Leaching of UO2 pellets doped with alpha-emitters (238/239Pu) in synthetic deep Callovian-Oxfordian groundwater

Abstract

The reactivity of a polycrystalline UO₂ surface under alpha irradiation in contact with groundwater is investigated, in the hypothesis of direct disposal of spent fuel in a deep geological repository. Two series of plutonium-doped UO₂ samples (specific alpha activity of 18 and 385 MBq·g⁻¹_UO2) were leached in a synthetic Callovian-Oxfordian deep groundwater under anoxic conditions (Ar/CO₂ 3000 ppm, 3.5 bar relative pressure) to assess both the impact of alpha radiolysis of water and the complexing capacity of the groundwater ions on the dissolution of UO₂. This study follows a prior one performed in pure and carbonated waters. Firstly, technical developments were necessary for the analyses in the groundwater solution because of its high salt concentrations: quantification limits were determined for the measurement of uranium and radiolytic H₂O₂ traces in this medium. Secondly, given the very high reactivity of these samples in the presence of air and in order to minimize any prior surface oxidation, a strict experimental protocol was followed, based on high-temperature annealing in Ar + 4% H₂ with preleaching cycles. Each type of UO₂ pellet was then leached under static conditions for 30 days (anoxic conditions, deep groundwater solutions). Results on the evolution of uranium releases are presented. For the lowest alpha activity (18 MBq·g⁻¹_UO2), uranium releases in groundwater were below the quantification limit of 2 × 10⁻⁸ mol·L⁻¹ with a kinetic phosphorescence analyzer, even after 30 days. However, for higher alpha activity (385 MBq·g⁻¹_UO2) the uranium releases begin to exceed the quantification limit after 14 days of leaching and then increase exponentially. This increase is comparable to results previously obtained in carbonated solutions.