Dissolution mechanism of UO2 in groundwater

The dissolution mechanism of UO2 in groundwater under the disposal conditions of spent fuel has been investigated. The effects of different phenomena affecting the dissolution mechanism have been discussed based on the literature. These include the oxidative dissolution, formation of secondary phases (precipitation) on the surface of fuel matrix or elsewhere in groundwater, adsorption of the uranium species on particles in groundwater and formation of colloids. Finally, the modelling of UO2 dissolution has been discussed. In the experimental part the dissolution of the UO2 pellets under different parametric conditions in two synthetic groundwaters has been investigated. The synthetic groundwater I is simulatingthe undisturbed chemical conditions deep in granitic bedrock and the synthetic groundwater II is simulating the effects of bentonite. The pH and redox conditions were varied. The experimental solubility in synthetic groundwater I under oxidizing conditions varied 5.0 x 10-4...4.0 x 10-3 g U/l. Adsorbed and/or precipitated uranium was found in the waters. The surface analyses did not reveal the formation of secondary phases on the surface of the pellets in synthetic groundwater I. No increase in the uranium solubility in synthetic groundwater II was observed in spite of the higher carbonate content, the values varied 9.0 x 10-6...4 x 10-3 g U/l. The formation of secondary phase was observed in synthetic groundwater II in the progress of dissolution, probably having passivating effect on further dissolution. The lowest solubility values were measured at the end of the latest dissolution period. The experimental solubilities in synthetic groundwaters I and II varied under reducing conditions 2.7 x 10-6...6.9 x 10-5 and 9.0 x 10-7...3.9 x 10-5 g U/l, respectively. The analyzing of the oxidation state of uranium revealed that dissolved uranium was partly in the oxidation state of VI. The formation of secondary phase was observed in both groundwaters