Reductive hydrothermal conversion of uranyl oxalates into UO$_{2+x}$ monitored by in situ XANES analyses

Hydrothermal conversion of actinide oxalates has recently gained attraction as an innovative fabrication route for nuclear fuels but remained mainly limited to tetra-of tri-valent cations. We report herein the reductive conversion of mixtures of uranyl and oxalates ions into UO$_{2+x}$ oxides under mild hydrothermal conditions (T = 250°C). A multi-parametric study first led to specify the optimal conditions in terms of pH, oxalate/U ratio and duration to provide a quantitative precipitation of uranium in the hyper-stoichiometric dioxide form with : pH = 0.8; R = n$_{oxalate}$/n$_U$ = 3, and t = 72 hours. Particularly, pH was evidenced as a key-parameter, with 3 different compounds obtained over a range of only 0.4 units. The mechanism leading to the formation of UO$_{2+x}$ was then investigated thanks to an in situ XANES study. Analysis of the supernatant showed that U(VI) was quickly reduced into U(IV) thanks to the presence of oxalates and/or of their decomposition products in solution, following a first-order kinetics. Tetravalent uranium was then hydrolysed, leading to the precipitation of UO$_{2+x}$ as the only crystalline phase. This study thus demonstrates that the hydrothermal conversion of actinide oxalates into oxides is an extremely versatile tool that can be implemented in a large variety of chemical systems, particularly in terms of the oxidation state of the cations initially present in solution.