High burnup changes in UO2 fuels irradiated up to 83GWd/t in M5 claddings

Since the beginning of the 90's EDF and AREVA NP have irradiated lead assemblies with M5 cladded fuels up to very high burnups. An overview of the main post irradiation examination results obtained on high burnup UO2 pellets show, the increase of the fission gas release rate, the increase in the fuel swelling, the formation of fission gas bubbles all over the pellets. The amounts of xenon were quantified and various phenomena leading to this gas bubble formation identified. All these examinations give valuable data on the complex state of the fuel during irradiation. They show the good behavior of these fuels, exhibiting various microstructures at very high burnups, none of which is likely to lead to particular problems during irradiation