R. A. Dewberry, S. R. Salaymeh, and F. S. Moore
Westinghouse Savannah River Company
Aiken, SC 29808

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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 were 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 of uranium residue in the cooling hut HEPA bank of the 321-M facility. Our results indicated that the HEPA bank contains 17± 12 g enriched uranium. This report discusses the methodology, non-destructive assay (NDA) measurements, assumptions, and results of the Uranium holdup in this item.

Keywords: NDA, Multichannel, Far field, Assay, Holdup

The 321-M facility was used to fabricate enriched uranium fuel assemblies, lithium-aluminum target tubes, neptunium assemblies, and other 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 residue 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

²³⁵U 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 best estimates only, due to lack 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 highly enriched uranium (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 ²³⁵U 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 ²³⁵U 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 of uranium residue in the 321- M cooling hut high efficiency particulate air (HEPA) filter bank. Because holdup values are extremely difficult to determine, conservative assumptions are usually made to report the ²³⁵U gram values. Relative uncertainties for this kind of measurements are generally quoted as +100% and –50%.² Our results indicated that the HEPA bank contains 17± 12 g enriched uranium.

A 2" x 2" NaI detector system and a portable high purity germanium (HPGe) detection system were used to conduct nondestructive assay (NDA) measurements of HEU holdup on the cooling hut HEPA filter housing. The NaI detector system uses a 2" x 2" 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 For the NaI acquisitions and analyses we used Canberra Genie-2000 software. The HPGe detection system uses an EG&G Dart package that contains a high voltage power supply and signal processing electronics. A personal computer with Gamma-Vision acquisition software was used to provide space to store and manipulate multiple 4096-channel g-ray spectra. This system is described in reference 9 and has been used extensively in HEU holdup measurements for FDD.

Photographs of the cooling hut housing are shown in Figures 1 - 4. To accomplish the assays of it, we obtained 37 NaI close-coupled acquisitions from a distance of eight inches from the face of the housing. These acquisitions were obtained from all four sides of the HEPA filter housing. Each NaI acquisition was assumed to be an area source configuration, where the detector was viewing a planar source of uniform activity. From a range of eight inches with the detector recessed one inch inside of the steel-clad lead shield, the detector has a field of view equal to approximately 100 in².^6,7 The data obtained in the 37 NaI acquisitions is summarized in Table 1, and the acquisition points are designated in Figures 1 - 4. A typical NaI quality control spectrum obtained with a 4.41-g HEU standard is shown in Figure 5, and a typical component acquisition (location number 18) is shown in Figure 6. The housing unit is approximately a rectangle with dimensions of 81.75" x 110" x 51". The third column in Table 1 lists the surface area of each acquisition component. Each component is an area of approximately uniform contamination with an area much larger than the 100 in² field of view of the detector. Thus each acquisition qualifies as an area source configuration. With some discussion below, we sum the results of these component assays for one of the determinations of HEU holdup in this item.

The HPGe acquisitions are summarized in Table 2. We obtained thirteen HPGe acquisitions. Five acquisitions were in the far-field configuration from a range of 48 inches or more. One of the acquisitions was a Cs-137 source check acquisition,⁸ one was a ²³⁵U source check acquisition, and one (731BKG) was a background. The ²³⁵U QC spectrum is shown in Figure 7, and a component spectrum is shown in Figure 8. The acquisitions obtained from 48" were treated as area source configurations in which the detector field of view is 4166+570 in², as documented in reference 9. Each of the four vertical faces that we assayed in the area source configuration with the HPGe detector has a surface area of at least 5000 in², and thus each qualifies as an area source. With some discussion below, we sum the results of these four vertical face assays for a second determination of HEU holdup in this item.

Two of the twelve HPGe acquisitions of Table 2 were obtained in the point source configuration from a range of 266 inches, and one (T₀821) was a transmission source measurement obtained from a distance of 423 inches. These point source acquisitions were obtained several weeks after the area source acquisitions. Spectrum 821BKG is the background spectrum associated with the point source spectra. These three spectra taken together provided a point source transmission-corrected measurement of the filter bank as a unit. We use this point source measurement as a third determination of HEU holdup in this item.

Figure 8. HPGe Spectrum "farfield" of Table 2.

The holdup or surface contamination of each individual component observed in the 37 NaI measurements was determined from equation (1) and is listed in Table 1.

[HEU] = (1.20x10^-6)(cpm)(2.08)(A), (1)

where the first factor is the area source calibration constant of reference 6 in units of g-min/in², cpm is the measured detection rate in the 185-keV peak, 2.08 is the transmission correction factor for 3/8" of stainless steel, and A is the total area of each component. The transmission correction factor for a 185-keV g-ray was taken directly from reference 10. All 37 measurements taken together should represent a sum of the total surface area of the cooling hut. We present this sum (31.9 g) at the bottom of column five in Table 1.

The holdup or surface contamination for each of the four vertical faces of the cooling hut filter bank is determined using equation (2) from the HPGe acquisitions in Table 2.

[HEU] = (1.29x10^-5)(cps)(3.39)(A), (2)

where the first factor is the area source calibration constant of reference 9 in units of g-sec/cm², cps is the measured detection rate in the 185-keV peak, 3.39 is the measured transmission correction factor for the filter bank unit, and A is the total area of each of the vertical face components. In this case the transmission correction factor was determined from equation (4) using the measured values of Table 2. We describe that calculation below.

The HPGe detector system was QC checked using the ²³⁵U standard source wt2025a, which has an HEU content of 4.41 g. Acquisition T₀ in Table 2 was a point source acquisition obtained at a source to detector distance of 110 inches, as documented on page 60 of reference 7. The measured content for the source check yielded

[HEU] = (2.36x10^-5)(cps T₀)(110x2.54)² = 4.80 g, (3)

which is in satisfactory agreement with the known value.⁸

To determine the transmission of the 185 keV g-ray through the cooling hut, we obtained the acquisitions labeled farfield, Tbox, and T₀ in Table 2. Transmission is measured directly by

T = { cps(Tbox) - cps(farfield)}/cps(T₀) (4)

The correction factor is then taken to be (1/T)^1/2 = 3.393.

The sum for the four vertical faces measured by HPGe should ideally be equal to the sum of the 37 NaI measurements. We present the HPGe sum near the bottom of Table 2. We consider the two values (31.9 g and 21.7 g) to be in good agreement. Clearly, however, since there is measurable transmission through the cooling hut, the assumption that each acquisition measures only the face or component it is viewing introduces a positive bias in the area source assays. We believe this is a significant positive bias that might approximately double the assay results.

In order to reinforce the area source acquisitions, we obtained four point source acquisitions using the HPGe detection system. These data are listed separately in Table 2. The very-far-field point source acquisition 821FF266 was obtained at a distance of 292.5 inches and yielded a detection rate of 0.275 cps in the 185 keV peak. We convert this point source rate to a ²³⁵U content of 7.42+4.80 g using equation (5) with a transmission correction value of 2.07+1.33.

[HEU] = (2.36x10^-5)(cps sample)(292.5x2.54)²(2.07) = 7.42 g. (5)

The transmission correction value of 2.07 was not obtained by a direct measure of source transmission in this case. We acquired the necessary spectra to make that measurement, but since the source was at the large distance of 423 inches, our detection rates were very low, and the statistical uncertainty in the transmission measurement was very high. The transmission correction factor calculated from these data was unrealistically low and included 1.00 within its uncertainty. The value of 2.07+1.33 used above is an average with one sigma standard deviation of the two direct transmission values we measured and of the 2.08 value taken from reference 10. The uncertainty in the transmission correction factor is the dominant contributor to the total uncertainty in this point source measurement.

We have already discussed the positive bias in the area source acquisitions introduced by the transmission of back-side events. Even though these events from the opposite sides travel a much greater distance and are detected with a much smaller efficiency, it is important to recognize that the detector is able to view the opposite with a much larger field of view than that of the component it is facing. Therefore we believe the positive bias could be as much as 100%. It is very difficult to judge the bias or to design an experiment to determine it. One option would be to install a shield in the middle of the cooling hut to prevent cross talk between surface components. While installing such a shield is clearly impractical in this instance, it would be a very valuable experiment to perform for future measurements on 321-M HEPA filter housings.

The single point source measurement made on the cooling hut likely proves to be the most valuable measurement we made. It does not contain a systematic bias that we have recognized, however it is limited by poor statistics due to the very large acquisition distance. In determining the adopted value for overall content in the cooling hut filter housing, we used all three measurements. It is difficult to tell which provides the best value, but since the point source measurement contains no recognizable bias, we have given it twice the weight of the area source measurements. The reported content is {31.9 + 21.7 + 2(7.42)}/4 = 17+12 g.¹¹ This is in good agreement with the measured value of 19 g reported in 1995.²

We have performed three distinct g-PHA measurements of the ²³⁵U holdup content in the 321-M cooling hut HEPA filter housing. One measurement was a summed set of 37 NaI area source acquisitions. The second measurement was a summed set of 4 HPGe area source acquisitions. The third measurement was from a point source acquisition. All three measurements used transmission correction in the calculation. The reported content of ²³⁵U holdup is 17± 12 g.

In our discussion of results we recognize the likelihood of a positive bias introduced into the area source acquisitions by the measurable transmission of g-ray events through the sample. Shielding this transmission to remove this bias was not practical in these measurements. However we suggest an experiment to be performed on subsequent holdup measurements of the 321-M lathe exhaust HEPA filter housings to gain a measure of the effect. We believe the results will yield a very valuable contribution to subsequent nondestructive assay measurements in the DOE community.

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, "²³⁵U 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. R. A. Dewberry and S. R Salaymeh, "Efficiency Calibration Using HEU Standards of 2" x 2" NaI Detector ," WSRC-TR-2000-00269, July 2000.
6. Raymond A. Dewberry, Saleem R. Salaymeh, and Frank S. Moore, "K_a For a NaI Detector Using Two Methods", WSRC-TR-2001-00490, October 2001.
7. R. A. Dewberry, Laboratory Notebook, WSRC-NB-2000-00086, pages 52, 61.
8. R. A. Dewberry to Danny Smith, "Method Development Report for use of the Portable HPGe Detection System for Assay of Highly Enriched Uranium," SRT-ADS-2001-00388, September 2001.
9. 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.
10. D. R. Rogers, ed., Handbook of Nuclear Safeguards Measurement Methods, National Technical Information Service, (Springfield, VA) 1983.
11. R. A. Dewberry to Henry Burruss, SRT-ADS-2001-0387, September 2001.