A new method for solving the fission gas diffusion equations with time-varying diffusion coefficient and source term considering recrystallization of fuel grains

摘要

The fission gas swelling of nuclear fuels depends on the fission gas behavior in fuel grains, which will be accelerated by grain recrystallization occurred in some fuels. Under transient and accident conditions, the temperature and fission rate could vary greatly, which will affect the fission gas behavior in fuel grains significantly. It is necessary to develop a new method to solve the fission gas diffusion equations with these effects involved. In this study, for fission gas diffusion equations with time-dependent diffusion coefficient and source term, a new time-discrete method for solving the intergranular gas atom concentration and flux is proposed. With the grain recrystallization considered, the corresponding time-discrete solutions are obtained respectively for the grains in the recrystallized and un-recrystallized areas. The fission gas swelling can be subsequently calculated out. These new mechanistic models are imbedded in the finite element (FE) simulation of the irradiation-induced multi-scale thermo-mechanical coupling behavior in a composite fuel pellet. FE simulation results demonstrate that: (1) the proposed new method is an efficient method with high accuracy and acceptable computational cost; (2) the enlarged fission gas diffusion coefficient will lead to a considerable increase in fuel particle swelling; (3) for composite pellets with high volume fraction of fuel particles, their radial deformation will be heavily affected when the temperature dependence of diffusion coefficient is involved. It can be concluded that the fission gas swelling computation based on this new solution method is necessary to be adopted, when the in-pile behavior simulation is performed for fuel elements under transient and accident conditions.