Evaluation of BISON's Transient Fission Gas Release Model on High-Burnup UO₂ MiniFuel

Abstract

BISON's fission gas release (FGR) model was evaluated and tested over several hypothetical temperature transient conditions at high burnup and validated against FGR experimental data from an annealing test on high-burnup fuel. Diffusioncontrolled FGR is accounted for in the model by including several physical mechanisms such as the diffusion of fission gas to grain faces, grain boundary sweeping, grain growth, and intergranular bubble growth. Under temperature variations, the development of microcracks at grain faces increases FGR. Nevertheless, a fraction of the FGR during temperature transients such as loss-ofcoolant accident (LOCA) can be a result of different mechanisms (in particular, fuel fragmentation) and present an important dependency on local burnup and high burnup structure formation. This report documents a systematic study conducted to investigate the performance and necessary developments of the existing BISON fission gas release model for the analysis of LOCA-type transients. The model is applied to a UO2 Miniature Fuel (MiniFuel) example and shows an overall good qualitative agreement with experimental observations on FGR during annealing tests under different temperature conditions. It also accounts well for microstructural effects on FGR. When quantitatively compared with FGR data from previously irradiated 103 MWd/kgU UO2 discs under thermal annealing, the model shows a less satisfactory agreement with the experimental data. Finally, a UO2 MiniFuel test matrix is proposed to help to improve and validate the current FGR model in BISON.