Analytical Assessment of Far-Field Thermo-Poroelastic Response and Thermally Induced Failure Potential in the Host Rock Around Underground Radioactive Waste Repositories

Abstract The decay heat generated by high-level radioactive waste increases the temperature and leads to pore pressure buildup in the surrounding host rock. This so-called thermal pressurization may extend well beyond the excavation-damaged zone (EDZ) in the low-permeability formations typically considered for underground disposal and may lead to tensile or shear failure, thereby compromising the long-term integrity of the geological barrier. This study presents closed-form solutions for assessing the thermo-hydro-mechanical response of the host rock beyond the EDZ, far from the individual emplacement drifts (“far field”), based on a simplified one-dimensional model assuming linear material behavior. The applicability of the derived analytical solutions for practical design purposes is evaluated through comparative two-dimensional numerical analyses covering three representative European repository cases and avoiding several of the simplifying assumptions of the analytical model. The results show good agreement between analytical and numerical predictions of temperature, pore pressure, and stress evolution, particularly in the far-field region of interest. Using a 10% deviation criterion for assessing one-dimensionality, the thermal response in the three representative cases becomes practically one-dimensional no later than 75 years after waste emplacement and at distances above emplacement level not exceeding approximately one-fourth of the drift spacing. The excess pore pressure field becomes one-dimensional earlier. The potential for tensile and shear failure is assessed a posteriori by applying the respective failure criteria to the elastically computed trial stress state; post-failure behavior is outside the scope of the formulation. The main deviations between the analytical and numerical predictions occur in the near field, where multidimensional effects and excavation-induced plastic yielding are relevant and lie outside the intended scope of the formulation. The proposed solutions therefore provide a physically transparent and computationally efficient tool for assessing thermally induced hydro-mechanical effects under low-permeability, conduction-dominated far-field conditions and for supporting design assessments, sensitivity analyses, and the verification of numerical models in underground waste disposal applications.