DEFECT DISORDER AND OFF-STOICHIOMETRY IN UO<SUB>2</SUB>

The microstructure of UO₂ is altered significantly by irradiation during its operation as nuclear fuel. Being an oxide, defect disorder, vis-à-vis off-stoichiometry, in this important material is a major aspect of the microstructure change process under irradiation. In general, defect disorder is controlled by the thermodynamic state variables, i.e. temperature and stress, as well as the composition or the oxygen partial pressure in the surrounding environment. We investigate the off-stoichiometric variation in UO₂ in terms of atomic defects and electronic carriers. A defect model that describes the defect disorder in terms of temperature and oxygen partial pressure is developed. The underlying parameters are drawn from density-functional theory literature. We extend the model to include the stress and electrostatic fields. Such extension relates the spatial off-stoichiometric variation to the microstructure elements in the oxide. In this regard, the impact of free surfaces and voids forming inside the bulk of UO₂ are investigated. The details of the surface interaction with the environment are understood using the ionosorption theory. The results establish the ground for the study of the oxide's time response to transients and irradiation conditions, thus employing the inventory of results from atomistic methods for a better understanding of the oxides structural behaviour. This material is based upon work supported as part of the Center for Materials Science of Nuclear Fuel, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number FWP 1356.