REACTIVITY EFFECTS IN THE OXIDATIVE DISSOLUTION OF UO2 NUCLEAR-FUEL

The known energy-level scheme for UO2 and the nature of electron transport in UO2+x have been assessed with regard to their influence on electrochemical reactions. An unusually negative flat-band potential permits conventional electrochemical experiments to be conducted at considerably more cathodic potentials than would be feasible with a normal p-type semiconductor. The unique electronic properties of UO2+x can also be seen to accommodate cathodic and anodic photocurrents simply by changing the degree of hyperstoichiometry. Cathodic reduction of O2 in dilute alkaline aqueous solution has been studied intensively on two kinds of UO2+x (with p-type and n-type photocurrent response) and a UO2-based SIMFUEL The kinetics of this reaction appear to control the corrosion (oxidative-dissolution) rate of UO2 and are a sensitive probe of surface reactivity: a significant range of behaviour has been observed as a function of the properties of the electrode material and its interactions with solution. Anomalous features in Tafel plots can be correlated with qualitative changes in the composition of the surface oxide film. A theory developed to describe O2 reduction on transition-metal oxides with p-type semiconductivity has been adapted to provide a basis for understanding this process at UO2+x surfaces. Electron-transfer reactions are presumed to occur exclusively at active sites, formed by adjacent cations in different valence states, where O2 and any intermediates are adsorbed.