Solubility of Uranium Oxide and Calcium Uranate in Water, and Ca(OH)₂-bearing Solutions

The solubilities of uranium(VI) oxide and a calcium uranate were measured, at 300°C and 0.5 kbar, in pure water and a 0.01 mol dm^-3 Ca(OH)₂ solution. The starting materials were characterized by XRD, microprobe, XPS and SEM, and were found to be microcrystalline UO₃ and poorly crystallized CaU_1.6O_5.8 · 2.5 H₂O. The equilibrium solubilities measured ranged between 10^-5.9-10^-6.3 mol dm^-3 for both phases, and there was only minor variation between neutral and alkaline pH. Initial uranium concentrations in solution were higher in all experiments, suggesting reduction in solubility with time, as a result of better crystallisation of the starting materials. This was confirmed by XRD and SEM after the experiments. The experimental results for uranium oxide were compared with geochemical modelling predictions using EQ3NR [1]. Agreement was poor particularly in the alkaline region. This was attributed to lack of both high temperature stability constants for some aqueous species which may be important, and of an appropriate solubility product for the reacted phase.