A SIMPLE THERMODYNAMICAL MODEL TO DESCRIBE THE CONTROL OF THE DISSOLUTION OF URANIUM-DIOXIDE IN GRANITIC GROUNDWATER BY SECONDARY PHASE-FORMATION

Uranium dioxide leaching tests conducted at 96 degrees C in a syntheti c 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, RES and XPS) are given. The formation of a secondary phase has been clearly shown under oxidizing conditions for high SN ratio and long duration tests and only suspect ed under reducing conditions. A simple thermodynamical model based on chemical equilibria has been built to predict the total uranium conten t in the leachate in function of pH and total aqueous carbonate concen tration. Then, the model has been applied in the case of the formation of partially dehydrated schoepite [UO2(OH)(2)] under oxidizing condit ions. The results have been compared both with the experimental data a nd theoretical calculations from PHREEQC. The agreement obtained is re latively good. For reducing conditions, model predictions only agree w ith experimental data and do not fit PHREEQC calculation results. (C) 1998 Elsevier Science B.V. All rights reserved.