Evaluation of porewater chemistry in the buffer material for the second progress report H12

In the safety assessment for geological disposal of high-level radioactive wastes (HLW), porewater chemistry in buffer materials is used to estimate migration of radionuclides and corrosion of overpack materials. For the reference case in the second progress report on research and development for HLW disposal in Japan, entitled H12, porewater chemistry was evaluated by using a chemical model based on an experimental work by Oda and Shibata (1999) under the assumption of a thermodynamic system of groundwater with bentonite and corrosion products of carbon-steel overpack. This report provides the scientific information basis for the porewater chemistry evaluation, and describes the possible variations in porewater composition affected by following factors: - variations in groundwater composition relevant to the alternative geological environments cases and the perturbation scenario, and supplementary variations in groundwater composition. - model/data uncertainties associated with insufficient understanding of important processes with respect to the time-dependent behavior of a geological disposal system: in particular, how the surface reaction of smectite changes with time, how the impurities of bentonite affect porewater, and how the reactions like redox equilibria, kinetics of dissolution of accessory minerals in bentonite and precipitation of secondary minerals (including corrosion products of overpack materials) should be handled in the porewater calculations. - uncertainties of thermodynamic data of the geochemical elements. The results of calculation indicated that porewaters in the buffer material, as far as calcite is not exhausted, may vary within the range of pH from 6 to 11. It was found that important factors on the variations in porewater composition were the change of surface reactions of smectite with time, the degree of soluble impurities dissolution/dispersion and the amount of iron being supplied into the buffer region by corrosion of the overpack. Concerning only saline-reducing-high-pH groundwater, redox process of sulfur and carbon was calculated and the result suggested that it could significantly affect the porewater chemistry. (author)