Three-dimensional modeling of diffusion in bulk and grain boundaries: application to (U, Pu)O2 system

The diffusion of cations on a microscopic scale plays a crucial role in the evolution of the microstructure of oxides, which has a direct impact on the performance of the material during its use. In the case of the mixed oxide (U, Pu)O₂, the diffusion of plutonium, uranium and oxygen ions during pellet sintering is responsible for their final distribution in the fuel, contributing to its homogenization. This process is governed by the thermodynamic conditions of the sintering furnace, which influence the rate of diffusion via two main transport mechanisms: grain boundary diffusion and bulk diffusion.

A numerical model has been developed to simulate diffusion within the fuel during sintering and to better understand the impact of diffusion on fuel homogenization. This model is designed to simulate the diffusion of cations into the bulk, the slowest process, coupled with the faster diffusion of cations at grain boundaries, while taking into account the faster diffusion of oxide ions compared with metal cations.

Numerical results from the model have been compared and are in good agreement with experimental data obtained using the diffusion couple method. In addition, the model makes it possible to handle more complex experimental configurations (interface between spherical diffusion pairs). It can also be used to simulate the evolution of Pu concentration during heat treatment of a mixed oxide (U, Pu)O₂. This model could be used as a tool for the design of future fabrication routes or new fuel microstructures.