To understand the mechanism of fuel oxidation, in the event of exposure of spent nuclear fuel to groundwater in a final repository, the key parameters are the diffusion coefficients for oxygen and water in UO 2 . In the temperature range relevant for geological disposal (20-100 C), we conducted static corrosion experiments, for 3-6 months, with 18 O-labelled water on different UO 2 matrices (single-crystal, polycrystalline UO 2 , SIMFUEL) so that we could evaluate the penetration of the 18 O-tracer by means of secondary ion mass spectrometry depth profiling, measuring the 18 O/ 16 O ratio. Fick's second law was applied to fit the diffusion profiles in the single crystal and to determine the chemical diffusion coefficient of oxygen, which was found to be in good agreement with the Arrhenius plot extrapolation from high-temperature literature data. Profiles of polycrystalline UO 2 could be fitted with the Levine-MacCallum model, showing that grain boundaries behave like high-diffusivity paths. The presence of secondary phases precipitated along the grain boundaries, as in the case of SIMFUEL, showed a hindering effect for this grain-boundary diffusion, reducing the measured penetration depth.
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 18O-labelled water at room temperature. 18O 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, DL, and the quantity δDB – product of the grain boundary width, δ, and the grain boundary diffusion coefficient of water, DB – have been measured, resulting in DL = (2.5 ± 0.1) × 10-24 m2 s-1 and δDB = (7.5 ± 0.3) × 10-24 m3 s-1.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.