Benchmarking thermo-hydro-mechanical models for geological barrier integrity in clay-based repositories

Abstract This study investigates the thermo-hydro-mechanical (THM) behavior of argillaceous formations, particularly the Callovo–Oxfordian (COx) claystone, over extended timescales to evaluate the long-term safety of radioactive waste repositories. Numerical simulations were performed as part of a benchmark exercise to study the response of the COx formation under heating scenarios representative of high-level radioactive waste disposal. Six modeling teams from various institutions participated in this benchmark, using various numerical codes, providing valuable information on the evolution of temperature, pore water pressure, and stresses within the repository environment, particularly around the disposal cells. The results highlight that the COx formation exhibits significant thermal pressurization and stress relaxation because of its low permeability, whereas the excavation damaged zone (EDZ) remains confined to the near field and does not extend significantly under the thermal load considered. The study demonstrates the robustness of numerical tools for repository safety assessments and emphasizes the importance of validated THM formulations to ensure long-term containment of radioactive waste.