Assessing the role of spent fuel surfaces during leaching in presence uf hydrogen by using Cr(VI) as a redox marker

摘要

In many deep repository concepts spent nuclear fuel (SNF) will be disposed in canisters containing large amounts of iron. Intrusion of groundwater in a failed canister may occur under the presence of hydrogen, expected to be produced by the anoxic corrosion of iron. Compelling evidence now exists that hydrogen inhibits oxidative dissolution of SNF, the mechanism is however not fully understood. Hydrogen generally requires a catalyst in order to operate as a reductant. The metallic inclusions (ε-particles) present in SNF are a likely catalyst for this process due to their noble metal content. There is also evidence that the SNF UO_2 matrix or doping of the UO_2 with fission products can activate hydrogen. In most spent fuel experiments carried out under hydrogen, a decrease in concentration of all redox sensitive nuclides originating from a pre-oxidized layer is observed. Given their low concentrations and abundance in the fuel, it has however been difficult to detect any reductive precipitation on the fuel surfaces. In this study, Cr(VI) oxyanions were employed as a redox sensitive marker, as Cr(VI) is expected to precipitate as Cr(III) oxide on the catalyst that activates hydrogen. In the experiments PWR spent fuel (43 MWd/kgU) was leached in simulated groundwater (10 mM NaCl, 2 mM NaHCO_3) at 25 and 70 °C under 5 MPa of hydrogen and dissolved Cr(V1). Dark green, Cr(III)-oxide was found to precipitate; mapping by electron microscopy (SEM-WDS) evidenced a Cr rich layer covering the fuel, suggesting that the whole fuel surface is catalyzing the reduction of chromium.