AQUEOUS PROCESSING OF THORIUM FUELS

The status of aqueous processing methods for thorium fuels is summarized, with principal emphasis on the stainless steel-clad ThO/sub 2/UO/sub 2/ type. Data were obtained principally from laboratory-scale experiments with fully irradiated fuel samples and engineering-scale tests with unirradiated fuel. Stainless steel cladding was easily dissolved with 4 to 6M H/sub 2/SO/sub 4/ (Sulfex process) or 5M HNO/sub 3/-2M HCl (Darex process) in LCNA (Nionel type) or titanium equipment, respectively, in semicontinuous or batch equipment. Uranium losses to the decladding solutions were approximates 0.3% and 3 to 5% for the Sulfex and Darex processes, respectively, with fuel irradiated to approximates 20,000 Mwd/ton of core. The uranium was readily recovered from the Darex decladding solution in the acid Thorex extraction process. The ThO/sub 2/UO/sub 2/ core was dissolved in 13M HNO/sub 3/ -0.04M NaF-0.1M Al(NO/sub 3/)sub 3/. Uranium and thorium can be recovered from graphite-base fuels by disintegration and leaching with 90% HNO/sub 3/, grinding and leaching with 70% HNO3, or combustion followed by dissolution in fluoridecatlyzed nitric acid. Uranium and thorium were recovered from nitric acid solutions and separated from fission products by extraction with 30% tributyl phosphate in Amsco in the acid Thorex process. The use of an acid deficient feed (0.15M a.d.) induced high decontamination while injection of nitric acld at the fourth extraction stage provided high salting strength and ensured quantitative uranium and thorium extraction. Extensive studies with other organophosphorous extractants established the relations between metal extraction, radiation stability, and metal complex solubility with P-C or P-O bonding and chain branching. Di-sec- butyl phenylphosphonate (DSBPP) combined many of the advantages found and is applicable to the separation of uranium from thorium fission products. Since the thorium distribution coefficients (or Th/F.P. separation factors) were low, none of these extractants is potentially useful for recovery and decontamination of both thorium and uranium. (auth