The role of gas volume on the advective movement of gas through an engineered barrier system

The final experiment (full canister test, FCT) of the large-scale gas injection test (LASGIT) aimed to explore the impact of gas volume on gas transport behaviour. Unlike previous tests, the FCT involved pressurising a full-scale KBS-3 canister up to ∼7100 kPa when pressure was held, allowing excess water to drain and to establish gas entry. Once drained, pressure gradually decreased by 220 kPa, indicating gas moving into the fully saturated clay buffer. Observations showed different pore pressure and total stress behaviour compared with earlier tests, but no major changes were seen in the buffer’s response, suggesting gas migration was unaffected by gas volume. The slow pressure decay shows gas travelled through a limited number of narrow pathways, which were not formed through tensile fracturing. A gas leak early in the FCT led to depressurisation and later pressurisation of the canister, causing a 50 μm expansion in its radius. This mechanical loading on the buffer, greater than the bentonite’s drainage capacity, caused pore pressure and radial stress changes. The expansion of the canister by pressurisation or thermal effects should therefore be considered in performance assessments.