Long-term safety of radioactive waste disposal: source term for performance assessment of spent fuel as a waste form. Final report

The solid waste form 'spent fuel' constitutes both the dominant radionuclide source as well as a first radionuclide retention barrier of planned future high level nuclear waste isolation systems in deep geological formations. A source term shall quantify the release/retention of individual radionuclides from the spent fuel waste package in the case of groundwater access for as much as millions of years as a function of disposal time and disposal conditions. MOX fuel shows significantly higher release rates for Pu and other radionuclides than UOX-fuel. The general reaction mechanism of the spent UOX fuel matrix dissolution was found similar in various geological formations. Differences in UOX spent fuel corrosion rates were explained by the effect of groundwater constituents on solubility, surface complex formation and electrochemical corrosion potentials. Key parameters were radiation field, redox conditions and carbonate concentrations. Salinity and humic acid concentrations were less significant. In the presence of repository rock and iron (container material) spent fuel matrix dissolution rates are similar as in their absence, but the environmental materials retained up to 99% of the total released radioactivity. Partial pressures of hydrogen (generated by container corrosion) of 2.7 bar were observed to reduce spent fuel corrosion rates by a factor of 500. Coprecipitation was found to be an important retention mechanism for actinides. Model development (electrochemical, geochemical, solid solutions, reaction path, surface complexation and radiolysis) and modelling results of various aspects (rates, solution concentrations, reaction products, corrosion potentials, pH etc.) of the spent fuel dissolution process were confirmed experimentally. For spent fuel corrosion rates are expected to remain lower than 10-6/yr. Due to uncertainties in the prediction of the temporal evolution of radiolysis effects on surface oxidation potentials, it is not yet possible to describe the transition from radiolytically dominated anaerobic to reducing environments in a realistic manner. It is recommended to treat grain boundary inventories as part of the instant release term in a source term. (orig.)