WSRC-MS-98-00745

A Batch Process for Vitrification of Americium/Curium Solutions

Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161.

Abstract

Isotopes of americium (Am) and curium (Cm) were produced in the past at the Savannah River Site (SRS) for research, medical, and radiological applications. These highly radioactive and valuable isotopes have been stored in an SRS reprocessing facility for a number of years. Vitrification of this solution will allow the material to be more safely stored until it is transported to the DOE Oak Ridge National Laboratory for use in research and medical applications. The Savannah River Technology Center (SRTC) research and development (R&D) program supporting the stabilization of the Am-Cm solution began in 1995. A full-scale Pilot Facility has been in operation since late-1995. Originally, the vitrification process was to be accomplished using a Pt-Rh bushing melter similar to that used in the glass industry. Several equipment application challenges were addressed and significant progress was made on the original feed system, melter, drain tube, and off-gas design. However, in October 1997, the bushing melter failed and a batch flowsheet concept was proposed. Testing on the batch flowsheet has been ongoing since November 1997 and full-scale Pilot Facilities are currently in operation. This paper describes the batch flowsheet, discusses the equipment development, and presents the path forward for stabilizing the highly radioactive Am-Cm solutions.

Introduction

The Savannah River Technology Center (SRTC) is developing the equipment design bases and process operating parameters to vitrify a nitric acid solution containing isotopes of americium and curium (Am-Cm). The final glass form will be placed in interim storage until it can be transported to the U.S. Department of Energy's Oak Ridge National Laboratory for recovery of the Am-Cm from the glass. Technical problems associated with efforts to directly vitrify the nitric acid feed solution in a slab-type bushing melter led to scoping studies of a batch operation that included a new pretreatment step as well as a new melter design. Operating results and difficulties of the continuous feed Am-Cm slab melter pilot system have been documented by Smith, et al.[1] The alternative of batch vitrification and the simplification of the melter system are described by Marra, et al.[2] While a new induction-heated cylindrical melter was being installed, vitrification experiments with the product slurry from a new pretreatment step were carried out in a resistance-heated platinum melter to validate the technical feasibility of the new flowsheet.[3] These experiments demonstrated the initial feasibility of the process and provided the baseline for operations in the Cylindrical Induction Melter (CIM). This paper describes the batch vitrification process and discusses the results of Pilot Facility testing and a series of Integrated Demonstration Runs.

Oxalic Pretreatment Process Description

An oxalic acid precipitation of the nitric acid feed stream will be the only "in-cell" pretreatment step before the feed material is introduced to the melter vessel. The precipitation pretreatment flowsheet includes a precipitation with the addition of 8 weight percent oxalic acid. The precipitated oxalates (primarily rare earth oxalates) are settled and the free liquid decanted. The precipitated oxalates are washed with 0.1 molar oxalic acid, and again allowed to settle before the wash solution is decanted. The washed oxalate precipitate is then combined in the melter vessel with a glass-forming composition of frit, cullet, glass beads, or raw batch chemicals.

Operations at the Am-Cm Pilot Facility have been performed using two different surrogate feed compositions. The first surrogate feed was based on 1993 sample results of the F-canyon Am-Cm solution and used erbium as a substitute for both americium and curium. Recently, results of a 1998 sample have been obtained and the surrogate feed revised to incorporate the new sample results. Both surrogate compositions have been tested with no significant processing differences. The baseline glass forming composition (a strontium-aluminum borosilicate composition known as 25SrABS) and the two surrogate feed compositions are shown in Table 1.

Batch Vitrification Process Description

The batch vitrification process includes the three distinct steps of drying, calcination and vitrification prior to draining the glass into a storage canister. The drying step removes the free aqueous fraction required to transfer the precipitated feed slurry to the melter and the chemically bound waters of hydration. The drying step is expected to be complete by the time the bed temperature reaches 200°C. The amount of time required to dry a single batch varies between 1 - 3 hours depending upon the total batch size and the volume of flush water used. Water is the main constituent in the off-gas during the drying step. Nitrous oxides and a small quantity of carbon monoxide (CO) and carbon dioxide (CO₂) also are evolved.[3]

Calcination decomposes the precipitated solids to carbonates and oxides and is expected to occur as the bed temperature is raised from 200°C to ~700°C. The amount of time required to calcine a single batch is a function of the heating rate and the total batch size. Typical heating rates are in the range of 4-10°C/min. At a nominal rate of ~8°C/min, calcination requires approximately one hour to complete. During the calcination step, CO and CO₂ are evolved from oxalate decomposition.[3]

Vitrification occurs as the oxides resulting from the calcination step begin to react with the 25SrABS glass-forming composition. The softening point of the 25SrABS composition (which results in a high viscosity initial liquid phase) is around 1100°C. Additionally, it is known that cerium oxide (CeO₂) will reduce at high temperatures in lanthanide borosilicate glass.[4] However, the cerium is only partially reduced (Ce⁺⁴ Ce⁺³) at about 1150°C and is completely reduced at 1300°C.[4] The overlap of the 25SrABS softening point (high viscosity initial liquid phase) with the liberation of oxygen due to cerium reduction results in a frothy bubble accumulation at the glass surface. The time to complete the vitrification step may include a hold period to fine the bubbles before the vitrified glass is drained from the melter into a stainless steel canister. Bubbles in the final product would not affect the product performance criteria of recoverability.

Experimental Description

Drain Tube Test Stand Testing

Early testing of the integrated batch process (i.e. drying, calcination, and vitrification steps) was conducted in a test facility known as the Drain Tube Test Stand (DTTS).[3] The 11-inch high, open-top cylindrical DTTS melter is constructed of 90% Pt and 10% Rh alloy with an inside diameter of 2.5". The melter is heated by passing an electric current through the Pt-Rh shell.

In early 1998, Phase I of batch flowsheet development studies was completed in the DTTS melter vessel. Laboratory-scale crucible tests were completed in parallel with these larger scale DTTS tests. Phase I tests conducted in the DTTS melter were designed to identify the technical issues associated with the sequential drying, calcination, and vitrification of an oxalate slurry feed in a cylindrical melter. The identification of a processing issue, that of volume expansion/high temperature bubble formation, drove additional testing to identify process parameter variations to eliminate/mitigate the expansions. The increase in volume occurred as the batch transitioned from the calcination step (oxides and unmelted cullet) to the vitrification step (a molten glass). Batch volume increases resulted from two semi-related mechanisms, a sintered frit cap that trapped expanding air and evolving gases, and a frothy bubble accumulation due to the thermally induced reduction of cerium (oxygen evolution). Testing was also designed to evaluate the effects of heating rates, hold times and glass processing temperature on volume expansions, pour characteristics, and product glass quality.

The first mechanism for volume expansion associated with sequential drying, calcination and vitrification in the batch flowsheet focused on a frit size phenomena. The mechanism isolated was a sintered frit cap that evolved from unmelted 25SrABS frit particles and expanding gases below the cap surface. Initial testing of the batch flowsheet added a pre-dried oxalate feed to 25SrABS glass composition frit sized to -80+200 mesh. At approximately 1120°C, the volume of the batch in the melter increased greater than 5X. The domed, sintered surface was fairly thick, approximately one inch at its thickest point. The sintered material was porous and light, and cooled quickly near the open top of the melter vessel, effectively sealing evolving gases. Subsequent analysis of a cold cap sample showed that the material was primarily unreacted frit, with just a small amount of undissolved oxides from the surrogate feed.

Prior testing in the DTTS melter, during pour characterization studies, revealed a pattern to the sintered surface volume expansion noted in initial batch vitrification tests. Approximately one quarter of the previous melter runs that used small frit sizes (-14+30 and -80+200 mesh sizes) resulted in the sintered bed expansion. However, the sintered bed expansion had not been observed with the use of cullet. A batch was prepared with a 25SrABS-cullet composition added in place of the frit. No sintered frit surface increase was noted in any runs where cullet was used. However, solving this problem revealed the second processing issue. Repeatedly, between 1140°C and 1200°C, the bed would begin to pulse as large bubbles were released from beneath a bed surface of surrogate feed oxides and high viscosity glass (>500 Poise at this temperature), identifying the second mechanism of volume expansion.

The elimination of the sintered bed surface expansion revealed that the volume increase also was due to the evolution of gas at high temperature. This was evidenced by the presence of a pulsing oxide/cullet bed at ~1140°C, before the batch became a low viscosity molten glass pool. A review of the literature revealed that cerium is completely reduced from a +4 valence state to a +3 at temperatures in the 1120°C to 1300°C range.[4] Tests without cerium in the surrogate feed, combined with laboratory tests, confirmed that the high temperature gas generation was oxygen resulting from cerium reduction. Details of the reactions occurring during vitrification are discussed in a separate paper.[4]

Since cerium is a component of the incoming feed, oxygen liberation had to be addressed. It was theorized that a key to mitigating the volume expansion is to control the vertical location of glass softening and cerium reduction (minimizing the overlap of cerium reduction and the formation of a high viscosity liquid phase). If precipitate were loaded on top of a preloaded bed of glass cullet, heating from the top down, that is providing a vertical temperature profile with a hot top and cooler bottom, would appear to provide a lower viscosity glass near the top allowing for oxygen to escape. The DTTS vessel provided such a temperature profile with a hot spot approximately one inch above the calcined bed surface and a cool bottom. Temperatures of the platinum bottom are approximately 200°C to 240°C cooler than the platinum wall during the heat up. When the batch was intentionally segregated with the cullet on the bottom and the oxalate slurry batched on the top of the cullet, the temperature profile in the DTTS melter allowed for the sequential redox of cerium while fluxing and melting the glass from the top down. This resulted in minimal bubbles and significantly reduced the severity of the bubble accumulation to less than twice the final glass volume.

The successful demonstration of sequential drying, calcining and vitrifying an oxalate slurry in the DTTS melter vessel provided the basis for testing on a larger scale in a Cylindrical Induction Melter (CIM). Sintered frit cap expansions were eliminated with the use of cullet and volume expansions due to high temperature bubble formation (oxygen liberation from cerium reduction) were mitigated in the DTTS melter vessel through a vessel temperature profile that effectively separated the softening point of the glass cullet and the evolving oxygen from cerium reduction. A processing temperature of 1450°C and a 1-2 hour hold time to `fine' the glass reduced bubbles in the poured glass to an acceptable level. The success of the preliminary process demonstrations provided a workable process that was directly applicable to the CIM system, making the batch flowsheet the preferred option for vitrification of the Am-Cm feed stream.

Cylindrical Induction Melter Testing

The Cylindrical Induction Melter system installed at SRTC is a Pilot Facility demonstration version of a more robust and remotely operable system to be installed in the F-Canyon Separations Facility at SRS. It consists of an inductively heated Pt-Rh containment vessel, an induction heating system, a control system, and a simple off-gas filtering system. The difference between the CIM and previously tested Am-Cm systems was discussed in a previous paper.[2]

The oxalate precipitate produced from the Am/Cm surrogate solution, along with glass forming frit, is fed to the melter in a prescribed ratio. Volatilization products and other offgases are swept into a semi-circular slotted hood positioned above the top of the CIM vessel, then drawn through a moisture separator and high efficiency particulate air (HEPA) filter before being exhausted to the stack. The glass flows from the bottom of the Pt-Rh vessel by gravity through an inductively heated Pt-Rh drain tube into a steel canister. A separate water chiller provides cooling water to the induction coils and heat stations to prevent overheating. A schematic diagram of the CIM Pilot Facility system is provided in Figure 1.

Phase I testing of the 3" CIM began in April 1998 and comprised the initial heat-up of the melter and drain tube induction heaters. This phase of testing was completed in April 1998 and confirmed that the induction heating system was much more amenable to the remote canyon-processing environment than the previous bushing melter system. Further, the Phase I testing of the pouring operation demonstrated that the induction heated drain tube resulted in a pouring operation much improved over that observed with a resistance heated drain tube.

Figure 1 - Schematic diagram of Cylindrical Induction Melter Pilot Facility

Phase II testing involved development of the batch vitrification process with the CIM. Feed to the melter consisted of rare-earth oxalates precipitated from surrogate feed and 25SrABS cullet or frit. Initial runs used dried oxalate feed to the melter vessel for convenience. Subsequent runs used wet oxalate precipitate to enable the drying, calcination, and vitrification processes to be carried out in a single vessel. The objective of this phase of testing was to generate information on required drying time, heating rate during calcination and vitrification, and hold time at the melting temperature for homogenization and fining. The performance of the off-gas system components also was assessed.

Phase II testing on the 3" CIM began in May 1998. The early Phase II testing was performed using batch sizes similar to that used in the preliminary batch flowsheet work that was performed in the DTTS. Drying and calcination behavior in the CIM was similar to that observed in the DTTS. However, a larger amount of high-temperature bed expansion (characterized as a loose, foamy, frothy material) was observed in the CIM than had previously been observed in the DTTS. In the CIM, this foamy, frothy material often rose over 3" above the glass melt versus a height of only 0.5-1" in the DTTS. In no case, did the foam rise out of the CIM vessel. Several tests, including detailed temperature profiling, were performed to better understand the reason(s) for the increased bed expansion observed in the CIM. It was observed that the bottom of the CIM vessel was significantly hotter than the sidewalls and top. The hot bottom was due to the fact that the original CIM coil configuration had two turns from the vessel coil located on the conical taper of the vessel which resulted in a significant power input to the bottom of the vessel. This was significantly different than the desired thermal profile identified during DTTS testing as discussed above. The hot bottom resulted in O₂ being generated at the bottom of the bed (from reduction of CeO₂) which was subsequently trapped in the cooler, more viscous layer at the top of the bed resulting in bed expansion. While the amount of foaming observed in the 3" CIM met general project acceptance criteria, no volume increase outside the confines of the vessel, it was determined that altering the temperature profile to more closely resemble that of the DTTS would improve processing.

In order the better understand the observed temperature profile, and to facilitate potential redesign of the coils, a thermal model of the CIM was developed. The model utilized the MSC/THERMAL general-purpose finite-element heat transfer software, which employs the wall power distribution as an input variable. In the model, the melter was approximated as an axisymmetric body and is at the phase of operation (power, material loading, etc.) at which foam/froth formation has been observed. The power distribution in the model was developed such that the predicted temperature profile approximates that obtained experimentally. Using the parameters generated, the power distribution was varied to produce an acceptable temperature distribution in the bed. This data, coupled with consultation with the induction heating system vendor, led to a redesign of the coils. The coils on the 3" CIM were modified in August 1998 to address the undesirable temperature profile. This testing showed an improvement in processing but demonstrated that a third heating zone, dedicated to the conical section of the vessel, would provide additional process control.

During the summer of 1998, significant progress was also made on development and demonstration of an oxalate precipitation and transfer process suitable for remote operation. The precipitate cake is transferred to the melter as a slurry. Development work was performed to determine the best means for draining the precipitate vessel. This testing included an investigation of vessel geometry, agitator design, wash decant amount, and flush amounts. The overall purpose of the oxalate precipitate transfer testing was to determine the geometry and agitation requirements during oxalate transfer for the coupled precipitator to be installed in the CIM Pilot Facility. A specific objective of the testing was to develop a system where the transfer could be accomplished with a minimal amount of free liquid to avoid overfilling the melter.

The testing was accomplished using four glass oxalate transfer funnels (OTF) with varying bottom slope and outlet configurations with a digitally controlled agitator that displayed speed and torque. The glass funnels allowed the draining operation to be monitored and videotaped. The testing determined that the oxalate transfer can be performed within the volume constraints of the melter vessel and that a minimal amount of water is sufficient to rinse the vessel. Based on the analysis of data from this testing, parameters for design of the Pilot Facility system were identified. Equipment to demonstrate the precipitation and draining operation was designed, fabricated, and successfully demonstrated in an off-line Pilot Facility.

In September 1998, initial proof-of-concept testing was completed in the 3" CIM (CIM3) and the off-line oxalate precipitation transfer facility. Based on this milestone, the Department of Energy approved the restart of Project Conceptual Design activities in October 1998. The initiation of Conceptual Design activities is paralleling a vitrification process demonstration in a 5" CIM (CIM5), which is intended to be an integrated, production-scale process demonstration.

Installation of the CIM5 facility was completed in October 1998. This installation included relocation of the pilot feed system to the CIM facility in order to provide an integrated demonstration of the entire batch vitrification process. The CIM5 is similar to the CIM3, except that is has a higher capacity (5" vs. 3" diameter) and contained a separate heating zone for the conical section of the melter vessel. This heating zone allowed the thermal profile in the melter vessel to be `tailored' in a manner to mitigate the volume expansion observed in the 3" CIM. Testing in the CIM5 was initiated in October 1998 and quickly demonstrated the benefits of the additional heating zone. A series of four Integrated Demonstration Runs were completed in the CIM5 facility in December 1998. These tests met or exceeded all performance requirements and demonstrated successful operation of a system suitable for remote operations.

Conclusions

A batch vitrification process for stabilization of the SRS Am-Cm solution using an induction-heated melter system has been successfully demonstrated in a full-scale Pilot Facility. This demonstration included development of the process and equipment and complete characterization of the precipitation and vitrification processes. The development program was completed in less than 12 months and will provide a system suitable for remote operation. Based on this successful demonstration the DOE has authorized Preliminary Design. Design, construction, and qualification activities will continue over the next two years with radioactive operations currently scheduled to begin in 2002.

Acknowledgments

This paper was prepared in connection with work done under Contract No. DE-AC09-96SR18500 with the Department of Energy.

References