QUANTITATIVE PERFORMANCE ALLOCATION OF MU LTIBARRIER SYSTEM FOR HIGH-LEVEL RADIOACTIVE-WASTE DISPOSAL

Performance assessment of each barrier consisting of geologic disposal system for high-level radioactive wastes is carried out quantitativel y, and key radionuclides and parameters are pointed out. Chemical comp ositions 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 ma ss transfer theory and precipitation analysis have been done in order to determine the inner boundary condition for radionuclide diffusion t hrough a bentonite-filled buffer region, where multi-member decay chai n and isotopic sharing of solubility at the inner boundary are conside red. Natural barrier is treated as homogeneous porous rock, or porous rock with infinite planar fractures. Performance of each barrier is ev aluated 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, Pu-239 is the k ey nuclide to the performance, whereas at the exit of the natural barr ier, weakly-sorbing fission product nuclides such as Cs-135, I-129 and Tc-99 dominate the hazard.