The effect of dissolved hydrogen on spent nuclear fuel corrosion

Doctoral thesis, 2009

In case of a canister failure in a deep bedrock repository for nuclear fuel, the release of radiotoxic nuclides to the groundwater will depend on the chemical environment near the fuel surface. Due to the presence of large amounts of iron in the canister, hydrogen will be formed if the canister becomes groundwater flooded. To elucidate if this hydrogen influences the nuclear fuel corrosion, i.e., the release of radionuclides, an irradiated MOX fuel, a high burn-up UO2 fuel and an alpha-doped UO2 fuel have been studied under simulated repository conditions. It was found that the fuel surfaces became protected from oxidation by the presence of hydrogen and that the radionuclide release ended. It was further demonstrated that the amount of dissolved uranium in the leachate gradually decreased and, regardless of fuel type, reached a value below the reported solubility limit for UO2(am). Neither strong alpha-radiation fields, nor intense mixed radiation fields, were found to increase the concentration of U in solution. Furthermore, it was shown that a protecting effect of hydrogen is obtained in presence of pure 10% 233U-doped UO2 and that the dissolved uranium concentration in contact with this fuel decreased to 9*10-12 M. The solubility limiting phase governing this uranium concentration is likely to be more crystalline than any other previously observed UO2 precipitate formed in contact with water at near neutral pH. As a result, the hydrogen will completely protect the spent nuclear fuel from corrosion until its activity has reached below the alpha-activity threshold, i.e., until the activity of the fuel is too low to sustain fuel oxidation.