The gas permeability, breakthrough behaviour and re-sealing ability of Czech Ca–Mg bentonite

Abstract

Abstract The study's objectives were to test the gas permeability of unsaturated compacted bentonite, the gas-breakthrough behaviour of saturated bentonite, and the re-sealing ability compared with the material's dry density, hydraulic conductivity and swelling pressure. During the project's pilot phase, a series of laboratory experiments were conducted on samples of two Czech Ca–Mg bentonites (Bentonite 75 and Cerny Vrch). The testing procedure, employing repeated hydration and gas-breakthrough test phases, simulated conditions within deep geological repositories for radioactive waste, wherein the buffer will be progressively hydrated from the surrounding host rock and loaded with pressure exerted by gases created within the repository. It was found that the gas permeability of the bentonites tested was affected by dry density and, as with the two materials (FEBEX and MX-80) used for comparison, it correlated well with the accessible void ratio. Gas-breakthrough tests revealed correlations between time to breakthrough and both dry density and sample height. The recorded repeated test breakthrough times on the same samples varied, which implies that the pathways created are complex and differ for repeated gas-injection cycles. However, no systematic deterioration in sample properties was detected, thus indicating the extensive re-sealing ability of the materials tested.