Assessing the Gas Transport Mechanisms in the Swiss L/ILW Concept Using Numerical Modeling and Supporting Experimental Work

In low/intermediate-level waste (L/ILW) repositories, anaerobic corrosion of metals and degradation of organic materials produce hydrogen, methane, and carbon dioxide. Gas accumulation and gas transport in a L/ILW repository is an important component in the safety assessment of proposed deep repositories in low-permeability formations. The dominant gas transport mechanisms are dependent on the gas overpressures as with increasing overpressure the gas transport capacity of the system increases. The dominant gas transport mechanisms occurring with increasing gas pressure within the anticipated pressure ranges are: diffusion of gas dissolved in pore water (1), two phase flow in the host rock and the excavation damaged zone (EDZ) whereby no deformation of the pore space occurs (2), gas migration within parts of the repository (if repository materials are appropriately chosen) (3) and pathway dilation (4). Under no circumstances the gas is expected to induce permanent fractures in the host rock. This paper focuses on the gas migration in parts of the repository whereby materials are chosen aimed at increasing the gas transport capacity of the backfilled underground structures without compromising the radionuclide retention capacity of the engineered barrier system (EBS). These materials with enhanced gas permeability and low water permeability can supplement the gas flow that is expected to occur through the EDZ and the host rock. The impact of the use of adapted backfill and sealing materials on the gas pressure build-up and the major gas paths were assessed using numerical two-phase flow models on the repository scale. Furthermore, both the gas and water fluxes as a function of time and gas generation rate can be evaluated by varying the physical properties of the materials and hence their transport capacity. Results showed that by introducing seals with higher gas permeability, the modelled gas flow is largely limited to the access tunnels and the excavation disturbed zone for the case of a very low permeability host rock. The bulk of the gas flows through the repository seal and the adjacent EDZ into the tunnel system. In addition to the demonstration of the gas flow in the seal and access tunnel system by numerical models, laboratory results confirm the high gas transport capacity of the sand/bentonite mixtures. In a next step a multi year demonstration scale experiment (GAST) at the Grimsel Test Site is envisioned.