Source term modeling for spent fuel elements in a rock salt repository

Between the years 1996 and 2000, the BFS (federal Office for Radiation Protection, Germany) supported the development of an integrated source term for disposal of different wastes forms, including spent fuel elements in a salt formation. The aims of this project covered reviews on the present state of knowledge on waste form reactions in salt solutions, experimental and theoretical short term radionuclide release, and modelling of the long-term evolution of the geochemical environment and the maximal radionuclide concentrations. This presentation covers specific findings by a combination of experimental and theoretical results relevant for the source term (radionuclide concentrations) in the near-field of the spent fuel elements. Geochemical modeling is based on the concept which assumes a quasi closed system in the rock salt disposal including the spent fuel and all other components present in the near-field. Parts of these components are the solutions, primary and secondary solid phases, and gaseous components. Due to an effective functioning of the near-field barriers,one can assume that exchange processes between this quasi closed system and the open natural system are limited and controlled by slow diffusive processes. The benefit from this approach lies in the fact that thermodynamic reaction path models effectively describe the systems under consideration. Kinetically controlled radionuclide release processes account only for the easily soluble radionuclides (e.g. Cs), but not for the actinides. Thermodynamic/geochemical modeling show that under the conditions of a deep repository in rock salt, the dominating corrosion of steel canisters are Fe(II)-Fe(III) compounds. Fe(II) may be oxidized to Fe(III) and which compete for oxidizing species produced e.g. by radiolysis. Results of experiments in the presence and absence of corroded iron significantly: In the presence of iron, release rates were found to be lower and the measured actinide concentrations were lower,too. In the experiments, an increase of the pH was measured. This effect can be modeled by assuming an inhomogeneous Cs distribution on the fuel grains. (Author)