Finite element modelling of multi-gas flow in expansive clay

In a nuclear waste repository, the interaction of groundwater with barrier materials such as canisters, steel liners or waste packages can generate multiple gases. The accumulation of such gases can adversely affect the integrity of an Engineered Barrier System (EBS) and host rock. Therefore, efficient repository modelling requires having a mechanistic understanding and predictive capabilities of these multi-gas interactions and transport processes. This study presents a numerical framework to model gas mixture flow for two incondensable and inert-type gases in expansive clays under hydraulic conditions. The framework is an extension of the finite element code Thebes, incorporating new functions for water retention and gaseous flow behaviour. The current work is limited to only gas flows in soil. For model verification, the study utilises two simplified benchmark tests and one experiment. Each test focuses on specific flow mechanisms i.e., advection, gas phase diffusion or dissolved diffusion. The benchmark test employs COMSOL and/or semi-analytical approaches for verification. The experimental replication uses the diffusion experiment of dissolved gas in Boom clay at constant volume (CH4 and He), performed by SCK CEN. The presented results show a good match against the experimental data while giving critical insight into gas flow mechanisms in expansive clays.