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The Analytical Development Section of Savannah River Technology Center (SRTC) was requested by the Facilities Disposition Division (FDD) to determine the holdup of enriched uranium in the 321-M facility as part of an overall deactivation project of the facility. The 321-M facility was used to fabricate enriched uranium fuel assemblies, lithium-aluminum target tubes, neptunium assemblies, and miscellaneous components for the production reactors. The facility also includes the 324-M storage building and the passageway connecting it to 321-M. The results of the holdup assays are essential for determining compliance with the Waste Acceptance Criteria, Material Control & Accountability, and to meet criticality safety controls. Two measurement systems will be used to determine highly enriched uranium (HEU) holdup: One is a portable HPGe detector and EG&G Dart system that contains high voltage power supply and signal processing electronics. A personal computer with Gamma-Vision software was used to control the DART MCA and provide space to store and manipulate multiple 4096-channel g-ray spectra. The other is a 2"x 2" NaI crystal with an MCA that uses a portable computer with a Canberra NaI+ card installed. This card converts the PC to a full function MCA and contains the ancillary electronics, high voltage power supply, and amplifier required for data acquisition. This report covers holdup measurements in two uranium aluminum alloy (U-Al) casting furnaces and the riser crusher. Our results indicated a total amount of 21.6 g and 36.3 g of enriched uranium in the two furnaces and the riser crusher respectively. This report will discuss the methodology, non-destructive assay (NDA) measurements, assumptions, and results of the U holdup in the two furnaces and the riser crusher.

The 321-M facility was used to fabricate enriched uranium fuel assemblies, lithium-aluminum target tubes, neptunium assemblies, and miscellaneous components for the production reactors. The facility also includes the 324-M storage building and the passageway connecting it to 321-M. The facility operated for 25 years; during this time, thousands of uranium-aluminum-alloy (U-Al) fuel tubes were produced. After the facility ceased operations in 1995, all of the easily accessible U-Al was removed from the building and only residual amounts remained. The bulk of this residual is located in the equipment that generated and handled small U-Al particles and the exhaust systems for this equipment (e.g., Chip compactor, casting furnaces, log saw, lathes A & B, cyclone separator, Freonä cart, riser crusher, …etc). ¹

U-235 holdup measurements were performed in 1995 and documented in technical report WSRC-TR-95-0492.² The holdup values reported in WSRC-TR-95-0492 were only best estimates, due to lack of availability of time for conducting the measurements and analysis. Therefore, FDD has requested technical assistance from the Analytical Development Section (ADS) of the Savannah River Technology Center (SRTC) to determine the holdup of enriched uranium in the 321-M facility, as part of an overall deactivation project of the facility.³ This project includes the dismantling and removal of all held-up HEU to the extent practical. ADS was tasked to conduct holdup assays to quantify the amount of HEU on all components removed from the facility prior to placement in B-25 containers. The U-235 holdup in any single component of process equipment must not exceed 50 g in order to meet the B-25 limit.⁴ This limit was imposed to meet criticality requirements of the E-Area Low Level Vaults. Thus the holdup measurements are used as guidance to determine if further decontamination of equipment is needed to ensure that the quantity of U-235 does not exceed the 50 g limit. In summary, the results of the holdup assays are essential for determining compliance with the Waste Acceptance Criteria, Material Control & Accountability, and to ensure that criticality safety controls are not exceeded.

This report covers holdup measurements in two uranium aluminum alloy (U-Al) casting furnaces and the riser crusher. Our results indicated a total amount of 21.6 g and 36.3 g of enriched uranium in the two furnaces and the riser crusher respectively. This report will discuss the methodology, non-destructive assay (NDA) measurements, assumptions, and results of the U holdup in these components.

Since the 1995 assay indicated a total of 144.5 g and 35.5 g of U-235 holdup in the two furnaces and the riser crusher, a more precise value was required to determine their disposition with respect to the 50 g limit. The furnaces were used to cast U-Al alloy ingots. These casting furnaces have the dimensions 24" x 23.5" x 26". Figure 1 is a photograph of one of the two furnaces showing its side and top. The riser crusher was used to break risers cut from the top of U-Al castings so that the broken risers fit in scrap cans.

We used two measurement systems for determining the quantity and approximate location of the HEU holdup in the two furnaces and the riser crusher. One is a portable HPGe detector and EG&G Dart system that contains high voltage power supply and signal processing electronics. A personal computer with Gamma-Vision software was used to control the DART MCA and provide space to store and manipulate multiple 4096-channel g-ray spectra. The other is a 2"x 2" NaI crystal with an MCA that uses a portable computer with a Canberra NaI+ card installed. This card converts the PC to a full function MCA and contains the ancillary electronics, high voltage power supply, and amplifier required for data acquisition.⁵

Photographs of one of the two casting furnaces are shown in Figures 1 and 2. The top view in Figure 2 shows a five inch radius well of depth 18 inches. The following are the measured holdup values obtained for the two furnaces, which we identify as furnace A and furnace C to be consistent with the identification used in the 1995 report. We use two approaches in these calculations and show that the results agree well.

The furnaces were measured with an HpGe detector in the area source configuration at 19 inches and with a NaI detector at ranges of 8 inches and in close contact. We attempted a point source far field configuration measurement on furnace A using the HpGe detector at a range of 48 inches, but could measure no transmission through the furnace. Thus the point source far field measurements must represent a lower limit of content. The far field HpGe measurements are shown in Table 1. The lower limit of content was calculated by

where the first factor is the point source calibration constant in units of g-sec/cm², and the second factor is the square of the acquisition distance in centimeters.⁵

Table 1 lists data acquired from furnace A using the HpGe detector in the far field point source configuration at 48 inches. The last column shows the lower limit of content calculated in the point source configuration and applying no transmission correction factor.

The area source HpGe measurements obtained at 19 inches are shown in Table 2 for each furnace. We did not obtain data from all four sides, but only from two 26 x 24 inch sides. Using the area source counting configuration we determined the U-235 holdup in each face by

where the constant is the area source calibration constant in units of g-sec.⁵ At 19 inches, the detector observes an effective area of 4229 cm² (reference 6), while the 26 x 24 inch faces of the furnace have surface areas of 4026 cm². We assume for both furnaces that the two sides are representative of all four sides. Thus for furnace A we determine a surface area contamination of 0.73± 0.10 g for each side and for furnace C we determine a surface area contamination of 0.52± 0.06 g for each side. We use these contamination rates further below.

If we assume a constant rate of contamination per unit area, we can estimate the contamination on the 24 x 23.5 inch bottom to be equal to {23.5 x 24/26 x 24} times the assumed average for the 26 x 24 sides. Thus the bottom of furnace A is estimated to have surface holdup of 0.66± 0.11 g and the bottom of furnace C is estimated to have surface holdup of 0.47± 0.05 g.

The NaI measurements were obtained in several spots at a distance of eight inches and in several others at close contact. The data obtained are summarized in Table 3. Since we already have assigned measured values for each of the sides from the HPGe measurements, we can use a ratio method to obtain measured values for the measurements taken at 8 inches. That is, we have assigned the assay value of 0.73 g for side 2 of furnace A and of 0.52 g for side 2 of furnace C. We will use the ratio of rates measured in Table 3 to obtain values for the position 5 and top 8 NaI measurements.

By the ratio method, the holdup measured in position 5 of furnace A is calculated by

where 0.73 is the HpGe measurement of side 2 furnace A, 95 cps is the NaI measurement of position 5 at 8 inches, and 70 cps is the side 2 NaI measurement at 8 inches. From this ratio method the measured values for position 5 of both carts and for the tops of both carts are listed in Table 4. With this calculation we assume that the position 5 measurement obtains the U-235 holdup in the entire cylindrical well of each furnace and the top measurement obtains the U-235 holdup in the rest of the top of the furnace.

We then determine the U-235 holdup from the contact measurements. These determinations are done with an interactive fit of the Deming contact curve.^7,8 From the measured counts per second obtained at contact, the U-235 holdup for each contact measurement is listed in Table 5. The summed content for the four measurements at the bottom of the cylinder in furnace A is 0.268± 0.042 g, and the summed content for the four measurements at the bottom of the cylinder in furnace C is 0.267± 0.040 g.

Table 5 lists measured U-235 holdup for each contact measurement made with the NaI detector. The holdup values are taken from a direct interactive fit of the Deming contact curve of reference 7. Each contact measurement represents the activity seen in a circle with area p square inches.

Since each NaI contact measurement represents the activity observed in a surface with area p in², the summed contamination for the four contact measurements at the bottom of the two cylindrical wells is 0.268 g/4pin². With a radius of 5 inches, the bottom of each of the wells has a surface area of 25pin². Therefore we estimate the total surface contamination for each well bottom is 1.68± 0.26 g.

We finally make one more estimate of the surface contamination in the cylindrical wells and on the top surfaces of the two furnaces using another approach. In this approach we use the calculated surface areas of each face and determine the surface contamination or holdup by ratio-ing to other surface holdup measured values. This is the same approach used above to determine the holdup on the bottom 24 x 23.5 inch surface of each furnace.

Recalling the U-235 surface holdup in furnace A for a 26 x 24 inch face is 0.73 g, we observe a content of 0.00117 g/in². We use that rate of contamination to estimate U-235 holdup on the vertical sides of the cylinder. With a radius of 5 inches and a height of 18 inches, the total surface area for the inner vertical sides is 180p in². Thus we estimate a total surface contamination of 0.66 g. The top surface has a total surface area of 24 x 23.5 minus the hole cut out by the cylindrical well, thus a surface area of 24 x 23.5 – 25p in² = 485 in². This yields an estimated contamination of 0.57 g.

For furnace C we perform the same calculations with the same surface areas. The calculated rate of contamination for furnace C is 0.52 g/26x24 = 0.000833 g/in². For the sides of the cylinder we calculate a surface contamination of 0.47 g. For the 24 x 23.5 inch top minus the cylindrical well we calculate a surface contamination of 0.40 g.

We finally have two sums for each furnace to estimate the total surface area holdup. For furnace A the first sum comes from the area source calculations of Table 2, plus the calculation for surface contamination for the 24 x 23.5 bottom, plus the ratio method calculation used in equation (3) and Table 4 to determine the content of the cylindrical well and top of the furnace. By this sum we obtain a total content of

For furnace A the second sum comes from the area source calculation of Table 2, plus the calculation for surface contamination for the 24 x 23.5 bottom, plus the ratio of areas method of calculations of Table 5 to determine the surface contamination of the cylindrical well, plus the separate ratio of areas method to determine the surface contamination of the top. By this sum we obtain a total content of

By either method of summing the U-235 contents of each furnace we obtain a value near 6 g of holdup. For both furnaces either sum includes the HpGe area source measurements for the four vertical sides and the ratio of areas measurement for the 24 x 23.5 inch bottom. To determine which of the two sums is a more reliable measure of U-235 content requires selecting between the middle two or middle three terms of the two methods of summing.

We believe the four NaI contact measurements made at the bottom of the cylindrical well are among our most reliable measurements, and so we believe the calculation of 1.68 g at the bottom of the two wells is on very firm ground. In contrast, it is clear that the NaI measurements made at 8 inches in position 5 and in the position we label top 8 in Table 3 overlap in their fields of view. In addition, the position 5 measurement can not be considered to view the whole of the vertical sides of the cylindrical wells. For these two reasons we have selected the second set of sums as the more reliable measure of U-235 content.

For the sake of conservatism, we have applied a 100% uncertainty to our second set of measurements. Therefore the reported values are 11.6 g for furnace A and 10 g for furnace C. These results were reported in reference 9.

Figure 3 is a photograph of the riser crusher on its side demonstrating the complexity of the holdup measurement required. In order to meet the AOP milestone SC-FDD00-011, the holdup assay of this item was conducted with very limited time. We performed two transmission-corrected assays using the HPGe detector system as shown in Figure 4. We do not include a complete discussion of these measurements, but note they are very similar to the far field transmission corrected holdup measurements performed on the Freonä cart of reference 10. For the riser crusher we obtained experimental transmission correction factors of 1.18± 0.14, which we believe were negatively biased for the reasons we describe below. From the assay of one side of the riser crusher we obtained a measured value of 16 g, and from the assay of the other side we obtained a measured value of 21 g, for a precision of 20%.

We report a holdup value with uncertainty of 96% for the riser crusher. Below is a list of the significant contributors to the overall uncertainty of our measurement:

• The transmission correction factors had a precision of 10%.
• The two far field measured values had a precision of 20%.
• The source to detector distance was fixed by a best guess of the center of activity. We assigned an arbitrary uncertainty of 20% to this component.
• The riser crusher is extensive, non-uniform, and non-symmetrical. Detection efficiency and transmission is uncertain on all three of these grounds. We assigned an arbitrary uncertainty of 25% to this component.
• The background uncertainty was large because there was a hot B-25 in the near neighborhood of the detector. Over the duration of the g-ray acquisitions for these assays the B-25 had variable content of contaminated exhaust duct from the facility. We assigned an arbitrary uncertainty of 25% to this component.
• The experimental transmission correction is under-calculated because the transmission source was able to shine around some of the most massive portions of the crusher to have a straight through view of the detector. We assigned an arbitrary uncertainty of 50% to this component.

To calculate the overall uncertainty, we summed the first five bullet items geometrically. The last bullet item was then added directly to the geometric sum. This yielded a total uncertainty of 96%.

From the assay of side one of the riser crusher we obtained a measured value of 16 g, and from the assay of side two we obtained a measured value of 21 g. We reported a holdup value of 18.5± 17.8 g.⁹

Using a combination of far field transmission corrected measurements obtained with a portable high purity germanium detection system and close field and close contact measurements obtained with a sodium iodide detection system, we have provided holdup measurements of U-235 content in two casting furnaces and in the riser crusher used in Building 321-M. These holdup measurements were important contributions to assist FDD to accomplish the AOP milestone SC-FDD00-011. The measured values reported are 21.6 g U-235 in the two furnaces and 36.3 g in the riser crusher.

All of the measured values obtained were important to reduce the uncertainty in the U-235 holdup in the three items to below levels of nuclear safety concern. Previous holdup assays performed in 1995 had uncertainties of 400%, and therefore included U-235 contents of up to 140 g. Including the quoted uncertainty of the 1995 measurements nuclear safety procedures prevented FDD from discarding these items whole into B-25 solid waste. These assays reported here, including uncertainty, lift this limitation.

The authors would like to thank Frank S. Moore for the many discussions and for providing technical review. The authors would like to extend their sincere thanks to Henry Burruss and Ernie All for scheduling and providing the operations support that made it possible for us to conduct this work.

1. D. L. Honkonen, "Nuclear Criticality Safety Evaluation (NCSE): Enriched Uranium Removal From Building 321-M", N-NCS-G-00051, February 2000.
2. R. S. Thomason, "U-235 Holdup in Building 321-M Contamination Areas and Associated Exhaust Systems", WSRC-TR-95-0492, December 1995.
3. Deactivation Project Plan 321-M Fuel Fabrication Facility, V-PMP-M-00004, January 2000.
4. WSRC 1S Manual, Procedure 3.17, Section A.3.a, "E-Area Vaults Nuclear Safety", July 2000.
5. S. R Salaymeh and R. A. Dewberry, "Efficiency Calibration of the Portable High Purity Germanium Detection System for use in the M-Area Deactivation Assays", WSRC-TR-2000-00317, September 2000.
6. R. A. Dewberry, Laboratory Notebook WSRC-NB-2000-00086, pg 27.
7. R. A. Dewberry and S. R Salaymeh, "Efficiency Calibration Using HEU Standards of 2" x 2" NaI Detector", WSRC-TR-2000-00269, July 2000.
8. Deming Least Squares Curve Fitting for Windows, Version 1.13, Safeguards Science and Technology Group (NIS-5), Los Alamos National Laboratory.
9. S. R. Salaymeh to Henry Burruss, "Results of Assay to Meet Milestone", SRT-ADS-2000-00423, August 2000.
10. S. R Salaymeh, R. A. Dewberry, and V. R. Casella, "HEU Holdup Measurements in 321-M Freonä Cart", WSRC-TR-2000-00360 September 2000.