Compacted bentonite and compacted sand-bentonite mixture are attracting greater attention as buffer materials for disposal pits in the underground facilities for repositories of high-level nuclear waste. Buffer materials must have swelling characteristics and are expected to fill up the space between the material and surrounding ground by swelling. This role is called as "Self-sealing." To design the specifications, such as dry density, sand-bentonite mass ratio and size, of buffer material, this study proposed the new evaluation formulas for swelling characteristics of buffer materials containing bentonite. The applicability of the evaluation formula proposed in this study has been confirmed by comparing the predicted results with laboratory test results on the swelling deformation and swelling pressure of compacted bentonites and sandbentonite mixtures. The evaluation formulas proposed can obtain the maximum swelling pressure and the maximum swelling strain of buffer materials at various dry densities, kinds of bentonite, and bentonite contents. Therefore, the evaluation formulas can be used for selecting a kind of bentonite which is suitable for buffer material, and for designing the sand-bentonite mass ratio and compaction density from the viewpoint of "Self-sealing.
Some repositories of high-level nuclear waste are currently planned to be constructed to a depth of several hundred meters (Atomic Energy of Canada Limited, 1994; Ogata et al, 1999; Swedish Nuclear Fuel and Waste Management Co., 1992). Figure 1 shows an example of disposal facility and pit for high-level radioactive wastes in Japan (Ogata et al, 1999). The function of the buffer material is to create an impermeable zone around the containers since high-level radioactive wastes must be kept separate from the surrounding environment. The buffer material must have swelling properties. We also discussed the processes of swelling characteristics of bentonite in the buffer materials, and proposed the model of the swelling behavior.