Microhardness and local properties of high burnup UO2 fuel

This work presents Vickers microhardness indentation and Young's modulus results obtained from light water reactor (LWR) UO2 fuel at burnup ≥ 60 GWd/tHM. The hardness profiles are compared with porosity distribution profiles measured by Scanning Electron Microscopy image analysis, with particular focus on the pellet rim, where the high burnup structure (HBS) is present. The formation of HBS and porosity build-up leads to a noticeable change of apparent microhardness in the rim zone of the fuel pellet, primarily related to porosity increase. The microhardness was also measured on a longitudinal section. At each radial position, no significant difference was observed between the microhardness at the mid-pellet and at end of the pellet, close to pellet-pellet interface. The relationship between the local values of the microhardness and the Young's modulus is also evaluated. The current data provides the basis for an assessment of microhardness and other mechanical properties in relation with local fuel structure and conditions and supports the development of predictive models in fuel performance codes like TRANSURANUS.JRC.E.2-Safety of Irradiated Nuclear Material