On-line monitoring of the gas composition in the Full-scale Emplacement experiment at Mont Terri (Switzerland)

An on-line gas monitoring has been conceived and implemented to study the evolution of the composition of the free gas phase in the Full-scale Emplacement (FE) experiment in the Underground Rock Laboratory at Mont Terri (Switzerland). The FE experiment is a trial run for a spent-fuel emplacement drift for a repository according to the Swiss concept for radioactive waste disposal. The monitoring of gas species such as He, Ar, Kr, Xe, N-2, O-2, H-2, CH4, and CO2 was performed successfully over several months. The partial pressures of gases relevant for the operational safety such as H-2 and CH4 have been found to be below the concentration threshold for ignition. The combination of the on-line monitoring data and conventional noble-gas isotope measurements reveals rapid gas exchange between the pore space of the compacted bentonite granulate material used as backfilling and both the access niche and the host rock surrounding the FE tunnel (Opalinus Clay). Such fast gas exchange partly explains the disappearance of oxygen from the bentonite pore space detected by O-2 sensors even prior to sealing of the drift and the accumulation of a fraction of terrigenic gases such as He-4, Ar-40, CH4, and CO2.