Understanding hydrothermal conversion mechanism of mixed uranium-cerium oxalate into oxide thanks to in situ XANES analysis

New generations of nuclear reactors require the development of innovative methods to elaborate nuclear fuels. Within this framework, studies have been devoted to the hydrothermal conversion of An(IV) oxalates (An = Th, U, Np, Pu) which allows the direct and quantitative precipitation of hydrated oxides from solution. In this frame, our group is currently studying the hydrothermal conversion of uranium(IV)-cerium(III) mixed oxalates into oxides, cerium being used as a surrogate for plutonium. These two elements can exist in several oxidation state in solution, thereby inducing a complex redox behaviour during hydrothermal treatment. In situ HERFD-XANES experiments will be used to directly monitor the redox state and the speciation of both elements during hydrothermal conversion of oxalates under various conditions of temperature and pH. Understanding of these conversion mechanisms will then allow to deduce optimum experimental conditions for the synthesis of uranium-cerium oxides.