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A point defect model is developed in hyperstoichiometric uranium dioxide, UO2+x,in order to explain some experimental results of deviation from stoichiometry and electrical conductivity existing in the literature. This model takes into account both singly charged uranium vacancy, V'U , and two types of Willis clusters, characterized by two kinds of interstitial oxygen atoms that are located on two different sites and two oxygen vacancies, namely 2(Oi"Oi'Vȯ)4' and . The electron holes, ḣ , are also introcued to ensure the crystal global electroneutrality. An adjustment procedure of the most convincing experimental data of the literature, corresponding to the temperature and oxygen partial pressure dependencies of departure from stoichiometry,x, and electrical conductivity, [sigma] , has been undertaken. The fit model/experience obtained has been found fairly good, justifying, a posteriori, the relevance of the nature of the selected defects in the model.

Thermodynamic and transport properties of hyperstoichiometric uranium dioxide,UO2+x, constitute a key issue in the application of this material for nuclear fuel since they are closely related to many chemical and physical processes, e.g., diffusion or creep. Starting from a point defect model for UO2+x developed by the authors in a preceding work[1] and quantitatively consistent with some literature experimental measurements of deviation from stoichiometry [2] and electrical conductivity[3],the oxygen and uranium self diffusion processes have been studied in order to appreciate the influence of defects involved in the model and particularly, the two Willis complexes i.e., 2(Oi"Oi'Vȯ)" and 2(Oi"O1'Vȯ)4'. Such a result leads the way to a better understanding of the mechanisms involved in diffusion processes and also helps to define more accurately the nature, as well as the type of the preferential interactions between diffusing impurities and the defects responsible for these transport properties.

We compare the experimental interdiffusion constants DAlCu in liquid Al81.3Cu18.7 at% to the experimental Cu self diffusion constants DCu in a similar Al80Cu20 at% melt. DAlCu are measured by an X-ray radiography technique and DCu are measured by the quasielastic neutron scattering method. It was found that DAlCu is enhanced by a factor of 3 with respect to DCu. The relation between DAlCu and DCu is investigated in the context of Darken’s equation where the interdiffusion constant DAB in a binary A-B system is approximated by a simple linear combination of the self diffusion constants DA and DB, DAB = (DAXB + DBXA)Φ (with XA and XB the mole fractions of A and B particles, respectively, and Φ the thermodynamic factor representing the thermodynamic driving force for interdiffusion). We also present a calculation of Φ from the Gibbs free energy data in molten Al-Cu. With these calculated Φ and the DCu data, interdiffusion coefficients in liquid Al-Cu have been calculated. However, the experimental DAlCu are larger than the calculated ones by a factor of 2. These results suggest that the enhancement of interdiffusion in Al-Cu liquids can not be only attributed to the thermodynamic driving force. Here we propose that the missing factor of about 2 in the Darken equation comes from the contribution of cross correlations to interdiffusion in Al-Cu liquids.

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