Numerical investigations on buffer-host rock thermo-hydro-mechanical interactions in diverse geological settings: a comparative study

Abstract Understanding the coupled thermo-hydro-mechanical (THM) interactions between bentonite buffer and host rock is crucial for ensuring the safety of nuclear waste repositories (NWRs). While many site-specific studies exist, cross-comparative analyses that isolate the influence of potential buffer and host rock combinations on the THM response of an NWR under consistent boundary conditions remain limited. Therefore, this study analysed representative buffer-host combinations using numerical simulations with two bentonite varieties, FEBEX and MX-80, and five host rocks, including Callovo-Oxfordian claystone (COx), Opalinus Clay (OPA), Boom Clay, shale, and crystalline fractured and intact granite. The evolving buffer-host interactions were assessed using the Thermo-Richards-Mechanics module of the open-source finite element code OpenGeoSys (OGS). The simulations identified a thermo-hydraulic window, where peak buffer temperatures occur early after waste emplacement, whereas full buffer saturation occurs later. Hydration depended strongly on host rock properties; mid-buffer regions generally reached peak temperatures earlier and, in several cases, saturated earlier than the top buffer. The evolution of mean stress at the buffer-host interface was influenced by coupled interactions among suction-dependent swelling, thermal stresses, and pore pressure. Both bentonites exhibited broadly similar THM responses for a given host rock; however, their evolution differed due to contrasts in their THM properties. Likewise, the host rock THM characteristics were strongly influential, with argillaceous Boom Clay leading to earlier hydration and heat buildup, and intact crystalline granite showing gradual and delayed THM evolution. Overall, the study provides comparative insights into the coupled THM behaviour of buffer-host systems, offering a basis for future site-specific repository assessments.