Novel insights in the study of water penetration into polycrystalline UO2 by secondary ion mass spectrometry

Oxygen and water species (e.g., OH−, H2O) diffusion in polycrystalline UO2 are key parameters to investigate in order to predict spent fuel behaviour in a final repository in the event of exposure to groundwater. In this work, 18O tracer diffusion is evaluated by means of secondary ion mass spectrometry on a UO2 pellet exposed to 18O-labelled water at room temperature for 3 months. Depth profiling up to 22 μm beneath the pellet surface clearly indicates a combination of oxygen diffusion into the UO2 lattice and water species diffusion along grain boundaries, behaving as high-diffusivity-paths. The relevant coefficients have been estimated around 1.4 × 10−24 m2 s−1 for lattice diffusion and around 2.3 × 10−16 m2 s−1 for grain boundary diffusion.