Performance assessment of each barrier consisting of geologic disposal system for high-level radioactive wastes is carried out quantitatively, and key radionuclides and parameters are pointed out.
Chemical compositions and solubilities of radionuclides under repository conditions are determined by PHREEQE code starting from compositions of granitic groundwater observed in Japan. Glass dissolution analysis based on mass transfer theory and precipitation analysis have been done in order to determine the inner boundary condition for radionuclide diffusion through a bentonite-filled buffer region, where multi-member decay chain and isotopic sharing of solubility at the inner boundary are considered. Natural barrier is treated as homogeneous porous rock, or porous rock with infinite planar fractures. Performance of each barrier is evaluated in terms of non-dimensionalized hazard defined as the ratio of annual radioactivity release from each barrier to the annual limit on intake.
At the outer edge of the engineered barriers, 239Pu is the key nuclide to the performance, whereas at the exit of the natural barrier, weakly-sorbing fission product nuclides such as 135Cs, 129I and 99Tc dominate the hazard.
