Modelling of transient fission gas behaviour in oxide fuel and application to the BISON code

Experimental observations relative to both in-reactor irradiation and post-irradiation annealing of oxide nuclear fuel indicate that substantial fission gas release can occur on a small time scale during temperature transients (burst release). The rapid kinetics of the process cannot be interpreted as purely diffusion-controlled. Micrographs demonstrate the presence of patterns of grain-face separations (micro-cracks) in transient tested fuel, thus indicating micro-cracking as the basic mechanism of burst release. In this work, a new model for transient fission gas behaviour in oxide fuel is developed. The treatment extends a previously developed model for diffusion-controlled fission gas release to introduce the effect of micro-cracking, which is interpreted as a reduction of the grain-face gas inventory and storing capacity during transients. The process is characterized through an empirical temperature-dependent function, based on the experimentally observed characteristics of gas release during both heating and cooling transients. The model is implemented in the BISON fuel performance code and applied to the simulation of fuel rod irradiation experiments. The results are presented, pointing out a consistent representation of the kinetics of fission gas release during transients.