Modelling the Oxidative Dissolution of UO 2
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FRASER KING,* MIROSLAV KOLAR* AND DAVID W. SHOESMITH** *Atomic Energy of Canada Limited, Whiteshell Laboratories, Pinawa, Manitoba, CANADA "**Departmentof Chemistry, University of Western Ontario, London, Ontario, CANADA ABSTRACT An electrochemically based model for predicting the effects of a-radiolysis, the precipitation of U(VI) corrosion products and redox processes with Fe and Fe(II) on the dissolution of UO 2 is described. Various aspects of the model are presented, including: the underlying mechanism, the reaction-diffusion equations used to describe the mass transport and homogeneous reactions of the various species considered in the model, the geometrical grid used to simulate both experimental and used fuel/container geometries and the electrochemical boundary conditions used for the numerical solution of the reaction-diffusion equations. The results of preliminary simulations are also discussed. INTRODUCTION The oxidative dissolution of U0 2 and used fuel are currently the focus of many spent fuel programs worldwide [1]. Oxidants produced by et-radiolysis may cause the long-term oxidative dissolution of the UO, matrix once the f/ly-radiation fields have decayed after a period of 3001000 a. The ax-activity of used CANDU® fuel is predicted to slowly decrease from -1 Ci.kg-' after 10 a to 0.2 Ci-kg' after 10oa. The corresponding dose rates in the thin irradiated surface water layer (-35 jim thick) will be - 50 Gy-h' and -13 Gy-hI', respectively [2]. The impact of the oxidants produced by ot-radiolysis may be mitigated by the precipitation of U(VI) corrosion products on the fuel surface and by redox reactions between oxidized species and Fe(II) produced by the corrosion of C-steel container components or from corrosion of the container itself [3]. Precipitation may block the surface sites at which the dissolution of UO9 and the concomitant reduction of oxidant occur. In addition, the porous nature of the corrosion product [4] will retard the mass transport of reactants and products to and away from the corroding surface and affect the amount of cr-radiolysis products formed at the fuel surface. Because of the possible long-term effects of ax-radiolysis, several models have been developed to predict the oxidative dissolution behaviour of UO 2 [1]. Bruno and co-workers [5] have developed a model based on reductive-capacity principals in which radiolytically produced H20 2 is simulated by an equivalent amount of 02. The chemical effect of precipitation is taken into account (i.e., the corrosion product acts as a sink for dissolved U(VI)), but not the physical blocking effects. Christensen has developed a detailed radiolysis model that includes both radical and molecular radiolysis products, in which the interfacial reactions on the U0 2 surface are simulated by homogeneous reactions [6]. Shoesmith and Sunder [7] have developed a semiempirical electrochemical model and Neretnieks [8] has attempted to include the effects of redox reactions with Fe(II). The model described here is based on a detailed mechanisti
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