Redox-Based Separation of Americium from Lanthanides in Sulfate Media

Abstract

In the development of advanced nuclear fuel cycles, a primary motivation is the reduction of the long-term radiotoxicity of the wastes. From a few decades to several thousand years after removal of the fuel from the reactor, americium (Am) dominates the radiotoxicity of used fuel, thus its transmutation represents an attractive option for improved management of the residues. However, every viable scenario for transmutation of Am demands some degree of separation of Am from fission product lanthanides. Partitioning of Am from curium further simplifies the transmutation process. The mutual separation of these elements is very challenging due to their similar chemistry. The focus of this work is on the utilization of the upper oxidation states of americium (Am(V/VI)) to facilitate a more efficient separation of Am from fission product lanthanides and curium. A minimum 90% efficient separation of americium from the lanthanides and curium has been achieved in the laboratory by selectively oxidizing trivalent Am to the hexavalent state using Na2S2O8. At equilibrium, lanthanides are precipitated as sodium lanthanide sulfate double salts, while oxidized Am species remain predominantly in the supernatant phase. Trivalent curium is readily coprecipitated with the NaLn(SO4)2 solid. Parallel studies of oxidized uranium, neptunium, and plutonium solutions support the conclusion that Am(VI) has sufficient stability to allow the separation to be completed in a reasonable time frame. A particular advantage of this approach is the absence of readily oxidized species that can quickly reverse the oxidation of Am and thus negate the separation. A variety of chemical and physical characterization techniques have been applied to profile the performance of the system.