Corrosion of uranium dioxide in hydrogen peroxide solutions

The corrosion behaviour of specimens cut from nuclear grade CANDU pellets has been studied electrochemically and under open-circuit corrosion conditions in hydrogen peroxide containing, slightly alkaline (pH=9.5) sodium perchlorate solution with and without added carbonate. The compositions of the electrode surfaces were determined using X-ray photoelectron spectroscopy (XPS). Three distinct ranges of behaviour are observed as a function of H2O2 concentration. For H2O2 concentrations lower than 10−4 mol/L, the UO2 corrosion potential is directly proportional to H2O2 concentrations. For H2O2 concentrations between 10−4 and 10−2 mol/L, the UO2 surface appears to be redox buffered by the H2O2 decomposition. For H2O2 concentrations higher than 10−2 mol/L, the formation of U(VI) corrosion product deposits may block H2O2 decomposition. Under these conditions UO2 corrosion is driven by reaction with H2O2. When carbonate is present, the formation of U(VI) deposits is avoided and H2O2 decomposition continues to occur at high [H2O2]. When the pH is decreased, UO2 dissolution is accelerated and for pH⩽5 uranyl peroxide deposits may form on the electrode surface. The importance of H2O2 decomposition at low pH (i.e. ⩽6) is not fully understood.