Analysis of Long-Term Thermo–Hydro–Mechanical Behavior in the Near-Field of a Deep Geological Repository System

Abstract

The deep geological repository (DGR) system has been selected by most of the world’s nuclear waste management organizations for the long-term disposal of radioactive wastes. The DGR mainly consists of a multi-barrier system—comprising the natural host rock and an engineered barrier system—to contain and isolate high-level radioactive waste, including used fuel containers (UFCs), to protect humans and the environment. Bentonite materials and host rock are the main components of the DGR’s engineered and natural barrier system, respectively. It is crucial to understand the coupled behavior of bentonite and rock materials under various in situ conditions over long-term durations, as it supports safety assessments and enhances the overall safety level of DGR systems. This study presents a methodology for the numerical modeling of a hypothetical DGR using developed coupled models. The developed model was used to investigate the hydromechanical (HM) and thermomechanical (TM) response within the near-field (the area within a radius of 50 m near the UFC and multiple-barrier system) of a simplified hypothetical DGR, based on the proposed design concept of the Nuclear Waste Management Organization (NWMO) of Canada. The analysis results included the evolution of temperature, thermal stresses, saturation, and swelling pressure at different stages of the DGR system’s lifetime. The results indicated that it could take up to 10,000 years to fully saturate the bentonite materials with a corresponding swelling pressure of 2.7 MPa associated with a decrease in the rock’s strength/stress ratio near the placement room; however, the ratio did not indicate a significant system failure. Sensitivity analysis was also conducted to assess the impact of various parameters on the saturation time and the strength/stress ratio in a DGR. The results highlighted that saturation time was highly influenced by the permeability of both the rock formation and the bentonite, resulting in saturation times ranging from 500 to 20,000 years. Moreover, the strength/stress ratio was found to be sensitive to the model’s parameters, particularly the maximum swelling pressure. The results of the TM analysis show that temperature development around the placement of rooms in a DGR is highly influenced by room spacing, with a lower factor of safety (FOS) as time and temperature progressed due to elevated temperature, while the rock remained stable over the 150-year analysis period. The inclusion of temperature-dependent mechanical properties produced negligible changes to the overall stability of the rock around the placement rooms of the DGR.