Dopant effect on the spent fuel matrix dissolution of new advanced fuels: Cr-doped UO2 and Cr/Al-doped UO2

To improve the fundamental understanding of the processes controlling spent fuel alteration under deep geological conditions, the influence of dopants (Cr and Cr/Al) on UO ₂ matrix stability is evaluated for “modern” types of light water reactor (LWR) fuels with additives. The uranium dissolution behavior of as-prepared 0.06 wt%Cr and 0.05 wt%Cr/0.02 wt%Al doped UO ₂ pellets is studied in simplified groundwater containing 19 mM HCO ₃ ⁻ in autoclaves under hydrogen atmosphere. Sintered disks were exposed to simulated highly carbonated conditions, representative of a repository scenario of water intrusion after a hypothetical canister failure. The uranium concentration released was ∼10 ⁻⁷ M for the 0.05 wt%Cr/0.02 wt%Al doped UO ₂ pellet and ∼10 ⁻⁶ M for 0.06 wt%Cr doped UO ₂ pellets, after 170 days. The results indicate that the amount of dissolved uranium is slightly lower compared to previous studies in absence of a reductant gas phase, but clearly above the solubility of UO ₂ (am, hyd). The initial measured pH was 8.9 ± 0.1, which gradually approached a constant value of ∼9.2 ± 0.1. Solid characterization at the end of the dissolution experiment, by SEM, Raman spectroscopy and XRD shows that the surface of all pellets remains almost unaltered. The experimental results indicate a potential oxidative dissolution of UO ₂ , which could be attributed to the presence/intrusion of dissolved oxygen in the prepared synthetic groundwater. In order to identify the mechanism of uranium release, the datasets from the batch experiments are simulated with a PHREEQC model previously calibrated with results of existing spent fuel UOx leaching experiments. The model includes the geochemical processes that are relevant for the studied experimental conditions: (i) non-oxidative dissolution of UO ₂, (ii) UO ₂ oxidation with O ₂, (iii) dissolution of U(VI) by carbonate water, (iv) reduction of oxidized U(VI) on the surface pellet by activated H ₂. The ability to activate the dissolved H ₂ is studied by implementing a kinetically controlled process of H ₂ activation on the Cr surface in the model.