Oxidative dissolution mechanism of both undoped and Gd₂O₃-doped UO₂(s) at alkaline to hyperalkaline pH

The dissolution rates of unirradiated UO₂ and unirradiated UO₂ doped with Gd₂O₃ were determined as a function of pH using flow-through experiments in the presence of O₂(g) and bicarbonate. The dissolution rate of non-doped UO₂ was very low under hyperalkaline conditions (pH 12-13) whereas it increased drastically as the pH decreased to 9. The dissolution of non-doped UO₂ in the pH range of 9-13 was consistent with the oxidative dissolution mechanism already described for UO₂ dissolution in the presence of bicarbonate and oxygen. XPS analysis performed on the solid after dissolution experiments at pH 10 and 13 supported the bicarbonate effect to complex UO₂²⁺ and accelerate dissolution. Moreover, UO₂ doped with Gd₂O₃ (5 wt% and 10 wt%) showed dissolution rates as low as non-doped UO₂ under hyperalkaline conditions, which were maintained throughout the pH range studied (9-13). No substantial differences in the dissolution rates between these two doping levels were found. XPS analysis evidenced a similar surface composition both at pH 10 and 13, with U(V) being the dominant oxidation state. The low dissolution rates were assumed to be a consequence of the gadolinium capacity to retard the oxidation of U(V) to U(VI). The slight increase in dissolution rates observed in the hyperalkaline region was attributed to a shift in the oxidative dissolution mechanism, in which the presence of OH^- promotes the formation of soluble uranyl hydroxo complexes.