Thermal and under irradiation diffusion of chlorine in uranium dioxide

This work concerns the study of the thermal and radiation enhanced diffusion of ³⁶Cl in uranium dioxide. We simulated the presence of ³⁶Cl by implanting a quantity of ³⁷Cl comparable to the impurity content of chlorine in UO₂. In order to evaluate the diffusion properties of chlorine in the fuel and in particular to assess the influence of the irradiation defects, we performed two kinds of experiments: - the influence of the temperature was studied by carrying out thermal annealings in the temperature range 900 - 1300 C; we showed that implanted chlorine was mobile from temperatures as low as 1000 C and determined a thermal diffusion coefficient D1000 C around 10^-16 cm²s^-1 - the influence of the irradiation by fission products were studied by irradiating the samples with ¹²⁷I (energy of 63.5 MeV). We could determine that the diffusion of the implanted chlorine under irradiation and in the range of temperature 30 - 250 C was not purely athermal. We calculated a diffusion coefficient under irradiation D250 C of about 0-14 cm².s^-1. We showed the importance of the implantation and irradiation defects as preferential paths for a fast chlorine transport. We carried out ab initio calculations showing that chlorine is preferentially located in a substitutional site. This is in favour of a Frank-Turnbull diffusion mechanism or a vacancy/chlorine. (author)