Reply to the comments by J.H. Evans about two papers `Thermal treatment of UO 2 irradiated in a pressurized water reactor: swelling and release of fission gases' and `Microstructural analysis and modelling of intergranular swelling of an irradiated UO 2 fuel treated at high temperature' by I. Zacharie, S. Lansiart, P. Combette, M. Trotabas, M. Coster and M. Groos

Abstract

In the rim zone of UO2 nuclear fuel pellets, the combination of high burnup and low temperature drives a microstructural change, leading to the formation of the high burnup structure (HBS). In this work, we propose a semi-empirical model to describe the formation of the HBS, which embraces the polygonisation/recrystallization process and the depletion of intra-granular fission gas, describing them as inherently related. For this purpose, we performed grain-size measurements on samples at radial positions in which the restructuring was incomplete. Based on these new experimental data, we infer an exponential reduction of the average grain size with local effective burnup, paired with a simultaneous depletion of intra-granular fission gas driven by diffusion. The comparison with currently used models indicates the applicability of the herein developed model within integral fuel performance codes.