2025 年 29 巻 3-4 号 p. 47-55
Performance of bentonite buffer materials used in the construction of geological repositories for high-level radioactive waste in coastal areas is significantly affected by salinity, thereby making it essential to evaluate these effects. This study aimed to investigate the effects of salinity on the swelling, outflow, and permeability characteristics of bentonite, a buffer material, in NaCl solutions at various concentrations. To this end, one-dimensional-swelling, radial-outflow, and backpressure loading-type falling-head-permeability experiments were conducted on a compacted bentonite specimen (Kunigel-V1; diameter: 50 mm; height: 10 mm; dry density: 1.5 Mg/m3) for 180 days. The experimental results show that as the salinity of the NaCl solution increased, the swelling pressure and outflow volume of bentonite decreased, while the hydraulic conductivity increased. Here, the salinity effects were analyzed based on (1) the interlayer swelling due to the hydration reaction of montmorillonite and (2) the swelling behavior associated with the diffuse electric double layer on the surface of montmorillonite particles. The observed trends in the experimental results are generally consistent with these mechanisms. Thus, this study serves as a guide for optimizing materials to enhance the safety of geological disposal systems. Our findings have policy implications and practical applications, as they help develop strategies to mitigate salinity effects in coastal and saline environments for radioactive waste containment.
