19E-09 5.42E-09 4.39E-09 3.76E-09 3.40E-09 1.55E-09 3.43E-09 2.04E-09 2.04E-09 9.97E-10 6.19E-10 5.00E-10 1.24E-10 1.25E-10 Saturation Degreea (%) 58.95 33.30 16.99 15.21 13.83 13.50 13.25 12.90 12.79 12.64 12.55 12.39 12.17 12.17 11.43 11.21 11.01 10.18 10.02 9.93 8.17 6.68 0.53 0.37 Time for Diffusion through Invertb (year) 1.02E+02 3.81E+02 5.09E+03 9.20E+04 3.02E+04 3.32E+04 4.70E+04 3.05E+04 2.77E+04 3.06E+04 9.41E+04 3.80E+04 4.69E+04 5.48E+04 6.06E+04 1.33E+05 6.01E+04 1.01E+05 1.01E+05 2.07E+05 3.33E+05 4.12E+05 1.66E+06 1.65E+06 No. 8: Colloids F-6 Revision 2 September 2003 Revision 2 F.5 REFERENCES F.5.1 Documents Cited BSC (Bechtel SAIC Company) 2003. EBS Radionuclide Transport Abstraction. ANL-WIS-PA- 000001 REV 01F. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0199. NRC (U.S. Nuclear Regulatory Commission) 2002. Integrated Issue Resolution Status Report. NUREG-1762. Washington, D.C.: U.S. Nuclear Regulatory Commission, Office of Nuclear Material Safety and Safeguards. TIC: 253064. Reamer, C.W. 2001. “U.S. Nuclear Regulatory Commission/U.S. Department of Energy Technical Exchange and Management Meeting on Total System Performance Assessment and Integration (August 6 through 10, 2001).” Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), August 23, 2001, with enclosure. ACC: MOL.20011029.0281. Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. F.5.2 Data, Listed by Data Tracking Number MO0002EBSDDC02.003. The Determination of Diffusion Coefficient of Invert Materials. Submittal date: 02/16/2000. September 2003 F-7 No. 8: Colloids INTENTIONALLY LEFT BLANK F-8 No. 8: Colloids Revision 2 September 2003 SCREENING CRITERIA FOR ATTACHMENT OF RADIONUCLIDES TO COLLOIDS (RESPONSE TO RT 1.03 AIN-1, ENFE 3.05 AIN-1, AND ENFE 4.05 AIN-1) No. 8: Colloids APPENDIX G Revision 2 September 2003 Revision 2 Note Regarding the Status of Supporting Technical Information This document was prepared using the most current information available at the time of its development. This Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were prepared using preliminary or draft information reflect the status of the Yucca Mountain Project’s scientific and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the AMRs and other references will be reflected in the License Application (LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to reflect changes in the supporting references prior to submittal of the LA. September 2003 No. 8: Colloids Revision 2 APPENDIX G SCREENING CRITERIA FOR ATTACHMENT OF RADIONUCLIDES TO COLLOIDS (RESPONSE TO RT 1.03 AIN-1, ENFE 3.05 AIN-1, AND ENFE 4.05 AIN-1) This appendix provides a response for additional information need (AIN) requests for Key Technical Issue (KTI) agreements Radionuclide Transport (RT) 1.03, Evolution of the Near-Field Environment (ENFE) 3.05 and ENFE 4.05. These KTI agreements relate to providing a technical basis for selection of radionuclides that are transported via colloids in Performance Assessment analyses. G.1 KEY TECHNICAL ISSUE AGREEMENTS G.1.1 RT 1.03 AIN-1, ENFE 3.05 AIN-1, and ENFE 4.05 AIN-1 Agreement RT 1.03 was reached during the U.S. Nuclear Regulatory Commission (NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management Meeting on Radionuclide Transport held December 5 through 7, 2000, in Berkeley, California. RT KTI subissues 1, 2, and 3 were discussed at that meeting (Reamer and Williams 2000). NRC used four DOE documents in its review of the response to KTI agreement RT 1.03: Inventory Abstraction (CRWMS M&O 2000a); Waste Form Colloid-Associated Concentrations Limits: Abstraction and Summary (CRWMS M&O 2001); Total System Performance Assessment (TSPA) Model for Site Recommendation (CRWMS M&O 2000b); and Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000c). Additional information was requested by the NRC after the staff’s review of these reports was completed, resulting in RT 1.03 AIN-1 (Reamer 2002). Agreements ENFE 3.05 and 4.05 were reached during the NRC/DOE Technical Exchange and Management Meeting on Evolution of the Near-Field Environment held January 9 through 12, 2001, in Pleasanton, California. ENFE KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). NRC used three DOE documents in its review of the response to KTI agreement ENFE 3.05: Total System Performance Assessment (TSPA) Model for Site Recommendation (CRWMS M&O 2000b); Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000c); and Waste Form Colloid-Associated Concentrations Limits: Abstraction and Summary (CRWMS M&O 2000d). Additional information was requested by the NRC after their review of these reports was completed, resulting in ENFE 3.05 AIN-1 (Schlueter 2002). NRC used four DOE documents in its review of the response to KTI agreement ENFE 4.05: Inventory Abstraction (CRWMS M&O 2000a); Waste Form Colloid-Associated Concentrations Limits: Abstraction and Summary (CRWMS M&O 2000d); TSPA for the Site Recommendation (CRWMS M&O 2000c); and TSPA Model for Site Recommendation (CRWMS M&O 2000b). Additional information was requested by the NRC after their review of these reports was completed, resulting in ENFE 4.05 AIN-1 (Schlueter 2002). September 2003 G-1 No. 8: Colloids Revision 2 The wording of these agreements is as follows: RT 1.03 Provide the screening criteria for the radionuclides selected for PA. Provide the technical basis for selection of the radionuclides that are transported via colloids in the TSPA. The screening criteria for radionuclides selected for TSPA are contained in the AMR Inventory Abstraction. DOE is documenting identification of radionuclides transported via colloids for TSPA in the AMR Waste Form Colloid-Associated Concentration Limits: Abstraction and Summary, in the TSPA-SR Technical Report, and in the TSPA-SR Model Document. These documents will be available to the NRC in January 2001. RT 1.03 AIN-1 Provide clarification and justification of radionuclides for which reversible and irreversible colloidal transport is modeled. ENFE 3.05 Provide the technical basis for selection of radionuclides that are released via reversible and irreversible attachment to colloids for different waste forms in the TSPA. The technical basis for the selection of radionuclides released via reversible and irreversible attachments to colloids for different waste forms is provided in Section 3.5.6.1 of the Total System Performance Assessment (TSPA) Model for Site Recommendation (MDL-WIS-PA-000002) Rev 00. This document will be provided to the NRC in January 2001. ENFE 3.05 AIN-1 1. Provide clarification and justification of radionuclides for which reversible and irreversible colloidal release is modeled. 2. Provide a stronger technical basis that release by irreversible attachment can be neglected for spent nuclear fuel.” ENFE 4.05 Provide the screening criteria for the radionuclides selected for PA. Provide the technical basis for selection of radionuclides that are transported via colloids in the TSPA. The screening criteria for radionuclides selected for TSPA are contained in the AMR Inventory Abstraction (ANL-WIS-MD-000006) Rev 00, ICN 01. The DOE is documenting identification of radionuclides transported via colloids for TSPA in the AMR Colloid-Associated Concentration Limits: Abstraction and Summary (ANL-WIS-MD-000012) Rev 0, in the Total System Performance Assessment for the Site Recommendation (TDR-WIS-PA-000001) Rev 00 ICN 01, and in the Total System Performance Assessment (TSPA) Model September 2003 G-2 No. 8: Colloids Revision 2 for Site Recommendation (MDL-WIS-PA-000002) Rev 00. These documents will be available to the NRC in January 2001. ENFE 4.05 AIN-1 1. Provide clarification and justification of radionuclides for which reversible and irreversible colloidal release is modeled. G.1.2 Related Key Technical Issue Agreements None. G.2 RELEVANCE TO REPOSITORY PERFORMANCE Determining which radionuclides can reversibly and irreversibly attach to colloids is an important consideration in establishing the relative importance of colloid-facilitated radionuclide transport. Once the concentration of colloids and the available surface area per unit volume is determined, the mechanism for the attachment of dissolved radionuclides to colloids will establish the mass of radionuclides that can be transported via colloids. The technical basis for the response for this KTI agreement is presented in Section 3 of this technical basis document. This KTI agreement is related to the waste package as shown in Figure G-1. G.3 RESPONSE Section 6.3.3.1 of Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003) provides detailed discussion of the screening criteria for radionuclides undergoing reversible sorption and Section 6.3.3.2 for radionuclides undergoing irreversible sorption. The rationale for selecting radionuclides was based on a combination of long half-life, relatively strong sorption characteristics, relatively large abundance during the initial 10,000 years in the repository, and observed field and laboratory behavior. The sorption of Pu, Am, Th, Cs, and Pa, are modeled using a linear isotherm model. Kd values for uptake of cesium (Cs), thorium (Th), protactinium (Pa), plutonium (Pu), and americium (Am) by colloids are provided in Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003, Table 10). These radionuclides were modeled because they are considered to be important to dose. A major fraction of Pu and Am (greater than 90 percent) are assumed to attach to smectite colloids irreversibly. Evidence from sorption experiments with Pu and Am with colloidal hematite and goethite show that the rates of desorption (backward rate) of Pu and Am are significantly slower than the rates of sorption (forward rate). More importantly, over a significant time period (up to 150 days in some experiments), the extent of desorption is considerably less than the extent of sorption. Pu and Am are considered so strongly sorbed to colloids that, for practical purposes, they can be considered irreversibly sorbed and are modeled in this manner within the engineered barrier system. September 2003 G-3 No. 8: Colloids Revision 2 Figure G-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System Components September 2003 G-4 No. 8: Colloids Revision 2 Recent long-term corrosion tests with commercial and DOE spent nuclear fuel have demonstrated alteration products containing low concentrations of uranium-based colloids (and low concentrations of actinides), dissolution of the uranium-based colloids over a short time duration (less than several months), and sorption dominating the behavior of the actinides (particularly onto the vessels’ metal surfaces). The information in this report is responsive to agreements RT 1.03 AIN-1, ENFE 3.05 AIN-1, and ENFE 4.05 AIN-1 made between the DOE and NRC. The report contains the information that DOE considers necessary for NRC review for closure of these agreements. G.4 BASIS FOR THE RESPONSE G.4.1 Screening of Radionuclides for Reversible and Irreversible Attachment to Colloids Section 6.3.3.1 of Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003) provides detailed discussion of the screening criteria for radionuclides undergoing reversible sorption and Section 6.3.3.2 for radionuclides undergoing irreversible sorption. G.4.1.1 Reversible Sorption Transport of radionuclides on colloids is potentially important for radioisotopes that 1) have low solubility; 2) have long half-lives, 3) can be entrained in waste-form colloids, or can be sorbed onto waste-form colloids, engineered barrier materials colloids, or geologic barrier materials colloids; 4) represent a major portion of the inventory in terms of total activity; and/or 5) can contribute significantly to radioactivity during the 10,000-year regulatory period. Considering these five criteria as part of radionuclide screening in Section 6.3.3.1, Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003) evaluated eight radionuclides for reversible sorption onto to colloids: Pu, Am, Th, Cs, Pa, Np, U, and Sr. Below is the rationale used in that report to include or exclude these radionuclides from the model for reversible attachment to colloids: • Plutonium–A large quantity of Pu will exist in the repository. Pu is sparingly soluble but sorbs strongly to oxide mineral surfaces (generally less strongly to silicates). Pu is observed to sorb strongly to soil minerals, and laboratory investigations have shown that it sorbs readily to colloids as well. • Americium–Am will be a significant contributor to radioactivity during the 10,000 years. As is Pu, Am is sparingly soluble but strongly sorbs to mineral surfaces, including colloids. Laboratory investigations have shown that it sorbs strongly to colloids. • Cesium–135Cs has a long half-life and can attach strongly to certain sheet silicates (including clays) by means of ion exchange. For this reason, 135Cs has been observed to sorb to soil minerals, and it could potentially form pseudocolloids particularly with groundwater and defense high-level radioactive waste glass-derived clay colloids. September 2003 G-5 No. 8: Colloids Revision 2 • Protactinium–Pa will be a significant contributor to radioactivity during the first 10,000 years. Because of this, and the fact that relatively little is known of the colloid behavior of Pa, it was included in this analysis. • Thorium–Th will be a significant contributor to radioactivity during the first 10,000 years. Because of this, and the fact that there is evidence that Th sorbs strongly to oxides, it was included in this analysis. There is relatively little known of the colloid-related behavior of Th. • Neptunium–Because Np will be the most significant contributor to radioactivity beyond the first 10,000 years, it was considered for inclusion in this analysis. Np is more soluble under anticipated repository conditions than many of the other important radionuclides, and it sorbs considerably less strongly than, for example, Pu and Am. It would appear then that the mobility of Np is influenced mostly by its solubility. For these reasons, and to simplify the modeling, Np was not included in the reversible-sorption portion of the colloid-associated radionuclide transport analysis. • Uranium–U will be by far the most abundant radioactive element in the repository and primarily for this reason was considered for the analysis. U is more soluble under anticipated repository conditions than many of the other important radionuclides, and it sorbs considerably less strongly than, for example, Pu and Am. As with Np, the mobility of U is influenced mostly by its solubility. Field observations at U deposits and mine sites have indicated that little or no colloid U transport occurs. For these reasons, and to simplify modeling, U was not included in the reversible-sorption of the colloid-associated radionuclide transport analysis. • Strontium–Sr is expected to be a potentially significant contributor to dose but because of its very short half-life was not considered further in this analysis. G.4.1.2 Irreversible Sorption 2 Defense high-level radioactive waste glass degradation experiments show that plutonium is probably irreversibly attached to smectite colloids generated during the experiments. Further, evidence from sorption experiments with Pu and Am (Lu et al. 2000) with colloidal hematite and goethite show that the rates of desorption (backward rate) of Pu and Am are significantly slower than the rates of sorption (forward rate). More importantly, over a significant time period (up to 150 days in some experiments), the extent of desorption is considerably less than the extent of sorption. Pu and Am are considered so strongly sorbed to colloids that, in essence, they can almost be considered irreversibly sorbed and are modeled in this manner within the engineered barrier system. Pu transport velocities in soils reflect the fact that Pu binds strongly to soils, leaving very little, if any, soluble Pu available for groundwater transport or plant uptake. Coughtrey et al. (1985) estimated exchangeable Pu to be less than one percent. At Rocky Flats, soil Pu is largely bound to soil metal hydroxides. Litaor and Ibrahim (1996) used 0.01M CaCl as an extractant and measured Pu in Rocky Flats soil to be 0.04 to 0.08 percent exchangeable. Bunzl et al. (1995) measured exchangeable 239Pu and 240Pu (0.5 to 1 percent) and 241Am (1.5 to 15 percent) from fallout-contaminated soils in Germany using 1M C2H7NO2 (ammonium acetate NH4C2H3O2) as the extractant. Laboratory experiments of Pu sorption onto iron oxides have September 2003 G-6 No. 8: Colloids Revision 2 shown that only approximately one percent of the initially sorbed Pu can be desorbed into solution, even after months of time have elapsed (Lu et al. 2000), which is broadly consistent with field observations. For these reasons, Pu and Am are modeled as irreversibly attaching to colloids. No other radionuclides are considered to be irreversibly attached to colloids. In order to accommodate these observations in the colloid abstraction, the TSPA-LA model calculates irreversible and reversible sorption of Pu and Am onto corrosion colloids as functions of specific surface area (SA), site density (NA), mass of corrosion colloids, dissolved concentration of Pu and Am, target-flux out ratio, and other parameters (such as flow and diffusion rate) internal to the TSPA-LA model. This is done in such a way that a majority (90 to 99 percent) of Pu and Am are irreversibly sorbed. The reversibly sorbed portion is determined according to an equilibrium Kd value model. G.4.2 Colloid Formation from Commercial and Defense Spent Nuclear Fuel Long-term corrosion testing of commercial spent nuclear fuel and defense spent nuclear fuel under unsaturated, oxidizing conditions has been performed to examine the release of radionuclides and, specifically for the purpose of this report, the release in the form of colloids. Testing was designed to simulate a variety of Yucca Mountain repository relevant water-exposure conditions for several spent nuclear fuels with a range of fuel burnups and compositions. Results from the unsaturated testing of commercial spent nuclear fuel and defense spent nuclear fuel indicated spallation of alteration products containing low concentrations of uranium-based colloids (and low concentrations of actinides), dissolution of the uranium-based colloids over a short time duration (less than several months), and sorption dominating the behavior of the actinides (onto clay colloids or onto metal surfaces). Assessment of the importance of potential colloid formation from commercial spent nuclear fuel was based on four major observations: (1) very low colloid concentrations were observed in the commercial spent nuclear fuel degradation tests, at least an order of magnitude less than concentrations observed in the defense high-level radioactive waste glass degradation tests (based on dynamic light scattering measurements) (Mertz et al. 2003); (2) the fraction of uranium in the colloid mass was uniformly low in the commercial spent nuclear fuel tests, the only deviation from this occurring immediately following one of two changes in vessel configuration in which the uranium fraction increased but rapidly decreased to the approximate level of earlier values (Mertz et al. 2003); (3) suspensions of meta-schoepite and UO2+x colloids in J-13 groundwater appear to exhibit decreasing stability in short-term saturated (with respect to the uranium phases) tests; their stability in unsaturated solutions has not been tested (Mertz et al. 2003); and (4) field studies at uranium-bearing deposits indicate generally that under oxidizing conditions at near-neutral pH, colloid particles contain little uranium and there is little sorption of uranium complexes to colloids. One of the reasons hypothesized for the low colloid release in the commercial spent nuclear fuel tests was the test configuration in which the Zircaloy-4 support for the fuel fragments had 7-micron holes. However, the results from the unirradiated UO2 tests also show few colloids after the formation of alteration products (Wronkiewicz et al. 1997). The unsaturated tests on unirradiated UO2 had a test configuration with large 2 to 3 mm holes at the holder base allowing for the spallation of UO2+x particulate during initial corrosion; however, the formation of a dense September 2003 G-7 No. 8: Colloids Revision 2 mat of alteration products during the UO2 corrosion apparently reduced particulate release by trapping particulates in the altered products (Wronkiewicz et al. 1997). A similar mechanism whereby the alteration products minimize particulate release may be applicable to the commercial spent nuclear fuel unsaturated tests. The concentration of released particulates or colloids from the commercial spent nuclear fuel tests is very low except during test conditions corresponding to disruptive events, such as movement of the fuel from one retainer to another (Mertz et al. 2003). In that case, colloid and particulate concentrations increased temporarily but returned to very low concentrations after the disruption (Mertz et al. 2003). While this indicates that disruptive events may contribute to the release of particulates and colloids from commercial spent nuclear fuel, it also indicates that the longevity of the colloids in the leachate is short. Of the different types of fuel in the defense spent nuclear fuel inventory, metallic uranium fuel comprises approximately 85 percent (by weight) of that inventory and thus was selected for corrosion testing. An irradiated uranium metal fuel from the N-Reactor at Hanford was tested in an experimental setup similar to that used at Argonne National Laboratory for testing commercial spent nuclear fuel. Additional details on this testing can be found elsewhere (DTN: MO0306ANLSF001.459). Corrosion testing of metallic uranium samples resulted in rapid oxidation (within a few months) of the uranium primarily to an oxide sludge consisting of UO2 and higher oxides of uranium (DTN: MO0306ANLSF001.459). Although the uranium fuel disintegrated rapidly, corrosion testing was continued to determine the effect of groundwater leaching on the fuel sludge. Results from the corrosion tests showed that the composition of the defense spent nuclear fuel colloids evolve over time from an initially UO2-rich population, to a mixed colloid population containing UO2 and higher oxides of U as well as smectite clays, to a population that appears to be dominated by U-containing smectite clays. After approximately one year of testing on the defense spent nuclear fuel, the total quantity of uranium in the sludge represented approximately 99 weight percent of the original uranium fuel sample, while that in the colloid size group corresponded to 0.002 to 0.006 weight percent of the original uranium fuel sample. The quantity of uranium in the fraction attached to the stainless steel vessel was 0.1 to 0.3 weight percent of the original fuel sample (DTN: MO0306ANLSF001.459). The attached material is measured by washing the stainless steel vessel in HNO3; the attached material includes sorbed solutes, sorbed colloids, and precipitates (DTN: MO0306ANLSF001.459). The disposition of Pu during defense spent nuclear fuel corrosion showed that Pu is associated predominantly with the colloidal, particulate, and sorbed size fractions. The 239Pu/ 238U ratios in the colloid fraction are significantly larger than those in the other fractions (sorbed, particulate, and dissolved) and is the only fraction that showed enrichment of Pu in comparison to that in the fuel prior to corrosion (DTN: MO0306ANLSF001.459). Results from the testing support a model of defense spent nuclear fuel in which Pu is significantly adsorbed to the surface of colloids (such as corrosion products of the waste package or groundwater clays) but does not occur as an embedded radionuclide in waste form colloids (DTN: MO0306ANLSF001.459). The results of the tests conducted for commercial spent nuclear fuel and defense spent nuclear fuel along with the corroborative field information indicate that the impact of colloids from the degradation of these waste forms is not going be significant compared to the potential impact of September 2003 G-8 No. 8: Colloids Revision 2 colloids from defense high-level radioactive waste glass and from pseudocolloids. Therefore, it is reasonable to exclude true colloids from the TSPA modeling. G.4.3 Response to Additional Information Needed Requests In response to the AINs listed in Section G.1.1, clarification and justification of radionuclides for which reversible and irreversible colloidal release is modeled has been provided. In addition, recent tests in commercial spent nuclear fuel and defense spent nuclear fuel indicate that colloids forming from the degradation of those waste forms will not be significant; thus, providing a stronger technical basis that colloid-associated radionuclides release can be neglected for commercial spent nuclear fuel and defense spent nuclear fuel. G.5 REFERENCES G.5.1 Documents Cited BSC (Bechtel SAIC Company) 2003. Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary. (MDL-EBS-PA-000004 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030626.0006. Bunzl, K.; Flessa, H.; Kracke, W.; and Schimmack, W. 1995. “Association of Fallout 239+240Pu and 241Am with Various Soil Components in Successive Layers of a Grassland Soil.” Environmental Science & Technology, 29, (10), 2513-2518. Washington, D.C.: American Chemical Society. TIC: 234160. Coughtrey, P.J.; Jackson, D.; Jones, C.H.; Kane, P.; and Thorne, M.C. 1985. Radionuclide Distribution and Transport in Terrestrial and Aquatic Ecosystems, A Compendium of Data. Volume 6. Rotterdam, The Netherlands: A.A. Balkema. TIC: 240299. CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating Contractor) 2000a. Inventory Abstraction. ANL-WIS-MD-000006 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20001130.0002. CRWMS M&O 2000b. Total System Performance Assessment (TSPA) Model for Site Recommendation. MDL-WIS-PA-000002 REV 00. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20001226.0003. CRWMS M&O 2000c. Total System Performance Assessment for the Site Recommendation. TDR-WIS-PA-000001 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20001220.0045. CRWMS M&O 2000d. Waste Form Colloid-Associated Concentrations Limits: Abstraction and Summary. ANL-WIS-MD-000012 REV 00. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20000525.0397. CRWMS M&O 2001. Waste Form Colloid-Associated Concentrations Limits: Abstraction and Summary. ANL-WIS-MD-000012 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20010130.0002. September 2003 G-9 No. 8: Colloids Revision 2 Litaor, M.I. and Ibrahim, S.A. 1996. “Plutonium Association with Selected Solid Phases in Soils of Rocky Flats, Colorado, Using Sequential Extraction Technique.” Journal of Environmental Quality, 25, (5), 1144-1151. Madison, Wisconsin: American Society of Agronomy. TIC: 252783. Lu, N.; Conca, J.; Parker, G.R.; Leonard, P.A.; Moore, B.; Strietelmeier, B.; and Triay, I.R. 2000. Adsorption of Actinides Onto Colloids as a Function of Time, Temperature, Ionic Strength, and Colloid Concentration, Waste Form Colloids Report for Yucca Mountain Program (Colloid Data Summary from 1999 to 2000 Research). LA-UR-00-5121. Los Alamos, New Mexico: Los Alamos National Laboratory. TIC: 249708. Mertz, C.J.; Finch, R.J.; Fortner, J.A.; Jerden, J.L., Jr.; Yifen, T.; Cunnane, J.C.; and Finn, P.A. 2003. Characterization of Colloids Generated from Commercial Spent Nuclear Fuel Corrosion. Activity Number: PAWTP30A. Argonne, Illinois: Argonne National Laboratory. ACC: MOL.20030422.0337. Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy Technical Exchange and Management Meeting on Evolution of the Near-Field Environment (January 9–12, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), January 26, 2001, with enclosure. ACC: MOL.20010810.0033. Reamer, C.W., 2002. “Radionuclide Transport Key Technical Issue Agreements.” Letter C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), February 6, 2002, with enclosure. ACC: MOL.20020920.0318. Reamer, C.W. and Williams, D.R. 2000. Summary Highlights of NRC/DOE Technical Exchange and Management Meeting on Radionuclide Transport. Meeting held December 5-7, 2000, Berkeley, California. Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20010117.0063. Schlueter, J. 2002. “Evolution of the Near-Field Environment Key Technical Issue Agreements.” Letter from J. Schlueter (NRC) to S. Brocoum (DOE/YMSCO), February 14, 2002, with enclosure. ACC: MOL.20020607.0086. Wronkiewicz, D.J.; Buck, E.C.; and Bates, J.K. 1997. “Grain Boundary Corrosion and Alteration Phase Formation During the Oxidative Dissolution of UO2 Pellets.” Scientific Basis for Nuclear Waste Management XX, Symposium held December 2-6, 1996, Boston, Massachusetts. Gray, W.J. and Triay, I.R., eds. 465, 519-526. Pittsburgh, Pennsylvania: Materials Research Society. TIC: 238884. G.5.2 Data, Listed by Data Tracking Number MO0306ANLSF001.459. Colloids Generated from Irradiated N Reactor Fuel, Data Report. Submittal date: 06/04/2003. September 2003 G-10 No. 8: Colloids Revision 2 APPENDIX H CHANGES IN COLLOID CONCENTRATIONS DUE TO SHIFTS IN pH AND IONIC STRENGTH (RESPONSE TO TSPAI 3.42) September 2003 No. 8: Colloids Revision 2 Note Regarding the Status of Supporting Technical Information This document was prepared using the most current information available at the time of its development. This Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were prepared using preliminary or draft information reflect the status of the Yucca Mountain Project’s scientific and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the AMRs and other references will be reflected in the License Application (LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to reflect changes in the supporting references prior to submittal of the LA. September 2003 No. 8: Colloids Revision 2 APPENDIX H CHANGES IN COLLOID CONCENTRATIONS DUE TO SHIFTS IN pH AND IONIC STRENGTH (RESPONSE TO TSPAI 3.42) This appendix provides a response for Key Technical Issue (KTI) agreement Total System Performance Assessment and Integration (TSPAI) 3.42. This KTI agreement relates to providing a sensitivity analysis on changes in colloid concentration due to changes in pH and ionic strength. H.1 KEY TECHNICAL ISSUE AGREEMENT H.1.1 TSPAI 3.42 Agreement TSPAI 3.42 was reached during the U.S. Nuclear Regulatory Commission (NRC)/U.S. Department of Energy (DOE) technical exchange and management meeting on total system performance assessment and integration held August 6 through 10, 2001, in Las Vegas, Nevada. TSPAI KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). The wording of this agreement is as follows: TSPAI 3.42 DOE should provide a sensitivity analysis on the potentially abrupt changes in colloid concentrations due to shifts in modeled pH and ionic strength across uncertain stability boundaries. This analysis may be combined with plans to address ENFE Agreement 4.06 and RT Agreement 3.07. DOE will complete sensitivity analyses to investigate the effects of varying colloid concentration due to shifts in model predicted pH and ionic strength across uncertain stability boundaries. These analyses will be documented in TSPA for any potential license application expected to be available to NRC in FY 2003. H.1.2 Related Key Technical Issue Agreement ENFE 4.06 and RT 3.07. H.2 RELEVANCE TO REPOSITORY PERFORMANCE The stability of colloidal suspensions impacts the significance of colloid-facilitated radionuclide transport. The pH–ionic strength relationship is one of several phenomena that impact the concentration of colloids in suspension. In addition, temperature and relative humidity also have an impact on the stability of colloid suspensions. The technical basis for the response for this KTI agreement is presented in Sections 4, 5, and 6 of this technical basis document. This KTI agreement is related to the invert, saturated zone, and unsaturated zone as shown in Figure H-1. September 2003 H-1 No. 8: Colloids Revision 2 Figure H-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System Components September 2003 H-2 No. 8: Colloids Revision 2 H.3 RESPONSE The sensitivity analysis required by this KTI agreement was not performed. However, as discussed below, performance of the repository system is not believed to be sensitive to abrupt changes across the pH-ionic strength stability boundaries. Only during the first 300 years following waste package breach can pH-ionic strength values for both smectite and iron-oxyhydroxide colloids fall within the range for stable colloid suspensions (BSC 2003). After 300 years, all possible pH-ionic strength combination of values would be within the range for which the colloid suspensions are unstable (BSC 2003). As shown in Section H.4, even during the first 300 years of the postclosure period only a very small number (less than 6 percent) of pH-ionic strength values will fall within the stable colloid suspension region. In addition, within the drift environment, temperature will be high during the early postclosure period (e.g., the first 1,000 years) as shown in part (a) of Figure 3-2 of this technical basis document. These conditions will contribute to making colloid suspensions unstable. Therefore, even if the pH-ionic strength values were to be favorable to stable colloids, the high temperature will destabilize the colloid suspensions. Based on these considerations, the total repository system performance will not be impacted by abrupt changes across pH-ionic strength stability boundaries. Therefore, a detailed sensitivity analysis as stated in this KTI agreement is not necessary. The information in this report is responsive to agreement TSPAI 3.42 made between the DOE and NRC. The report contains the information that DOE considers necessary for NRC review for closure of this agreement. H.4 BASIS FOR THE RESPONSE Ranges of sampled values of ionic strength and pH for the co-disposal waste package with N-Reactor fuel and no-drip conditions are shown in Table H-1 (BSC 2003). It is shown in the table that for the first 300 years after the waste package breaches there is a small chance (less than 6 percent) of colloids being stable within the waste package.1 Note that the analysis in In- Package Chemistry Abstraction (BSC 2003) assumes no water evaporation during waste package degradation. As discussed in Section 3.1 (Figures 3-2 and 3-3), radiation heat-induced water evaporation will likely concentrate any solution available inside the drift and lead to a high ionic strength environment (greater than 0.05 M, a threshold for colloid instability) environment for a significant portion of the regulatory time period. The evaporation effect will further reduce the chance for the formation of a stable colloid suspension with the drift. In summary, the probability of forming a stable colloid suspension is low (less than 5 percent), and so is the chance for “the potentially abrupt changes in colloid concentration due to modeled pH and ionic strength across uncertain stability boundaries”. This means that changes in pH and ionic 1 Similar data for a CSNF package under no-drip conditions indicate that there is a similarly small chance of sampling a pH-ionic strength combination that would yield a stable colloid suspension persisting over the entire 10,000-year regulatory period, not just the first 300 years. These data were not presented since it has been concluded that CSNF colloid contribution is negligible. September 2003 H-3 No. 8: Colloids strengths across uncertain stability boundaries will have little, if any, impact on colloid concentrations. 0 to 55 55 to 300 N-Reactor Fuel, No-Drip Ionic Strength Range Time Period (yr) (M) 4x10-6 to 0.85 0.01 to 0.85 0.05 to 0.85 300 to 10,000 Table H-1. Ranges of Sampled Values of Ionic Strength and pH for Co-Disposal Packages with pH Range H-4 4.1 to 9.6 4.1 to 9.6 4.1 to 9.6 Probability Stable 0.06 0.05 0.0 Unstable 0.94 0.95 1.00 September 2003 Source: BSC 2003. Within the drift environment, temperature will be high during the early postclosure period (e.g., the first 1,000 years) as shown in part (a) of Figure 3-2 of this technical basis document. The high temperature will make colloid suspensions unstable. Therefore, even if the pH-ionic strength relationship was to be favorable towards stable colloids (first 300 years of the postclosure period), the high temperature will destabilize the colloid suspensions. H.5 REFERENCES BSC (Bechtel SAIC Company) 2003. In-Package Chemistry Abstraction. ANL-EBS-MD- 00037 REV 01D. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030617.0024. Reamer, C.W. 2001. “U.S. Nuclear Regulatory Commission/U.S. Department of Energy Technical Exchange and Management Meeting on Total System Performance Assessment and Integration (August 6 through 10, 2001).” Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), August 23, 2001, with enclosure. ACC: MOL.20011029.0281. No. 8: Colloids Revision 2