A candidate environment for deep geological storage of spent nuclear fuel is sparsely fractured crystalline rock, since these typically offer low-permeable and long-term stable conditions. However, canister deposition holes and the repository tunnel system need to be back-filled with buffer material, typically containing bentonite clays. Thereby, upon closure, the repository will undergo a multiphase flow process as groundwater seepage re-saturates the subsurface tunnel system.

The prevalence of those transient unsaturated conditions in porous and fractured media generally has a considerable influence on the flow field and thereby also on transport. Specifically, the occurrence of mixed gas-water flows may influence the environment near the deposition holes as well as the physical and biogeochemical processes along potential transport pathways from repository depth. There are numerous scientific investigations of two-phase flow behaviour in soil systems, but much fewer observations for these types of engineered bentonite-clay buffer systems. In particular, effects of buoyancy and bubble trapping may differ in fractured bedrock from those in classical soil environments. A comprehensive understanding of multiphase flows in a coupled bedrock-bentonite system is therefore necessary.