Anodic dissolution of UO2 in aqueous alkaline solutions

In the nuclear fuel reprocessing, the dissolution of spent fuels is one of the key processes, because uranium dioxide (UO2) as the main component is poorly soluble in aqueous solutions and its soluble conditions are limited. For example, in the PUREX method, the dissolution of the spent fuel is carried out in HNO3 solutions of high concentrations at boiling temperature. Such severe conditions should not be desirable considering the corrosion of dissolution vessel. Our earlier study on the dissolution mechanism of UO2 using 17O-NMR showed that the dissolution of UO2 is controlled by an electron transfer from UO2 to oxidant. This mechanism indicates that UO2 could be dissolved under mild aqueous conditions, if a strong oxidant exists in solutions. As a mater of fact, UO2 powders were dissolved in 3M (M=mol/dm3) HNO3 solutions at room temperature in the presence of Ce(IV) and dissolved more easily in 1 M HCl solutions with a bubbling of ClO2. These results suggest that the dissolution of UO2 might be possible in non-acidic solutions with the aid of an oxidant. However, a strong oxidant is difficult to obtain in non-acidic aqueous solutions. In the present study, the dissolution behavior of UO2 has been investigated by means of the anodic oxidation instead of strong oxidants in alkaline solutions. In order to avoid the formation of uranium precipitate, NaHCO3 solutions in which U(VI) forms stable and soluble carbonate complexes are used as an electrolytic solution. (author)