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

Abstract

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 UO2 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 UO2 pellets is studied in simplified groundwater containing 19 mM HCO3− 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 UO2 pellet and ∼10−6 M for 0.06 wt%Cr doped UO2 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 UO2(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 UO2, 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 UO2, (ii) UO2 oxidation with O2, (iii) dissolution of U(VI) by carbonate water, (iv) reduction of oxidized U(VI) on the surface pellet by activated H2. The ability to activate the dissolved H2 is studied by implementing a kinetically controlled process of H2 activation on the Cr surface in the model.