Radionuclide release from spent nuclear fuel in sealed glass ampoules

Abstract

Radiation from spent nuclear fuel affects the redox chemistry of water in the proximity of the fuel surface. The overall effect of the radiolysis is oxidizing so that U(IV) in the spent nuclear fuel is oxidized to U(VI). The dissolution of U(VI) is relatively fast, especially when the water contains bicarbonate. Based on this, radiolytic oxidation is considered the main driving force for spent fuel dissolution and radionuclide release in a spent fuel repository environment, and many studies over the years have aimed to further our understanding of this process and how it is influenced by available redox active components in open and closed systems. Here, data from sealed glass ampoules in which spent fuel is leached for one and five years, are compared with published data from a previous similar experiment. With regards to evolution of radiolytic gases, the results are comparable to the results from the previous experiments: a steady state with regards to oxygen and hydrogen composition is observed. An unexpected observation is that some ampoules with old, pre-oxidized fragments produced less radiolytic gases. These older fragments release a larger fraction of the radionuclide inventory of the samples, yielding concentrations that are on the same level as previously published data. The data from the old fragments presented here are affected by the pre-oxidation and prolonged washing procedure, which confounds the interpretation of those data. New, recently prepared fragments yield data that are easier to interpret. The radionuclide concentrations in the ampoules with new fragments are much lower; uranium concentrations of ca 5E-6 M after one year, and ca 3E-5 M after five years. Using the calculated radionuclide inventory in the fuel samples, an apparent radionuclide release rate in these initially anoxic systems, based on U release, is ca 3E-5 per year.