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Since the mid-1980's the Pacific Northwest National Laboratory (PNNL) has used a value of 0.85 as a correction factor for the self absorption of activity for particulate radioactive air samples collected from building exhaust for environmental monitoring. More recently, an effort was made to evaluate the current particulate radioactive air sample filters (Versapor 3000, 47-mm diameter) used at PNNL for self absorption effects. There were two methods used to characterize the samples. Sixty samples were selected from the archive for acid digestion to compare the radioactivity measured by direct gas-flow proportional counting of filters to the results obtained after acid digestion of the filter and counting again by gas-flow proportional detection. Thirty different sample filters were selected for visible light microscopy to evaluate filter loading and particulate characteristics. Mass-loading effects were also considered. Large error is associated with the sample filter analysis comparison and subsequently with the estimation of the absorption factor resulting in an inadequate method to estimate losses from self-absorption in the sample filter. The mass loading on the sample filter as determined after digestion and drying was approximately 0.08 mg cm; however, this value may not represent the total filter mass loading given that there may be undetermined losses associated with the digestion process. While it is difficult to determine how much material is imbedded in the filter, observations from the microscopy analysis indicate that the vast majority of the particles remain on the top of the filter. In comparing the results obtained, the continued use of 0.85 as a conservative correction factor is recommended.

This report describes the caustic leaching test conducted on Hanford Tank T-110 sludge during FY 2002 at the Pacific Northwest National Laboratory. The data presented here can be used to develop the baseline and alternative flowsheets for pretreating Hanford tank sludge. The U.S. Department of Energy funded the work through the Efficient Separations and Processing Crosscutting Program (ESP; EM-50). The T-110 sludge sample was first subjected to washing with dilute sodium hydroxide solution at ambient temperature. Following the dilute hydroxide washing, several aliquots of the washed solids were taken for leaching tests. The washed solids were subjected to leaching with 1, 3, or 5 M NaOH at 60, 80, or 100°C for up to 168 h. The leachates were sampled at 4, 8, 24, 72, and 168 h. The leached solids were dried to constant mass at 105°C and then analyzed. Bismuth, Fe, Na, P, and Si are the dominant elements present in the T-110 sludge. As expected, Na is largely (> 90%) removed by dilute hydroxide washing. However, dilute hydroxide washing is ineffectual at removing Bi, Fe, or Si. For this particular sludge, the behavior of P is of major concern due to the relatively low tolerance for this element in the high-level waste (HLW) immobilization process and the high concentration of P in the waste. Only 33% of the P was removed by dilute hydroxide washing, resulting in washed solids that were 8.8 wt% P. This is presumably because the P is present as bismuth phosphate in the T-110 solids. More rigorous pretreatment (e.g., caustic leaching) will be required to remove enough P so that it is not a limiting component in the sludge solids. The minor sludge component, Cr, can also adversely affect the HLW immobilization process. The Cr in the T-110 sludge was largely insoluble in 0.01 M NaOH, with only 3% being removed by dilute hydroxide washing. The solution obtained by washing the T-110 solids with dilute hydroxide could likely be immobilized as a Class A low-level waste (LLW), even without removing 137 Cs. The work presented here indicates caustic leaching to be a very effective method for pretreating Hanford Tank T-110 sludge, primarily because this method essentially quantitatively removes P from the water-washed T-110 solids. Assuming a P 2 O 5 limit of 3 wt% in the immobilized high-level waste (IHLW) glass, it is estimated that caustic leaching will result in an ~80% reduction in the IHLW mass. Unlike high-Al tanks (see for example, Lumetta et al. 2001), relatively mild leaching conditions (1 M NaOH at 60°C) should sufficiently remove P from the T-110 solids. However, more rigorous leaching conditions (or oxidative leaching) may be needed to avoid encountering the Cr limit in the glass formulation. The leaching of P from the sludge solids is rapid and largely independent of temperature and NaOH concentration. On the other hand, the leaching of Cr is much slower and is highly dependent on temperature and NaOH concentration. Some of the caustic-leaching solutions contained significant concentrations of transuranic (TRU) el...