Corrosion and dissolution studies of UO2 containing alpha-emitters

摘要

Corrosion and dissolution of UO2 doped with short-lived alpha-emitters, the so called alpha-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 alpha-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 similar to 10 and similar to 0.1 wt. % Pu-238 were used. These compositions simulate the alpha-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 alpha-doped materials compared with undoped UO2. Relatively large amounts of plutonium were found in the leachate of the materials doped with Pu-238. 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 4f(7/2) uranium peak obtained from the XPS spectra. In the case of undoped UO2, after similar to 1000 h of leaching the increase of the U(VI)/U(IV) ratio stabilized to a level corresponding to similar to UO2.33. The XPS results for both alpha-doped materials showed a continuing increase of the U(VI) fraction on the surface with increasing leaching times after the first similar to 10 h. The rate of increase depended on the alpha-activity of the fuel sample, hence on the amount of radiolysis in the water. After similar to 1000h of leaching, essentially only U(VI) was detected on the surface of the high activity alpha-doped UO2. The presence of a yellowish layer of reprecipitated U(VI) phases covering the surface could be observed optically already after similar to 100 h of leaching. [References: 20]