Sensitivity analyses of fission gas release modelling

Fission gas release modelling is a key aspect of fuel performance assessment, and quantifying the associated uncertainties remains a challenge. This work performs two sensitivity analyses to build confidence in recently developed athermal and burst fission gas release models. The computational framework adopted for fuel performance simulations couples the fission gas behaviour module of SCIANTIX with the TRANSURANUS fuel performance code. This system is interfaced with Dakota software to perform a global sensitivity analysis based on variance decomposition. Five parameters are investigated for the athermal release in the US PWR 16x16 base-irradiation case, i.e.: fabrication porosity, grain-edge length, burn-up, temperature, and fission rate. Results highlight the dominant role of fabrication porosity, suggesting the need for future modelling efforts on fuel densification. For the burst gas release from fuel microcracks five parameters are studied in the HATAC C2 rod irradiation experiment, i.e.: grain-boundary energy, bubble radius and internal pressure, temperature, and hydrostatic stress. Grain-boundary energy, which controls the microcracking threshold, emerges as the most relevant parameter. Furthermore, the analyses highlight the need for enhanced lower-scale physical modelling to further reduce empirical assumptions. The outcome of this study is twofold: an operational range of parameter values resulting in a calculated fission gas release within a factor two from the experimental value, and a methodology for testing and interpreting model behaviour within the TRANSURANUS–SCIANTIX framework prior to engineering-scale validation.