Development and application of models for the safety analysis of repositories with regard to the clearance of radioactive materials for landfilling

This paper describes the results of an assessment of the radiological impact of the disposal of Very Low Level Waste (VLLW) from NPP decommissioning on landfills without mixing with conventional wastes. The transport of pollutants by unsaturated water flow using the van Genuchten approach for saturated/unsaturated porous media was calculated with the programs SiWaPro DSS and SPRING. The release of the considered radionuclides from the waste body as well as the retardation inside the landfill is modelled by the so called two-site model and with the Kd-concept using Kd-values (distribution coefficients), where the Kd-values are an expression of inherent mobility of chemicals in soil. The models of the landfills, inert (German 'Deponieklasse' DK 0), non-hazardous (DK I) and hazardous (DK II) are constructed on basis of relevant German technical standards. As radionuclides in a typical waste from decommissioning of nuclear facilities Ni-63, Sr-90, Cs-137, Pu-238, Pu-239/240, Am-241 and Uranium are considered. The calculations show that only small concentrations of Uranium, Ni-63, Sr-90, Pu-238 and Pu-239/240 penetrate through the ground of an inert landfill. For all landfill types no release of Cs-137 and Am-241 is observed. The threshold of 10 μSv/a for a member of the public will not be exceeded in the long term. (orig.)