Bentonite homogenisation and swelling: The effect of salinity

Hazardous radioactive waste must be removed from the biosphere, and geological storage is the universally favoured option for accomplishing this. In many cases, the repository designs include a clay buffer as part of the engineered barrier system (EBS) that surrounds the individual waste canisters and seals the disposal galleries. The emplacement of the EBS around the waste will generate small void spaces that must be closed to ensure that high permeability pathways do not develop. Many of the groundwaters at proposed disposal sites are saline, and this porewater chemistry, combined with the presence of technological voids may present a technical challenge to repository designers. In this study, a suite of experiments was conducted to examine the impact of fluid salinity in combination with the presence of a void space on the swelling behaviour of barrier bentonites. Both sodium and calcium bentonites were studied and the sample lengths were varied to provide an understanding of the role of axial strain on the homogenisation and swelling pressure development in the bentonite. After 100 days of testing the clay had swelled into the void space and differential swelling pressures (difference between the maximum and minimum swelling pressure recorded at a given time) had reduced substantially, highlighting the ability of the bentonite to expand into a void. However, clay that swelled into the void never generated significant swelling pressures during the testing period and differential pressures were still significant at the end of the tests. The calcium bentonite demonstrated much smaller peak swelling pressures than the sodium bentonite under the same salinity conditions, and in all tests, the generation of significant swelling pressure in the clay in the low-density end of the sample did not occur during the testing period. These findings suggest that the suppression of clay swelling at higher salinities is likely to strongly impact the void-filling process.