FGR model improvement for high burnup fuel analysis

NFI has developed and has been utilizing FPAC code (Fuel Performance Analysis Code) to analyze fuel behavior. Two main models in FPAC were revised in 1988 to expand the capability of the code based on the available high burnup fuel data at that time. Fission gas release from pellets (FGR) is modelled in FPAC based on diffusion theory. Diffusion constant in the model consists of three terms: K1, K2 and K3. K1 models the gas channels through grain boundaries and was revised to be more sensitive to fuel temperature. K2 models FGR enhancement by burnup increase and was adjusted to be less sensitive to burnup. K3 models the gas channels through open porosities. As a total, these constants were adjusted to minimize the error of code prediction. A new pellet relocation model was introduced into the code to predict the fuel temperature and clad deformation accurately. FGR is sensitive to fuel temperature and the code prediction is closely related to these models. 7 refs, 8 figs, 3 tabs