Coupled thermo-hydro-mechanical model for simulating thermal history-induced voids in buffer material

Abstract

The thermo-hydro-mechanical processes in a deep geological repository (DGR) influence the bentonite buffer’s long-term performance. The buffer’s temperature evolution over a repository’s service life imposes a thermal history in the DGR. The developed thermal history cause shrinkage of the buffer and generates the technological voids (i.e., annular gaps) in a DGR. The void formed compromises the intimate contact between the dissimilar materials within the DGR and thus impedes the containment ability of the buffer in the long run. Therefore, to investigate the complex coupled processes involved, the present study developed a coupled thermo-hydro-mechanical (THM) model to evaluate thermal history’s influence on buffer’s hydro-mechanical performance. The developed THM model was validated using data from an infiltration test conducted under isothermal conditions. Furthermore, the THM simulations on thermal history indicated radial and vertical shrinkage within bentonite. The formed voids were sealed due to rehydration occurring through the bottom boundary. The hydro-mechanical parameters, such as swell deformation in radial and vertical directions, porosity, permeability, water intake, and swell pressure, were assessed during the hydration process. A sudden increase in the swell pressure was observed when the void gets sealed completely due to rehydration. The rate and magnitude of swell pressure generated were proportional to the peak temperature imposed at the non-yielding boundaries and the consequent void size formed. The increased swell pressures could have impaired the structural integrity of the waste contained canister at a DGR scale. Given this, the implications were comprehended based on the outcome of numerical investigations for the plausible adverse effects inside a DGR due to thermal history.