A simple thermodynamical model to describe the control of the dissolution of uranium dioxide in granitic groundwater by secondary phase formation

Abstract Uranium dioxide leaching tests conducted at 96°C in a synthetic granitic water under oxidizing or reducing conditions are discussed. Results concerning the total uranium content in the leachates (ICP-MS) and solid surface characterization (SEM, RBS and XPS) are given. The formation of a secondary phase has been clearly shown under oxidizing conditions for high S/V ratio and long duration tests and only suspected under reducing conditions. A simple thermodynamical model based on chemical equilibria has been built to predict the total uranium content in the leachate in function of pH and total aqueous carbonate concentration. Then, the model has been applied in the case of the formation of partially dehydrated schoepite [UO 2 (OH) 2 ] under oxidizing conditions. The results have been compared both with the experimental data and theoretical calculations from PHREEQC. The agreement obtained is relatively good. For reducing conditions, model predictions only agree with experimental data and do not fit PHREEQC calculation results.