Humidity-induced water absorption and swelling of highly compacted bentonite in the project KBS-3H

In the KBS-3H concept for disposal of nuclear waste, the canisters and bentonite blocks are placed in horizontal boreholes in the bedrock. In this concept, large perforated supercontainers, containing the copper canisters and buffer materials, are separated by a number of distance blocks of highly compacted bentonite. One of the critical design issues for the distance blocks is the humidity-induced swelling, absorption and cracking when the blocks are exposed to high relative humidity. The objectives of the laboratory tests were to determine the water absorption rate, swelling rate and cracking of the bentonite with respect to time at different conditions when a bentonite block is placed above a free water surface separated by an air gap. The main variables are the gap width, the initial water content of the bentonite, the initial density, the temperature and the sample size. The material is a commercially available sodium bentonite with the quality symbol MX-80 (Wyoming bentonite from American Colloid Co.). It was here demonstrated that a lower initial water content yields higher rates of absorption and swelling. It was also shown that a smaller size of the gap between the water surface and the sample yields higher absorption rate and swelling rate. Some of the tests were theoretically modelled with a FEM code. The influence of the initial water content and the size of the air gap on water absorption was well captured by the models.