Dissolution of Th_1−xU_xO₂: Effects of chemical composition and microstructure

Graphical abstract: Dissolution of Th_1−xU_xO₂ samples in concentrated nitric acid showed that thorium enriched solid solutions follow the same trend than thoria while uranium enriched ones followed the behavior of urania, due to the important role of uranium (IV) oxidation. - Abstract: The dissolution of Th_1−xU_xO₂ solid solutions was studied by varying independently several parameters such as chemical composition, leachate acidity, leaching temperature, firing temperature, and densification state. The chemical durability of the samples was significantly affected by the uranium mole loading probably due to the oxidation of tetravalent uranium into uranyl during the dissolution process. The influence of nitric acid concentration and temperature also showed that the behavior of Th_1−xU_xO₂ solid solutions with higher uranium incorporation fractions (x = 0.52, 0.75 and 1) differed significantly from that of Th_0.84U_0.16O₂. Strong modifications of the dissolution mechanism occurring at the solid/liquid interface for the higher uranium loadings could explain this difference. It could result in the existence of the fast oxidation of U(IV) to U(VI) at the solid/solution interface then of the detachment of activated complexes formed with U(VI). Furthermore, initial normalized dissolution rate slightly depended on the elimination of crystal defects for firing temperatures below 900 °C but was almost independent of the crystallite size (T ⩾ 900 °C). Finally, dissolution tests on sintered Th_0.84U_0.16O₂ samples showed that R_L(Th) values decreased by an order of magnitude when the relative density increased from 79% to 89%