Dissolution of Th1−xUxO2: Effects of chemical composition and microstructure

Abstract

The dissolution of Th1−xUxO2 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 Th1−xUxO2 solid solutions with higher uranium incorporation fractions (x=0.52, 0.75 and 1) differed significantly from that of Th0.84U0.16O2. 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 Th0.84U0.16O2 samples showed that RL(Th) values decreased by an order of magnitude when the relative density increased from 79% to 89%.