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Oxygen potential of hypo-stoichiometric (U 0.8 La 0.2)O 2-x was measured as a function of oxygen-to-metal ratio (2-x) at the temperature range from 1173 K to 1473 K. Comparing with uranium oxides doped with other lanthanides, e.g. Lu, Er, Gd and Nd, the relationship between 2 O ΔG and the oxygen-to-metal ratio obtained for this sample showed almost the same trend, and for the relatively large oxygen non-stoichiometry x, its significance was comparable with that of

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Diffusion phenomena of actinides and oxygen are relating to various properties of oxide fuels, e.g. actinide and oxygen redistribution, pore migration, sintering behavior. Although lots of experimental data have been obtained for U and O diffusions, these seem to be a little scattered among experiments. In addition, there are few studies of transuranium elements. Recently, we experimentally showed that diffusion coefficients of Am and Pu were almost comparable with that of U and the contribution of grain boundaries (GB) were relatively large in oxide fuels. On the other hand, the molecular dynamics (MD) calculation showed that U ions migrated via its vacancies and the O diffusion mechanism changed with temperature in bulk. In this study, the GB diffusion coefficients and the mechanism were systematically evaluated for U, Am and O in oxide fuels by MD calculations. For actinides and oxygen, their GB diffusion coefficients are significantly larger than bulk ones, and the diffusion mechanisms in GBs are almost independent on temperature. Furthermore, the GB diffusion is strongly dependent on its structure and the interfacial potential energy. The lower oxygen-to-metal (O/M) ratio gives the larger GB diffusion coefficients for actinides and oxygen.

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