Post-Irradiation Examination to Assess Performance and Safety of Nuclear Fuel

Nuclear fuel performance and safety are inseparable, necessary attributes of successful operation in the reactor and of spent fuel management. The combined effects of the irradiation conditions and of the thermo-mechanical operating regime determine the evolution of key properties during nuclear fuels irradiation (e.g. fission products distribution and behaviour, structure alterations, thermophysical and mechanical properties). A high degree of heterogeneity characterizes irradiated fuel structure and composition. Further property evolution may affect spent fuel during extended storage, albeit to a smaller extent than during reactor operation. This work presents an overview of results from testing campaigns performed at JRC-ITU on irradiated fuels and analogues aimed at determining safety- and performance-relevant properties. Their applications to describe fuel behaviour during irradiation and spent fuel evolution during storage are highlighted. High burnup light water reactor (LWR) UO2 fuel (≥ 60 GWd/tHM) was used for mechanical property studies. Radial profiles of microhardness and porosity distribution obtained at different pellet axial region were compared with corresponding Young's modulus data. The effects related to the presence of the high burnup structure (HBS) were observed, namely a decrease of the apparent hardness and an increase of porosity. This data was complemented by new microstructural characterization of HBS specimens, which revealed a complex intra-granular configuration of the sub-micron grains. Additionally, possible ageing effects due to accumulation of microstructural decay damage during storage were investigated and compared to the results obtained under accelerated ageing conditions using alpha-doped analogues testing. No significant hardness increase was observed on the same high burnup fuel after >10 years of storage in hot cell; this supports the finding that hardening saturation occurs relatively early during spent fuel storage. The ultimate goal of these experimental campaigns, ...