Characterization of the penetration mechanisms of water into polycrystalline UO2

摘要

Following containment failure in the scenario of geological disposal of spent nuclear fuel, the penetration rate of groundwater into the UO2 matrix could cause a rapid increase of the fraction of inventory becoming available for prompt dissolution. In this respect, oxygen and water diffusion mechanisms are key issues to investigate. In this work, secondary-ion-mass-. spectrometry (SIMS) depth profiling has been applied to characterize a polycrystalline UO2 pellet exposed to ~(18)O-labelled water at room temperature. ~(18)O depth profiling up to 25 μm beneath the pellet surface clearly indicates a combination of oxygen diffusion into the UO2 lattice and water diffusion along grain boundaries, behaving as high diffusivity paths. The lattice diffusion coefficient of oxygen, D_L, and the quantity δD_B - product of the grain boundary width, δ, and the grain boundary diffusion coefficient of water, D_B - have been measured, resulting in D_L = (2.5±0.1) ·10~(-24)m~2s~(-1) and δD_B = (7.5± 0.3) ·10~(-24) m~3 s~(-1).