Résumé
The characteristics of the fuel microstructure during and after irradiation are influenced by the manufacturing process, particularly its final stage, sintering. During this phase, uranium dioxide and plutonium dioxide pellets are processed in an accurately controlled atmosphere and at carefully monitored temperatures. Cationic inter-diffusion can achieve a homogeneous distribution of cations in the pellets by the end of the manufacturing process. This homogenization is also ensured by processing steps such as sieving, mixing and milling. An extra slight contribution to homogenization is the recoil (~ 8µm) of heavy nuclei due to fission during in-pile operation. This homogenization enables to avoid fission rate heterogeneities within the pellet during irradiation. Also, an additional homogenizing thermal treatment could be applied to high Pu bearing samples of (U,Pu)O2 for instance, in order to eliminate insoluble particles during recycling (areas with Pu/(U+Pu)>30%). For a better understanding of the impact of diffusion on fuel homogenization, we developed a numerical model to simulate diffusion within the fuel during sintering. This model includes bulk diffusion, the slowest process (acting on a ~5µm scale), as well as surface and grain boundary diffusions, which are faster (acting on a ~150µm scale), while accounting for differences in diffusion rates between oxide ions and metallic cations. In this study, we analyzed microscopic homogeneity, assessed by the distribution of cations within the crystal lattice under various manufacturing conditions, using both grain boundary and bulk diffusion coefficients corresponding to these conditions. The goal is to quantify the time required to achieve cationic homogeneity at a defined observation scale within a three-dimensional microstructure. For this purpose, we used the relative standard deviation of cation concentration as an indicator of homogeneity. In summary, the numerical model developed in this study will be an essential tool for determining optimal conditions to reduce areas of high plutonium concentration and to increase the ratio of plutonium bearing phases.