Corrosion of uranium dioxide in hydrogen peroxide solutions

Abstract

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 H 2O 2 concentration. For H 2O 2 concentrations lower than 10 −4 mol/L, the UO 2 corrosion potential is directly proportional to H 2O 2 concentrations. For H 2O 2 concentrations between 10 −4 and 10 −2 mol/L, the UO 2 surface appears to be redox buffered by the H 2O 2 decomposition. For H 2O 2 concentrations higher than 10 −2 mol/L, the formation of U(VI) corrosion product deposits may block H 2O 2 decomposition. Under these conditions UO 2 corrosion is driven by reaction with H 2O 2. When carbonate is present, the formation of U(VI) deposits is avoided and H 2O 2 decomposition continues to occur at high [H 2O 2]. When the pH is decreased, UO 2 dissolution is accelerated and for pH⩽5 uranyl peroxide deposits may form on the electrode surface. The importance of H 2O 2 decomposition at low pH (i.e. ⩽6) is not fully understood.