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 (FGR) 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 boundary separations (micro-cracks) in transient-tested fuel, thus indicating micro-cracking as the basic mechanism of burst release. Representing such effect is essential for the proper analysis of fission gas behaviour and of the multiple related aspects of fuel performance during reactor operational transients and design-basis accidents. Models employed in fuel performance codes typically allow for gas release only following extensive grain-boundary gas bubble interlinkage (ductile behaviour of the grain boundaries). In this work, a new model for transient fission gas behaviour in oxide fuel is developed, which introduces the alternative mechanism of gas release from the grain boundaries following micro-cracking (brittle behaviour). As a preliminary approach, a relatively simple, semi-empirical description is adopted. The treatment extends an existing model for diffusion-controlled fission gas release and swelling. The effect of micro-cracking is interpreted as a reduction of the gas storing capacity of grain boundaries during transients, effectively leading to an increase of FGR and to a corresponding decrease of fission gas swelling. The fraction of cracked grain surface is described by a temperature-dependent sigmoid function, which reproduces the characteristics of transient gas release observed experimentally both during increasing as well as decreasing temperatures. Relative to existing transient release models, no discrete temperature threshold for burst release activation is involved, thus guaranteeing continuity of the calculated fission gas concentrations in both time and space. Thus, the model is compatible with a physically sound description of the coupled fission gas release and swelling. The model is implemented in the BISON fuel performance code and applied to the simulation of fuel rod irradiation experiments involving transients. The results are presented, pointing out an encouraging predictive accuracy and a consistent representation of the experimentally observed kinetics of FGR during transients.JRC.E.3-Materials researc