THE KINETICS OF DISSOLUTION OF UO2 UNDER REDUCING CONDITIONS AND THE INFLUENCE OF AN OXIDIZED SURFACE-LAYER (UO2+X) - APPLICATION OF A CONTINUOUS FLOW-THROUGH REACTOR

We have studied the kinetics of dissolution of uranium dioxide, UO2(s), under strongly reducing conditions (H2(g)/Pd). We have investigated the dependence of the rate of dissolution as a function of critical geochemical parameters: pH, pCO2, and carbonate concentration. By using a stirred batch reactor, the kinetics of dissolution of an UO2+x surface layer and the subsequent precipitation of UO2 have also been studied. Although the initial UO2 is pretreated before starting the experiments, it seems to be very difficult to avoid the formation of the oxidized surface layer. For this reason we have developed a thin layer continuous flow-through reactor in order to study the kinetics of dissolution of pure UO2(s). The dependence of the rate of dissolution of these solids on the proton concentration of the solution may be expressed according to the following equations: [GRAPHICS] The mechanism of dissolution of these solids is surface controlled. The rate of dissolution of UO2 in acidic solutions may be described in terms of an integer dependence on the activity of the protonated surface complexes: [GRAPHICS]