Dissolution of UO2 in Boom clay water in oxidizing conditions: an XPS study

The solubility behavior of uranium dioxide was studied under oxidizing conditions in the Belgian Boom clay water at 25°C up to 323 days. X-ray photoelectron spectroscopy analyses reveal that both U(VI) and U(IV) oxidation states are present in the altered layer. The U(VI)/U(IV) ratio in this altered layer increases up to 43 days and then remains constant throughout the whole test. In the leachate the uranium concentration also increases up to 43 days and then stabilizes. It is rather unlikely that the altered layer formed on the surface controls thermodynamically the uranium solubility. It is rather probable that the system has reached a pseudo-equilibrium governed by kinetics.