Hydrothermal Conversion of Uranium(IV) Oxalate into Oxides: A Comprehensive Study.

Within the development of future nuclear reactors, wet chemistry routes have been investigated for the fabrication of advanced oxide fuels. In this frame, a multiparametric study focused on the hydrothermal conversion of uranium(IV) oxalate U(CO)·HO into uranium oxides was undertaken in order to unravel the effects of temperature, pH, and kinetics. For pH ≤ 1, the lowest temperatures explored (typically from 180 to 200 °C) led to stabilized UO/UO mixtures exhibiting a global O/U ratio evaluated as 2.38 ± 0.10 from U M-edge HERFD-XANES experiments. Higher temperatures (220-250 °C) led the oxide stoichiometry to decrease down to 2.13 ± 0.04 which corresponds to a lower fraction of UO in the mixture. Additionally, increasing the temperature of the hydrothermal treatment efficiently improved the elimination of residual carbon species and water. Hydrothermal conversion of U(CO)·HO also led to a drastic modification of the powders morphology. With this aim, pH tuning could be used to shift from bipyramidal aggregates (up to pH 1) to microspheres (2 ≤ pH ≤ 5) and then to nanometric powders (pH > 5). Finally, a kinetics study showed that uranium oxides can be obtained from the hydrothermal decomposition of oxalate within only few hours. If the samples collected early during the treatment always presented the characteristic XRD lines of UO/UO fluorite-type structure, then they were found to be strongly oxidized (O/U = 2.65 ± 0.14) which suggested the existence of a U(VI)-bearing amorphous secondary phase. The latter further tended to reduce through time. Hydrothermal conversion then probably proceeds as a two-step mechanism composed by the oxidative decomposition of uranium(IV) oxalate followed by the reduction of uranium by organic moieties and its hydrolysis. It appears as an easy and efficient way to yield highly pure uranium oxide samples in solution.