Modelling intergranular fuel swelling in severe accidents

Fuel swelling during severe accidents in PWRs is considered to be insignificant for both core degradation behaviour and fission gas release. For this reason it is ignored by the major system codes, while the complementary process of fission gas release is treated as a simple diffusion from a sphere. Recent experiments showed that fuel swelling might accelerate core degradation, and the retention of fission gases might lead to burst release in later accident phases. Modelling of swelling has been confined to full mechanistic bubble behaviour modelling (microscopic modelling), which is not suitable for the major plant codes because of its high CPU consumption. In this paper a simple analytic model based on macroscopic observations will be presented. This model uses gas diffusion from a spherical grain model for the gas atom flux into the grain boundaries, and vacancy diffusion from the pellet surface model for the vacancy flux to the grain boundaries. The total vacancy volume and the gas atom number are coupled by the Xe-equation of state. The gas release or fuel swelling is then pressure controlled. The model gave good results for both release and swelling when compared with experimental observations. This made clear that the application of the gas diffusion model alone is not enough to describe the fission product release.