# Numerical Simulation of Thermo-Hydro-Mechanical Coupling of Model Test for Nuclear Waste Disposal
## 1 Introduction
Numerical modeling is an indispensable tool to fully understand these coupling processes, since the safety of the repository must be guaranteed for 100,000 years or more. However, modeling THM coupling processes in buffer materials can be challenging. On the one hand, the constitutive relation of a single process is complex and difficult to obtain; on the other hand, the choice of constitutive relation and parameters has a great impact on the simulation results. [PERSON] presented the laboratory results concerning the THM behavior of the FEBEX bentonite [26]. Results regarding water holding capacity, permeability and expansibility were obtained during the project. The THM behavior of the indurated Callovo-Oxfordian and Opalinus Clay rocks has been extensively investigated in laboratory under repository-relevant conditions [10]. By monitoring the temperature, relative humidity and mechanics of buffer materials under long-term heating conditions, the changes in the buffer materials under coupled THM conditions were revealed in [27]. [PERSON] investigated the applicability of a thermo-elasto-plastic model, using a TOUGH2-MP/FLAC3D simulator to reproduce the coupled hydro-mechanical action at the engineered barrier system emplacement test in Opalinus Clay, and the coupled THM behavior at the full-size engineered barrier system test in a crystalline host rock [28]. In order to study the THM processes occurring in the barrier system of an HLW, [PERSON] used the computer code TOUGH2 to simulate the multidimensional fluid and heat flows of multiphase and multi-component fluid mixtures in unsaturated media [29]. For MX-80 bentonite, SKB adopted different numerical software, ABAQUS, CODE_BRIGHT and COMPASS, respectively, and put forward a method for the numerical simulation of thermo-hydro-mechanical evolution under the condition of the simulated repository [30; 31; 32]. Mock-up and in situ tests were carried out in Madrid, Spain, and Grimsel, Switzerland, respectively, and the FEBEX bentonite multi-field coupling theoretical model and corresponding simulation methods were established [23; 33; 34]. In our country, Gaomiaozi bentonite is the preferred deposit for the buffer material of repository, and many studies on permeability, thermal conductivity, mechanical properties and multi-field coupling theory models have been carried out [22; 35; 36; 37; 38; 39; 40; 41]. In view of the theory of mixtures and mechanics of continuous media, a numerical model of porous media was established for coupled THM processes in light of the conservation laws of momentum, mass and energy over a solid-liquid-gas three-phase system. Based on an axisymmetric model, finite element modelling of the long-term performance of the buffer material in the Mock-up test bench under the condition of THM multi-field coupling was carried out by using COMSOL Multiphysics 5.6. The spatial distribution characteristics and change laws of suction, porosity and swelling pressure in the buffer layer were investigated. By comparing the real-time monitoring of the changes in the buffering material under long-term heating and water conditions, it was feasible to verify the numerical simulation of the COMSOL application design. This study is of some significance for the safety evaluation and design parameter selection of the barrier system of HLW repositories in China, and can provide a theoretical basis for the construction of HLW repositories.