We propose and test a disposition path for irradiated nuclear fuel using ammonium carbonate and hydrogen peroxide media. We demonstrate on a 13 g scale that >98% of the irradiated fuel dissolves. Subsequent expulsion of carbonate from the dissolver solution precipitates >95% of the plutonium, americium, and curium and substantial amounts of fission products, effectively partitioning the fuel at the dissolution step. Uranium can be easily recovered from solution by any of several means, such as ion exchange, solvent extraction, or direct precipitation. Ammonium carbonate can be evaporated from solution and recovered for reuse, leaving an extremely compact volume of fission products, transactinides, and uranium. Stack emissions are predicted to be less toxic, less radioactive, chemically simpler, and simpler to treat than those from the conventional PUREX process.
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.
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.
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