SPECIATION, STABILITY AND MOBILITY OF URANIUM AND THORIUM IN HYDROTHERMAL ENVIRONMENTS

Understanding dissolution, speciation and reactivity of actinide-bearing phases in water-mediated processes at elevated temperature (T) and pressure (P) conditions is critical for tackling a range of issues in actinide science and technology from disposal of high-level nuclear wastes in a geological repository to development of robust accident-tolerant nuclear fuels. Despite the importance, studies of the interactions of actinides with water at elevated T/P are scarce. In this presentation, we will highlight our recent studies on speciation and stability of U/Th in aqueous solutions containing different ligands (chloride, sulfate) at high T/P using a suite of techniques including solubility measurements, synchrotron X-ray absorption, UV-Vis and Raman spectroscopy, combined with the autoclave or hydrothermal diamond-anvil cell technique. Our results demonstrate that the neutrally charged U/Th-ligand species are highly stable at hydrothermal conditions, and U (IV) and Th (IV) - traditionally considered to be immobile - become highly mobile in hydrothermal environments.