Dissolution of unirradiated uranium oxide pellets at various parametric conditions

The solubility of unirradiated UO2 fuel pellets in synthetic groundwater has been investigated. A sequential dissolution procedure was used, and the extent of dissolution was measured by activation analysis for uranium in groundwater. Plated-out and/or precipitated material were included in the analysis. Dissolution was performed under different pH and redox conditions in two synthetic groundwaters, one simulating the undisturbed chemical conditions deep in granitic bedrock and the other simulating the effects of bentonite. The measured uranium concentrations in oxidizing conditions in the undisturbed synthetic groundwater and bentonite water varied 1-4 and 1-2 mg U/l, respectively. The concentration of uranium under reducing conditions decreased rapidly already during the shorter dissolution periods being 1-25 μg U/l in the undisturbed synthetic groundwater and 10-190 μg U/l in bentonite water. The dissolution as a function of pH gave the lowest solubilities for uranium at the neutral or slightly alkaline pH region 8-10 expected for deep groundwater