Thermal Diffusion in UO2-PuO2 Mixtures

A macroscopic segregation of PuO2 and UO2 in mixed oxide fuel elements caused by thermal gradient diffusion would have very unfavourable consequences. The contribution of 238U to the Doppler coefficient in a fast reactor would be affected. If the migration of plutonium occurs in the direction of the higher temperature, the central temperature of the fuel element increases. Our experiments were performed to investigate the thermal diffusion effect. Preliminary experiments on thermal diffusion in stoichiometric (U0,85, Pu0,15)O2 were made at a temperature gradient with 2300°C as the highest temperature. At Tmax = 2300°C a very small increase of plutonium concentration at the hot end was noticed after 110 h: This seems to be in agreement with the plutonium re-location found by p.E. Novak et al. in a highly rated fuel rod at temperature above 2300°C. These results will be confirmed by further experiments. Applying these results to the conditions of a fuel element in the fast reactor prototypes at present considered we conclude that with an inpile time of 10 000 h in the temperature range below 2300°C no disturbing effect of thermal diffusion is to be expected. (author)