Thermo-Hydro-Mechanical Simulation of Unsaturated Bentonite Buffers under High Temperatures
Compacted bentonite is widely investigated as an engineered buffer material for deep geological disposal systems because of its low hydraulic conductivity, swelling capacity, and self-sealing behavior. Under elevated temperatures, unsaturated bentonite buffers may undergo coupled thermo-hydro-mechanical responses involving heat transfer, liquid water flow, vapor transport, liquid–vapor phase change, suction redistribution, and thermally induced deformation. These processes are most significant near the heat source, where strong thermal gradients can cause drying, vapor generation, changes in transport properties, and time-dependent volume change. Philip and de Vries [1] and Cleall et al. [2] showed that nonisothermal moisture migration in bentonite is governed by coupled liquid–vapor transport, while Tang and Cui [3] and Lu and McCartney [4] showed that the associated response is affected by suction, stress state, temperature, and hydraulic history.
This paper presents a COMSOL Multiphysics® framework for simulating high-temperature thermo-hydro-mechanical processes in compacted MX80 bentonite using a two-dimensional axisymmetric tank-scale heater–bentonite configuration. Heat Transfer in Solids is coupled with Richards’ Equation for liquid water flow, Transport of Diluted Species in Porous Media for water vapor diffusion, and Solid Mechanics for constrained deformation. The hydraulic response is represented using nonisothermal water-retention and relative-permeability relationships based on the Mualem–van Genuchten framework, following Mualem [5] and van Genuchten [6]. Effective vapor diffusivity is evaluated using the porous-medium diffusion relationship proposed by Millington and Quirk [7], and liquid–vapor mass exchange is incorporated through finite-rate phase-change source terms following Başer et al. [8]. Temperature- and saturation-dependent thermal and hydraulic properties are implemented using user-defined material functions. No Application Library model is used; the geometry, material functions, coupling variables, and boundary conditions are developed specifically for the bentonite heater problem.
The mechanical formulation distinguishes instantaneous thermal expansion from time-dependent contraction during heating. A temperature-dependent creep contraction model based on Kim and McCartney [9] is implemented through an external strain feature superimposed atop thermal expansion, and yield-related state variables based on the Tang–Cui thermo-elasto-plastic framework are evaluated from the evolving stress, suction, and temperature fields. The model is applied primarily to the heating stage, with selected later-stage hydration and rewetting simulations used to examine the transition from vapor-dominated drying to suction-driven liquid redistribution. Simulation results evaluate temperature propagation, near-heater drying, vapor concentration gradients, phase-change activity, suction evolution, transport-property changes, creep strain accumulation, and local volumetric strain redistribution. The framework provides a COMSOL-based approach for interpreting tank-scale heating tests and extending numerical simulations toward high-temperature engineered barrier performance assessment.
