Statistical Model for Grain Boundary and Grain Volume Oxidation Kinetics in UO 2 Spent Fuel
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STATISTICAL MODEL FOR GRAIN BOUNDARY AND GRAIN VOLUME OXIDATION KINETICS IN U02 SPENT FUEL
R. B. Stout*, H. F. Shaw* and R. E. Einziger** *University of California, Lawrence Livermore National Laboratory P 0 Box 808, L-200, Livermore, CA 94550 "**Battelle - PNL, P 0 Box 999, Richland, WA 99352
ABSTRACT The Yucca Mountain Project of the U.S. Department of Energy is investigating the suitability of a site in the unsaturated zone at Yucca Mountain, NV, for a high-level nuclear waste repository. Most of the waste will consist of U02 spent fuel in Zircaloy-clad rods from nuclear reactors. If failure of both the waste containers and the cladding occurs within the lifetime of the repository, then the U02 will be exposed to oxygen in the air and higher oxides of uranium may form. The oxidation state of the spent fuel may affect its dissolution behavior if later contacted by water. A model for the kinetics of spent fuel oxidation under repository-relevant conditions is thus necessary to predict the behavior of the waste form for assessing the performance of the repository with respect to the containment of radionuclides. In spent fuel experiments, the U0
2
oxidation front initially propagates along grain
boundaries followed by propagation into grain volumes. Thus, the oxidation kinetics is controlled by two processes and the oxidation of spent fuel fragments will depend on the density and physical attributes of grain boundaries. With this in mind, concepts from statistical mechanics are used to define a density function for grain boundaries per unit volume per unit species in a spent fuel fragment. Combining the integral forms of mass conservation and this grain boundary density function, a model for the global rate of oxidation for a spent fuel fragment is obtained. For rapid grain boundary oxidation compared to grain volume oxidation, equations of the model are solved and results compared to existing data.
INTRODUCTION In any proposed nuclear waste repository for spent fuel from nuclear power reactors, the potential release rates of many radionuclides over a 10000 year design lifetime depends on the oxidation rate and the oxidation state of any irradiated U02 fuel pellets that may be exposed to the atmosphere [Einziger and Woodley, 1985; Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
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Einziger, 1985; Oversby, 1987]. This is because the oxidization state of U02, into U307 or possibly other oxides, could influence the low temperature dissolution rates of any spent fuel that may be exposed to water in a repository. Therefore, experiments to provide both data and a physical basis for rational model development for U02 oxidation kinetics are necessary in order to eventually predict potential radionuclide release rates from spent fuel in a repository. Experimental tests of spent fuel oxidation to provide data and to identify a physical basis for model development were proposed by Einziger [1986a, 1986b]. Results from the tests completed [Einziger, et al., 1987; 1988] at this time imply that th
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