The kinetics of dissolution of Th1−xUxO2 solid solutions in nitric media

The dissolution of thorium–uranium (IV) dioxide solid solutions has been investigated in nitric media as a function of several parameters (leaching time, acidity of the leachate, temperature and uranium substitution rate in the solid solution) by using batch experiments. The normalized dissolution rates were evaluated for Th1−xUxO2 with x<0.5, leading to the determination of the partial order related to the proton concentration, n, and to the corresponding normalized dissolution rate constant at pH=0, k′T,298 K. The normalized dissolution rate of Th1−xUxO2 increases with the acidity of the leachate, and with the amount of uranium in the solid for a given pH. As the thermodynamic equilibrium is not yet reached in acidic media after two years for the solid solutions with x<0.5, only kinetics of the dissolution is described. The stoichiometry of the release of both actinides was verified until the precipitation of thorium occurred in the leachate for pH > 2, while uranium was released in the solution under uranyl form. The partial order related to the proton concentration was determined for ThO2 and three compositions of solid solutions. The variation of the normalized dissolution rate with temperature allowed to determine the activation energy following the Arrhenius law (20, 33 and 16 kJ mol−1 for ThO2 Th0.63U0.37O2 and Th0.47U0.53O2 respectively) at pH 2. The dissolution mechanism of Th1−xUxO2 solid solutions was explained by three steps: initial oxidation of the uranium at the surface, protonation at the U(VI), Th(IV), U(IV) sites, then finally detachment of the surface complexes.