Thermal influence on compacted bentonite for nuclear waste disposal

Compacted bentonite is attracting greater attention as buffer material for repositories of high-level nuclear waste. The swelling characteristics of compacted bentonite is expected to fill up the cracks in the surrounding rocks in order to make an impermeable zone around the wastes. However, it is feared that the swelling characteristics of bentonite declines by the decay heat of nuclear wastes. To contribute to this problem, this study investigated the swelling deformation of bentonite experienced some thermal histories in the laboratory. From the test results, it is found that the swelling deformations of bentonites decrease by thermal histories. It is also found that the decline of swelling deformation by thermal history is caused by the reduction of absorbing power of bentonite and elution of exchange cations from clay-layer surfaces. These results indicate that we must design the buffer material in full consideration of the influences of decay heat of wastes.