Effect of Alpha Radiolysis on UO 2 Dissolution under Reducing Conditions
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Effect of Alpha Radiolysis on UO2 Dissolution under Reducing Conditions T. Mennecart (1),(2), B. Grambow (1), M. Fattahi (1), G. Blondiaux (3), Z. Andriambololona (2) (1) SUBATECH – Ecole des Mines de Nantes, 4 rue Alfred Kastler, BP 20722 – 44307 Nantes cedex 3. France (2) Agence Nationale pour la gestion des Déchets RAdioactifs, Parc de la Croix Blanche, 1/7 rue Jean Monnet, 92290 Châtenay-Malabry Cedex, France (3) Centre d’Etude et de Recherche par Irradiation (CNRS), 3A, rue de la Férollerie, 45071 Orléans cedex 2, France ABSTRACT Effects of water radiolysis on the dissolution rate of UO2 were studied by irradiating UO2 colloids with an alpha particle beam of a cyclotron. The solution was kept under reducing conditions by applying a current of -10 or -50 nA and an ultrahigh purity argon bubbling. The solution pH was fixed at 6 during experiment. The particle flux was 1.1x1011 α.cm-2.s-1 entering in solution with an energy of 5 – 6 MeV. Despite initial reducing conditions water radiolysis increased the dissolution rate of uranium dioxide due to reaction with oxidizing radiolytic species such as H2O2, O2, OH•, HO2•, ClO3-. The monitoring of the redox potential showed that the solutions became rapidly oxidizing. The dissolution rate values were between 0.01 and 26 mg.m-2.d-1 depending on the ratio of the irradiated UO2 mass to the solution volume. This dependency is caused by hydrogen peroxide consumption at the UO2 surface. The rate of hydrogen peroxide consumption by reaction with colloids corresponds to an equivalent rate of UO2 oxidation/dissolution between 2 and 60 mg.m-2.d-1 indicating that only a small fraction of produced H2O2 is consumed by the dissolution reaction. INTRODUCTION For assessment of the performance of spent fuel during geological disposal the interaction between uranium dioxide and groundwater must be understood. Usually, redox conditions in geological disposal are reducing but water radiolysis at the solid/solution interface must be taken into account because oxidizing conditions are locally created which favor fuel oxidation and dissolution of U(VI). After few hundred of years, the beta and gamma radiation fields will decrease significantly while the alpha field will remain strong for thousands of years. We studied the effect of alpha radiation on UO2 dissolution and H2O2 production with an alpha beam of a cyclotron. As solid phase we used colloidal UO2. Due to its high surface area radiation effects can easily be followed. EXPERIMENTAL UO2 colloids were prepared using an electrochemical method [1]. Few milliliters of a U(IV) stock solution were mixed with a 1 mol.L-1 NaCl solution at a pH of 6. During this step reducing conditions were assured (-500 mV / SHE) using platinum electrodes. The pH values were measured continuously with a combination glass electrode. By bubbling with ultrahigh purity
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argon the concentrations of oxygen and carbon dioxide in solution were minimized. Solid characterizations by TEM, XPS and XRD have already been reported [1]. The solid phase composition is UO2+
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