Lattice constant in nonstoichiometric uranium dioxide from first principles

International audienceNonstoichiometric uranium dioxide experiences a shrinkage of its lattice constant with increasing oxygen content, in both the hypostoichiometric and the hyperstoichiometric regimes. Based on first-principles calculations within the density functional theory (DFT)+$U$ approximation, we have developed a point defect model that accounts for the volume of relaxation of the most significant intrinsic defects of UO$_2$. Our point defect model takes special care of the treatment of the charged defects in the equilibration of the model and in the determination of reliable defect volumes of formation. In the hypostoichiometric regime, the oxygen vacancies are dominant and explain the lattice constant variation with their surprisingly positive volume of relaxation. In the hyperstoichiometric regime, the uranium vacancies are predicted to be the dominating defect,in contradiction with experimental observations. However, disregarding uranium vacancies allows us to recover a good match for the lattice-constant variation as a function of stoichiometry. This can be considered a clue that the uranium vacancies are indeed absent in UO$_{2+x}$ , possibly due to the very slow diffusion of uranium

Nature and Behaviour of Point Defects in UO2 Based on an Experimental and Theoretical Study of Electrical and Atomic Transport Properties

International audienceThermally or radiation induced transport properties impact practically all engineering aspects of nuclear oxide fuels, whether at the manufacturing stage, during in-reactor operation, or under long-term repository conditions. From a more fundamental standpoint, measuring transport properties is also a means of probing point or complex defects that are responsible for atomic migration. Although many studies relating to self-diffusion in UO2 have been carried out over the past forty years, these have not generally focussed on characterising these properties as a function of all the physical variables which determine it, i.e. temperature, the oxygen partial pressure and the impurity content, usually present in the form of bi- or tri-valent action impurities. In this talk, we show how electrical conductivity and self-diffusion property measurements may be combined in order to determine fundamental data relating to the nature of defects responsible for the property and their formation or migration energies. These data may then be compared to those obtained from first principles electronic structure calculations. The first part of the talk is dedicated to the development of a point defect model which captures the basic dependence of the electrical conductivity of UO2 upon temperature and oxygen partial pressure. The formation energies derived from this exercise are then compared to charged defect calculations using Density Functional Theory in the LDA+U approximation. The second part is concerned with oxygen self-diffusion and chemical diffusion coefficient measurements. We show that the point defect model is also compatible with the experimental data available. In the third part of the talk we examine uranium self-diffusion properties. The point defect model is specifically developed to account for uranium defects in a composition range close to stoichiometry. An analytical apparent activation energy is derived in this composition region and a numerical application is carried out based on basic formation and migration energy estimates obtained from first principles. The results compare favourably to existing data but highlight the need for additional experimental and theoretical work