LAUSR

Abstract Vickers indentation was used to evaluate the hardness and the apparent fracture toughness of unirradiated and irradiated UO2 fuels, with the objective to determine the effects of the irradiation on mechanical properties. Indentation tests were realized at room temperature on standard and doped UO2 fuels, irradiated in a pressurised water reactor up to a burnup of 81.6 GWd/tM. For the highest burnups, the fuels had restructured in a high burnup structure (HBS) on the periphery of the pellets. More than 100 valid impressions for the Vickers hardness determination were obtained on each fuel. The Vickers hardness of both UO2 increased with the local burnup, due to irradiation defects such as dislocations and fission products but decreased when the fuel restructured into the HBS. Valid crack patterns for the Vickers fracture toughness were only obtained for indentations inside the large grains of irradiated doped UO2 fuels or for both restructured fuels. The Vickers fracture toughness of the non-restructured doped UO2 was shown to increase quickly during the first irradiation cycle but was then constant with increasing local burnup. This increase of the apparent fracture toughness was mainly due to the decrease of the crack length from the corner of the indents, attributed to the interactions of the cracks with the defects such as bubbles or fission product precipitates and associated residual stresses. In the HBS, short cracks from the corner of the indents that followed grain boundaries of small grains led to high fracture toughness values. However, the use of the literature equations to calculate the apparent fracture toughness on porous samples such as HBS is questioned. All tests clearly illustrated the weakness of the grain boundaries in irradiated fuels.