Modelling Thermal-Hydraulic-Mechanical-Chemical Processes in Host Rocks and Buffer for Deep Geological Repositories

Abstract

The safe, long-term disposal of spent nuclear fuel is a critical challenge for the global nuclear industry. Deep Geological Repositories (DGRs) are widely recognized as the most viable solution, relying on a robust multi-barrier system to isolate radioactive waste from the biosphere. Among these barriers, the Engineered Barrier System (EBS), particularly the bentonite buffer surrounding the waste containers, plays a vital role in ensuring the repository's safety. The performance of the EBS depends on the coupled thermal, hydraulic, mechanical, and chemical (THMC) processes that govern the evolution of its components over time. Understanding and accurately modelling these complex interactions is essential for assessing repository safety and optimizing design.

This thesis develops, verifies, and validates a fully coupled THMC model to simulate the near-field evolution of the buffer and surrounding rock in the Canadian DGR concept proposed by the Nuclear Waste Management Organization (NWMO). The model integrates mass, energy, and momentum conservation principles to simulate heat transfer, moisture migration, mechanical deformation, and reactive transport within bentonite and host rock. Verification against benchmark analytical solutions, e.g., Terzaghi's one-dimensional consolidation (HM coupling) and Booker and Savvidou's thermal consolidation around a point heat source, confirms the model's accuracy in capturing coupled THM behaviour.

Comprehensive validation is conducted across laboratory and field scales to ensure predictive reliability. Laboratory validation includes (i) a sand–bentonite infiltration column test reproducing thermal–moisture migration in unsaturated media, (ii) a long-term MX-80 bentonite column test simulating radiogenic heating and subsequent hydration, and (iii) thermodiffusion experiments demonstrating temperature-dependent solute migration (Soret effect). Field-scale validation uses data from the DIGIT thermodiffusion experiment conducted at the Tournemire Underground Research Laboratory (France), confirming the model's applicability under in situ conditions. Additionally, bisulfide (HS⁻) transport, accounting for adsorption on bentonite and reaction with ferrous iron (Fe²⁺), is modelled and validated against laboratory diffusion tests, allowing the quantification of copper corrosion depth on the used fuel container (UFC).

The validated model is then applied to the Canadian DGR emplacement room to simulate long-term THMC evolution over one million years. The simulations capture the temporal and spatial variations of temperature, degree of saturation, relative humidity, swelling pressure, normal stress on the UFC, HS⁻ migration, and corrosion depth. A breach scenario is also analyzed to assess the robustness of the multi-barrier system under adverse conditions. The results demonstrate that the buffer and surrounding rock can maintain all safety function indicator criteria (SFICs) including temperature below 100 °C, hydraulic conductivity under 10⁻¹⁰ m/s and sufficient swelling pressure, confirming the long-term integrity and safety of the EBS.

Overall, this research delivers a validated, predictive framework for simulating coupled THMC processes in DGRs. By integrating mechanistic coupling, multi-scale validation, and application to Canada's repository design, this work significantly enhances the understanding of near-field evolution, strengthens performance assessment methodologies, and provides critical insights for ensuring the long-term safety of nuclear waste disposal systems.