The effect of [Fe]TOT on the dissolution of synthetic Pb-doped UO2 and Th-doped UO2

The dissolution of synthetic Pb-doped UO2 and Th-doped UO2 was systematically studied to determine the influence of leach parameters [Fe]TOT and ORP under standard leach conditions of: T = 50 °C, [H2SO4] = 15 g/L (0.15 M), and UO2 = 100 mg/L. Results demonstrated reduced uranium dissolution in both systems compared to pure UO2. This effect was greatest for Th-doped UO2. The decrease in uranium dissolution between the doped systems and pure UO2 was attributed to the formation of precipitate layers at the surface of the solid, slowing down or blocking uranium release. In the case of Pb-doped UO2, the formation of a Pb sulphate phase was directly detected but in the case of Th-doped UO2, no layer was found. For the latter system it was postulated that passivation of the Th-doped UO2 surface occurs due to the formation of oxidised Th-rich phases Th(OH)4, ThO2 and ThO2·nH2O at the surface of grains preventing uranium release. In tests varying the ORP, there was an approximately linear dependence of the dissolution rate on [Fe]TOT for both systems however the rate orders indicated a step change between an ORP of 420 and 460 mV. The specific influence of FeII showed that both Pb-UO2 and Th-UO2 exhibited two distinct regions of dissolution rate dependency similar to that previously noted for pure UO2.