BISON Fission Gas Behavior Modeling Capabilities at High Burnup in UO₂

While extending fuel burnup beyond current licensing practices would optimize fuel utilization, burnup extension remains limited by a lack of understanding and experimental data of high burnup (HBu) fuel performance. Beyond current burnup limits, significant microstructural changes, such as subgrain formation and increased pore density, occur in UO2 fuel, which affects its performance. Consequently, models developed for low burnup behavior become inadequate. Where restructuring happen, both in the rim region and in the dark zone deeper in the fuel, new mechanisms of fission gas behavior are enabled, which can lead to an increase in fission gas release (FGR), especially during transient conditions. The Nuclear Energy Advanced Modeling and Simulation (NEAMS) program is utilizing a multiscale approach to capture these new FGR mechanisms in the fuel performance code BISON.

In this talk, we will discuss the fission gas behavior modeling capabilities available in BISON and recent efforts to develop and validate models for high burnup conditions. The first part will begin with an overview of fission gas behavior capability for UO₂ at low burnup conditions, including validation against experimental data. Then, we will discuss recent advancements in modeling the high burnup structure and the so-called dark zone. The accuracy of these models will be quantified against experimental data. We will also highlight ongoing efforts and existing gaps in this area.