Modelling of Burst Release in Oxide Fuel and Application to the Transuranus Code

The substantial release of fission gas during temperature transients (burst release) can be critical during operational reactor transients and (design-basis) accidents. A purely diffusion-based model cannot explain the rapid kinetics of the process. In this work, we present a model for transient fission gas release in oxide fuel. This model arises from experimental observations relative to both in-reactor irradiation and post-irradiation annealing of UO2 fuel. In particular, micrographs demonstrate the presence of grain-face separations (micro-cracks) in transient-tested fuel, thus indicating that a basic mechanism of burst release is micro-cracking. The presented model extends a previously developed diffusion-based model, introducing the effect of micro-cracking. This is interpreted as a reduction of the grain-face gas inventory and storing capacity during transients. The process is modelled through an empirical temperature-dependent function based on the experimentally observed characteristics of gas release during both heating and cooling transients. The model also includes an irradiation-induced micro-crack healing process, which gradually restores the original grain-face gas storing capacity. This process is described by means of an empirical burnup-dependent function. The resulting extended model (diffusion-based fission gas behaviour together with transient release) is overall semi-empirical, but the burst release capability notably preserves the continuity in time and space as well as the consistent coupling of the calculated fission gas release and swelling. The new model was originally implemented in the fuel performance code BISON. In this work, we implemented the model in the TRANSURANUS fuel performance code, and applied it to the simulation of some light water reactors fuel rod irradiation experiments of the OECD/NEA International Fuel Performance Experiments database. The results point out a representation of the kinetics of burst release consistent with experimental evidence.