Impact of dopants and leachants on modern UO2-based fuels alteration under final storage conditions: Single and joint effects

Abstract

Doped-UO₂ fuels such as Cr- or Cr/Al-UO₂ (accident-tolerant fuels (ATF) or modern nuclear fuels) and Gd–UO₂ fuel (being Gd a burnable neutron absorber now in use in LWR fuels) need to be deeply studied not only relating to its advantages under normal and accident conditions in operation, but also its behavior under different repository conditions. After the geologic repository post-closure, once the spent nuclear fuel come into contact with groundwater after container failure, the release of some radionuclides will rely on the UO₂ matrix dissolution processes. The corrosion/dissolution behavior of doped UO₂ fuels, including ATF, in a deep geological repository is barely comprehended. The join influence of dopants and groundwater composition needs further enhance knowledge and understanding by filling gaps in the empirical databases. This study examines the impact of Cr, Cr/Al and Gd dopants on the corrosion of UO₂ fuel pellets in groundwater, based on the known benefits of adding certain soluble metal oxides to UO₂, depending on the nature of the doping element. Systematic dissolution experiments were conducted with Cr-, Cr/Al-, and Gd-doped UO₂ pellets in three aqueous media with varying pH and HCO₃⁻ concentrations. Groundwaters used with increasing the complexity of the system (i.e. contain many ionic species) were 20 mM NaClO₄ (pH 7.2), 19:1 mM NaHCO₃:NaCl (pH 8.9), and synthetic young cement water with Calcium (pH 13.5). The experiments revealed that the leachant attributes, particularly the combined effects of pH, redox conditions, and HCO₃⁻, had a more significant impact on uranium concentration and dissolution rates than the dopants themselves. No secondary uranium phases were observed on the surfaces of any post-leached samples. These findings contribute to the understanding of the combined effects of doping and aqueous composition on the dissolution behavior of modern nuclear fuels under long-term conditions anticipated in a deep geological repository.