Long-Term Coupled Thermal-Hydro-Mechanical Simulation of Heat and Gas Transport in a Deep Geological Nuclear Waste Repository in Opalinus Clay

ABSTRACT: Gases will appear in nuclear waste repositories because of anaerobic corrosion of metals and other processes. The accumulation and release of such gases from the repository system may affect several processes influencing long-term safety. This study provides thermo-hydro-mechanical (THM) coupled simulations using the TOUGH + FLAC3D simulator to forecast THM behaviors in a conceptual but realistic nuclear waste repository over a 100, 000-year period post-closure. A comparative simulation scenario was established, varying the steel corrosion rate, i.e., 0.1 µm/year, 1 µm/year, and 10 µm/year. The results show that thermal pressurization and gas accumulation substantially elevate pore pressure within the EBS and adjacent host rock until reaching a steady state. The peak pore pressure is likely to exceed the lithostatic stress in the case with the highest H2 generation rate. Tension failure and fracturing can occur at the tunnel wall because of gas buildup inside the tunnel. However, this simulation study indicates no risk of widespread hydro-fracturing in the host rock. The generated H2 continuously migrates outward and accumulates in the concrete liner and the excavation disturbed zone surrounding the tunnel. Meanwhile, the fluids that may contain radionuclides are not likely to migrate out of the high suction bentonite.