Room-temperature microindentation behaviour of LWR-fuels, part 1: fuel microhardness

Thorough microhardness (HV) measurements in LWR–UO2 fuels in the burn-up range 40–100 GWd/t M are described. Two softening processes are found to occur in the fuels during irradiation. The first one appears at the pellet periphery when the local burn-up exceeds ≈70 GWd/t M reflecting mainly the porosity build-up after rim restructuring. The second one affects the whole fuel when it reaches on average ≈70 GWd/t M denoting a bulk irradiation damage release. The rim material is found to acquire the stiffest possible configuration of porous solids showing the lowest property drop as a function of porosity and the largest admissible porosity (≈30%) without undergoing pore-phase opening. By assimilating the hardness data to other property vs. porosity relations, it is predicted that the porous rim material would show a higher Young modulus and thermal conductivity than a low burn-up fuel at equivalent porosity. This ensures not only a stiffer but also a cooler behaviour in reactor.