Dissolution of Unirradiated UO2 Fuel Under Oxidizing Conditions: Evaluation of Solubility (Steady State) Limiting Factors (EQ3/6)

Abstract

ABSTRACTThe solubility behavior of unirradiated UO2 pellets was studied under oxic, air-saturated conditions in deionized water, in NaHCO3 solutions and in two types of synthetic groundwater (25°C). The Allard groundwater represents natural fresh groundwater conditions at great depths in granite bedrock and bentonite groundwater simulates the effects of bentonite on granitic fresh groundwater. The release of uranium was measured during static batch dissolution experiments of long duration (6 years). A comparison was made with the theoretical solubility data calculated with the geochemical code, EQ3/6, in order to evaluate solubility (steady state) limiting factors. Various hypotheses for redox control (redox potential of the bulk solution or redox potential at the surface) were tested in the modeling calculations.The measured concentrations for uranium at steady-state in deionized water were equal to the solubility of schoepite (PO2 = 0.2 atm). In NaHCO3 solutions with lower carbonate concentrations (0.98 – 1.96 mmol/1) and in Allard groundwater they were close to the calculated solubilities of U at the U3O7/U3O8 redox potential. In bentonite groundwater, the results suggest the formation of a secondary phase with a lower solubility. Only uranium oxide with a crystal structure of uraninite (UO2 - U3O7) was identified in all waters, when analyzing particulate material in the solutions after contact with UO2 pellets.