Uranium Dissolution in Hyperalkaline TMA-OH Solutions: Preliminary Results

Leaching experiments were performed with depleted UO2 powders in tetramethylammonium solutions (TMA-OH) at pH 13.5 and 12.5, and at different UO2 surface area to volume of solution (SA/V) ratio's to determine the solubility and the dissolution kinetics of UO2 at high pH in absence of cations dominating cementitious waters (Ca,Na, K).The solubility of UO2 increased from pH 12.5 to 13.5 and by increasing the SA/V ratio up to 100 m-1. However, no known U secondary-phases were predicted by geochemical calculations to control the measured U-concentrations. A two-step dissolution process was put forward: 1-a fast initial rate, surface controlled and hydroxo promoted and 2-a sorption process at low SA/V ratio or a continuous residual dissolution process at high SA/V ratio.