Corrosion and dissolution studies of UO2 containing α-emitters

Corrosion and dissolution of UO2 doped with short-lived α-emitters, the so called α-doped UO2, was investigated in leaching experiments combined with X-ray Photoelectron Spectroscopy (XPS) analyses. These experiments are part of an ongoing set of tests aiming at evaluating the effects of α-radiolysis on the dissolution of the UO2 spent fuel matrix when it will become exposed to groundwater in a deep final repository. Pellets of UO2 containing ∝ 10 and ∝ 0.1 wt.% 238Pu were used. These compositions simulate the α-radiation field of different types of commercial LWR spent fuel after different storage times. Undoped UO2 pellets were also used as reference. Static sequential leaching experiments in demineralised water at room temperature under anoxic conditions confirmed that higher fractions of uranium are released in the case of α-doped materials compared with undoped UO2. Relatively large amounts of plutonium were found in the leachate of the materials doped with 238Pu. Significant contributions to the total amount released of uranium were found in the rinse solutions of the leaching vessels. The evolution of the U(VI)/U(IV) ratio on the leached surfaces as a function of contact time was studied by deconvolution of the 4f7/2 uranium peak obtained from the XPS spectra. In the case of undoped UO2, after ∝ 1000 h of leaching the increase of the U(VI)/U(IV) ratio stabilized to a level corresponding to ∝ UO2.33. The XPS results for both α-doped materials showed a continuing increase of the U(VI) fraction on the surface with increasing leaching times after the first ∝ 10 h. The rate of increase depended on the α-activity of the fuel sample, hence on the amount of radiolysis in the water. After ∝ 1000 h of leaching, essentially only U(VI) was detected on the surface of the high activity α-doped UO2. The presence of a yellowish layer of reprecipitated U(VI) phases covering the surface could be observed optically already after ∝ 100 h of leaching. (orig.)