Oxidation and dissolution of UO2 in bicarbonate media: Implications for the spent nuclear fuel oxidative dissolution mechanism

The objective of this work is to study the UO2 oxidation by O2 and dissolution in bicarbonate media and to extrapolate the results obtained to improve the knowledge of the oxidative dissolution of spent nuclear fuel. The results obtained show that in the studied range the oxygen consumption rate is independent on the bicarbonate concentration while the UO2 dissolution rate does depend on. Besides, at 10−4 mol dm−3 bicarbonate concentration, the oxygen consumption rate is almost two orders of magnitude higher than the UO2 dissolution rate. These results suggest that at low bicarbonate concentration (<10−2 mol dm−3) the alteration of the spent nuclear fuel cannot be directly derived from the measured uranium concentrations in solution. On the other hand, the study at low bicarbonate concentrations of the evolution of the UO2 surface at nanometric scale by means of the SFM technique shows that the difference between oxidation and dissolution rates is not due to the precipitation of a secondary solid phase on UO2.