SULFUR HEXAFLUORIDE TREATMENT OF USED NUCLEAR FUEL TO ENHANCE SEPARATIONS

Abstract

Reactive Gas Recycling (RGR) technology development has been initiated at Savannah River National Laboratory (SRNL), with a stretch-goal to develop a fully dry recycling technology for Used Nuclear Fuel (UNF). This approach is attractive due to the potential of targeted gas-phase treatment steps to reduce footprint and secondary waste volumes associated with separations relying primarily on traditional technologies, so long as the fluorinators employed in the reaction are recycled for use in the reactors or are optimized for conversion of fluorinator reactant. The developed fluorination via SF{sub 6}, similar to the case for other fluorinators such as NF{sub 3}, can be used to address multiple fuel forms and downstream cycles including continued processing for LWR via fluorination or incorporation into a aqueous process (e.g. modified FLUOREX) or for subsequent pyro treatment to be used in advanced gas reactor designs such metal- or gas-cooled reactors. This report details the most recent experimental results on the reaction of SF{sub 6} with various fission product surrogate materials in the form of oxides and metals, including uranium oxides using a high-temperature DTA apparatus capable of temperatures in excess of 1000{deg}C . The experimental results indicate that the majority of the fission products form stablemore » solid fluorides and sulfides, while a subset of the fission products form volatile fluorides such as molybdenum fluoride and niobium fluoride, as predicted thermodynamically. Additional kinetic analysis has been performed on additional fission products. A key result is the verification that SF{sub 6} requires high temperatures for direct fluorination and subsequent volatilization of uranium oxides to UF{sub 6}, and thus is well positioned as a head-end treatment for other separations technologies, such as the volatilization of uranium oxide by NF{sub 3} as reported by colleagues at PNNL, advanced pyrochemical separations or traditional full recycle approaches. Based on current results of the research at SRNL on SF{sub 6} fluoride volatility for UNF separations, SF{sub 6} treatment renders all anticipated volatile fluorides studied to be volatile, and all non-volatile fluorides studied to be non-volatile, with the notable exception of uranium oxides. This offers an excellent opportunity to use this as a head-end separations treatment process because: 1. SF{sub 6} can be used to remove volatile fluorides from a UNF matrix while leaving behind uranium oxides. Therefore an agent such as NF{sub 3} should be able to very cleanly separate a pure UF{sub 6} stream, leaving compounds in the bottoms such as PuF{sub 4}, SrF{sub 2} and CsF after the UNF matrix has been pre-treated with SF{sub 6}. 2. Due to the fact that the uranium oxide is not separated in the volatilization step upon direct contact with SF{sub 6} at moderately high temperatures (≤ 1000{deg}C), this fluoride approach may be wellsuited for head-end processing for Gen IV reactor designs where the LWR is treated as a fuel stock, and it is not desired to separate the uranium from plutonium, but it is desired to separate many of the volatile fission products. 3. It is likely that removal of the volatile fission products from the uranium oxide should simplify both traditional and next generation pyroprocessing techniques. 4. SF{sub 6} treatment to remove volatile fission products, with or without treatment with additional fluorinators, could be used to simplify the separations of traditional aqueous processes in similar fashion to the FLUOREX process. Further research should be conducted to determine the separations efficiency of a combined SF{sub 6}/NF{sub 3} separations approach which could be used as a stand-alone separations technology or a head-end process.« less