坑道閉鎖試験に基づく坑道掘削・閉鎖時の化学環境変化プロセスの考察

抄録

In assessing the safety of geological disposal of high-level radioactive waste, groundwater chemistry is an important factor influencing the performance of engineered materials and the behavior of radionuclides. It is essential to understand the hydrochemical changing process and stable condition after the closure of the disposal facilities. In this study, a simulated experimental drift was constructed in granite at a depth of 500 m at the Mizunami Underground Research Laboratory, and the hydrochemical condition and related processes after drift closure were observed. The groundwater flow and chemistry around the excavated drift changed owing to water inflow along fractures into the drift. The redox potential increased owing to the infiltration of oxygen from the drift. After closing the drift, the groundwater evolved chemically with time owing to mixing of the groundwaters derived from surrounding rocks, dissolution of shotcrete, and microbial reactions. The redox potential plunged to the initial baseline value in three months owing to the reducing activity of microbes, while the groundwater became alkaline owing to the influence of the dissolved shotcrete. If the content of a pH-buffer mineral in a rock is small compared with the cementaceous materials used, the alkaline groundwater probably spreads broadly in surrounding rock. In the future, it is necessary to verify the water chemistry evolved by the interaction between the alkaline groundwater and granite minerals.