Thermo-hydro-mechanical responses of the host rock in the context of geological nuclear waste disposal

Deep geological disposal facility has been considered as the most appropriated solution for the safe longterm management of high-level radioactive waste (HLW). Geologic disposal solution consists of isolating the radioactive waste from the biosphere. Argillaceous rock has been selected in several countries as host formation due to its favorable properties to isolate radionuclides and chemical contaminants (very low permeability, stable, high retention capacity, self-sealing, etc). Clays in their natural state is usually saturated. Disposal of the exothermic waste packages in the repository leads to an increase in temperature within the host rock, which induces the pore pressure build-up due to the difference in thermal expansion coefficients of the pore water and the solid skeleton. The excess pore pressure generally leads to a decrease in the effective stress and can provoke thermally hydraulic fracturing or shear failure. Therefore, understanding the thermo-hydro-mechanical (THM) responses of the saturated host rock due to the heat generated form waste packages is a key issue to assess the feasibility of the repository. This paper aims at presenting coupled THM constitutive equations for a saturated porous medium and its finite element (FEM) discretization and solution. The solution is validated against analytical solution and other numerical results from a benchmark within an international project. FEM program is then used to describe the THM behavior of the host rock around a HLW repository (i.e. near field responses). Sensitivity analysis were performed to evaluate effect of material anisotropy and hydraulic condition on the micro-tunnel wall