Cations redistribution in mixed oxides (U,Ce)O2 and (U,Pu)O2 under thermal gradient

Redistribution of Cerium and Uranium have been investigated in (Usub(0.7)Cesub(0.3))Osub(2-x) oxide subjected to a temperature gradient in laboratory experiments, in order to apply the results to the nuclear fuel (UPu)O2. Cylindrical specimens in sealed molybdenum containers were exposed to temperatures up to 24000C and longitudinal thermal gradients of 15000/cm generated by an image furnace. Stoechiometry of the samples has been determinated from measurements of the oxygen chemical potential with a solid electrolyte probe (ThO2-Y2O3). The chemical interdiffusion coefficient Dsub(UO2/CeO2) has been established in a temperature range from 14000C to 24000C. Our investigations of cations thermal diffusion in (UCe)O2 have given evidence of a fast cations migration by an evaporation-condensation mechanism, and emphasize the strong correlation between oxide stoechiometry and the degree of redistribution, as well as the role of the initial density. A model based on vapour transport of metal-bearing species is set up, which describes our experiments and the influence of the various parameters (temperature, density, (O/M) ratio) with a good agreement. This treatment, applied to the post-irradiation experimental data, gives a correct description of in-pile cations redistribution, even for high burnup, without introducing any hypothetic SORET effect