Kinetics of uranium dioxide dissolution in ammonium carbonate

The kinetics of UO2 dissolution in ammoniacal carbonate solutions were studied as a function of total carbonate concentration, oxygen pressure, temperature and pH value. The intrinsic leaching (no external potential) of UO2 agreed well with an electrochemical surface-reaction model, in which anodic oxidation of UO2 is coupled with cathodic reduction of oxygen. The experimental data corroborate an anodic dissolution sequence that includes both charge-transfer and chemical-reaction steps. Experimental evidence indicates that the mechanism involves consecutive one-electron-transfer steps. The rate equation that was developed embraces both electrode and adsorption processes. This equation provides an excellent description of the effects of oxygen pressure, total carbonate and hydroxide-ion concentrations in solution. For a potential region where charge transfer is rate-controlling, and at low total carbonate and high hydroxide-ion concentrations, the theoretical rate equation predicts a one-half and inverse one-half order dependence on total carbonate and hydroxide-ion concentration, respectively. Conversely, at high total carbonate and low hydroxide-ion concentrations, the theoretical rate expression is independent of total carbonate concentration and dependent on the one-half order of hydroxide-ion concentration. The dissolution was characterized by an experimental activation energy between 43 and 47 kJ mol-1. (author)