Sensitivity analysis for scenarios relevant to evolution of overpack and buffer material

Takeda, Seiji ; Watanabe, Masatoshi; Kimura, Hideo

This study focuses on a deterioration of expected barrier functions relevant to evolution of overpack and sand-bentonite buffer material in engineered barrier system in the geological disposal of high-level radioactive waste. Release rates for important radionuclides, Cs-135 and Se-79, were estimated from Monte Carlo-based analysis for the early failure of overpack and the buffer alteration scenarios. One of the possible early failure scenarios is the change of glass dissolution rate, effective diffusion coefficients and distribution coefficients in the buffer material, which are affected by early high temperature in the engineered barrier. The analytical result indicates that this scenario has no detrimental effect on the maximum release rates of these radionuclides. Another scenario is the occurrence of the increase of Se solubility correlated with the enhanced redox potential due to the radiolysis after the early failure of overpack. This scenario allows a conspicuous increase in the maximum release rates of Se-79 from the buffer, which is over one order of magnitude higher than that in the normal scenario. In addition, with the comparison of the maximum release rates between the early failure scenario and the buffer alteration scenario, it is suggested that the effect of the deterioration of the expected barrier functions relevant to the buffer alteration on the release rates is more remarkable than the effect on early failure.