Corrosion/Electrochemistry of Uranium Dioxide in Slightly Alkaline Hydrogen Peroxide Solutions
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Corrosion/Electrochemistry of Uranium Dioxide in Slightly Alkaline Hydrogen Peroxide Solutions Jon S. Goldik, James J. Noël, David W. Shoesmith Department of Chemistry, The University of Western Ontario, London, ON, Canada. N6A 5B7 [email protected], [email protected], [email protected] ABSTRACT The kinetics of H2O2 reduction have been studied in slightly alkaline (pH 9.7 NaCl) solution on 1.5 at.% SIMFUEL. Using cyclic voltammetric techniques, we have shown that the cathodic reduction of H2O2 is kinetically facile on UIVUVO2+x surfaces. Carbonate ions are found to have a significant effect on the kinetics of H2O2 reduction. Suppression of the reaction rate is observed at very cathodic potentials, and has been attributed to a competition between carbonate and hydrogen peroxide for catalytic sites on the electrode surface. A small enhancement of the reduction current occurs between –200 and +100 mV vs. SCE, which appears to be due to a surface adsorbed carbonate complex. The large values of the Tafel slopes for H2O2 reduction have been interpreted in terms of a chemical-electrochemical mechanism involving surface UV species. The results are discussed in terms of a mixed potential model for the prediction of nuclear fuel dissolution rates under permanent disposal conditions. INTRODUCTION Copper nuclear waste containers emplaced in deep granite rock repositories are expected to survive for considerable periods of time under permanent waste disposal conditions. Thus, gamma/beta radiation fields associated with the fuel waste form will have decayed to insignificant levels before container failure occurs and the fuel comes in contact with groundwater. Consequently, only the alpha radiolysis of water should produce oxidants to drive fuel corrosion in the long term. Since H2O2 is the primary oxidizing product of alpha radiolysis, an understanding of its influence on fuel dissolution is essential to the development of models to predict fuel corrosion behaviour under permanent disposal conditions. Here, the electrochemical reduction of H2O2 is studied on 1.5 at.% SIMFUEL, an unirradiated analogue of used nuclear fuel produced by doping the UO2 matrix with a series of stable elements in proportions appropriate to simulate the chemical effects caused by in-reactor irradiation [1,2]. In this material, the rare earth dopants replace a small fraction of the UIV in the fluorite lattice of UO2, and the noble metal dopants (Mo, Ru, Rh, Pd) congregate into uniformly distributed noble metal ε-particles. EXPERIMENTAL DETAILS Electrodes were cut from SIMFUEL pellets provided by Atomic Energy of Canada Limited (Chalk River, ON, Canada). Details of experimental procedure and solution preparation have been published elsewhere [3]. All potentials are measured and quoted against the saturated calomel electrode (SCE).
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RESULTS AND DISCUSSION Figure 1 compares the cyclic voltammograms obtained on a SIMFUEL electrode in a carbonate-free and a carbonate-containing (0.1 mol·L-1) solution. While not necessarily representative of anticip
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