The FY 2003 risk assessment (RA) (Mann et al. 2003) of bulk vitrification (BV) waste packages used 0.3 wt% of the technetium (Tc) inventory as a leachable salt and found it sufficient to create a significant peak in the groundwater concentration in a 100-meter down-gradient well. Although this peak met regulatory limits, considering uncertainty in the actual Tc salt fraction, peak concentrations could exceed the maximum concentration limit (MCL) under some scenarios so reducing the leachable salt inventory is desirable.

Experiments were conducted to determine sulfur solubility in Hanford low‐activity waste (LAW) glass melts by a sulfur saturation method. Sulfur‐incorporated glass melts were prepared by salt saturation and bubbling methods. The salt saturation method was performed by mixing crushed premelted baseline glasses with an excess amount of Na2SO4 prior to melting the mixture at 1150°C for 1 hour. The bubbling method involved bubbling the glass melt at 1150°C in a Pt crucible with an SO2/O2/N2 gas mix to equilibrate the melt at a known pressure of SO3. Preliminary results suggested that performing 1 cycle of mixing and melting was not sufficient to saturate the glass. The bubbling method successfully incorporated sulfur into the glass but caused significant losses of sodium from the melt. In order to saturate the glass melt with sulfate without causing noticeable sodium loss, a modified crucible salt saturation method was developed by repeating the mixing and melting of the glass and salt mixture. For all 3 representative LAW glasses tested in this study, it was found that after 3 mixing and melting cycles, the sulfur concentration reaches a plateau, indicating reasonable sulfur saturation.

This report describes the Hanford Tank S-110 sludge caustic leaching test conducted in FY 2001 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 S-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. The work presented here indicates caustic leaching to be a very effective method of pretreating Hanford Tank S-110 sludge. Because of the predominance of boehmite in the water-insoluble S-110 solids, high caustic and temperature are required to sufficiently remove Al. It would also be necessary to leach for several days to realize the full benefits of caustic leaching. As expected, Al removal improves with increasing temperature, NaOH concentration, and leaching time. The Cr behavior parallels that of Al. At a maximum of 0.5 wt% Cr 2 O 3 in the high-level waste form, the mass of immobilized high-level waste (IHLW) would be constrained by the Cr content of the leached S-110 solids. Nevertheless, an 80 to 90% reduction in IHLW mass from the S-110 solids should be readily achievable. The results of this work underscore the need to continue process optimization studies. If subjected to the baseline leaching approach (3 M NaOH, 80 to 90°C, for 8 h), only about 25% of the Al would be leached from the dilute hydroxide-washed S-110 solids. Clearly, this would not be sufficient to adequately reduce the IHLW mass.

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...

The S-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.The work presented here indicates caustic leaching to be a very effective method of pretreating Hanford Tank S-110 sludge. Because of the predominance of boehmite in the water-insoluble S-110 solids, high caustic and temperature are required to sufficiently remove Al. It would also be necessary to leach for several days to realize the full benefits of caustic leaching. As expected, Al removal improves with increasing temperature, NaOH concentration, and leaching time. The Cr behavior parallels that of Al.At a maximum of 0.5 wt% Cr 2 O 3 in the high-level waste form, the mass of immobilized high-level waste (IHLW) would be constrained by the Cr content of the leached S-110 solids. Nevertheless, an 80 to 90% reduction in IHLW mass from the S-110 solids should be readily achievable.