Characterization of the Penetration Mechanisms of Water into Polycrystalline UO₂

Abstract

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.

Room‐temperature diffusion coefficients for oxygen and water in UO2 matrices: a SIMS study

“…Depth profiles acquired on polycrystalline UO 2 and reported in previous works showed the presence of two different diffusion regimes. This is highlighted in Fig.…”

“…to retrieve a measurement of the volume diffusion coefficient . The value D L = (2.5 ± 0.1) ⋅10 −24 m 2 s −1 (which is in good agreement with Fayek and Kyser) was then obtained . The long‐range profile can instead be fitted with the Levine–MacCallum equation for diffusion into polycrystalline bodies, where α is a factor depending on the quantities t , D L , the grain‐boundary diffusion coefficient D B and the grain boundaries width, δ , through the equation: …”

“…Retrieving α from the fit shown in Fig. b and considering δ ~ 1 nm, we could calculate the value D B = 7.5 ⋅ 10 −15 m 2 s −1 for the grain boundary diffusion . As SIMS depth profiles followed the penetration of the tracer‐isotope 18 O regardless from its chemical form, it is unclear, at this stage, which one of the aforementioned water species this coefficient refers to.…”