# Numerical Simulation of Thermo-Hydro-Mechanical Coupling of Model Test for Nuclear Waste Disposal
## 1 Introduction  
The development of nuclear weapons and the efficient utilization of nuclear power produce radioactive waste, especially high-level radioactive waste (HLW), which is harmful to the biosphere. The radiation cycle of HLW can reach tens of thousands of years. HLW, due to its long half-life, high toxicity and strong radioactivity, is hazardous because it produces fatal radiation doses during short periods of direct exposure. Radioactive nuclides enter the environment and migrate with it, entering different types of food chains and causing radiation exposure to both human and non-human organisms [1, 2]. It is a topic of great concern and is the most difficult geological science problem in the world. At present, deep geological disposal is the only safe and reliable disposal method that meets practical conditions for the safe disposal of HLW, which is referred to as \"digging and burying\" [3, 4]. The geological disposal repository for HLW usually adopts the concept design of a \"multiple barrier system\", which consists of HLW wrapped in glass solidifiedbodies, disposal containers, buffer materials and surrounding rocks from inside to outside. The disposal containers are generally made of metal, with HLW source items on the inner side and buffer materials on the outer side. The parameters that affect the stability of HLW geological disposal include geological conditions, hydrogeological conditions, engineering parameters (size and material of waste containers, layout and depth of disposal facilities) and environmental parameters (decay heat, isotopic composition, type of assembly, radioactivity), etc. [5]. Multiple barrier systems such as natural barriers, engineering barriers and disposal containers are widely used internationally to achieve the deep geological storage of HLW [6; 7]. More than 20 countries around the world have launched geological disposal facility construction projects. Finland has launched the construction of the world's first deep geological disposal facility for HLW, Switzerland has set the goal of building a disposal facility by the 2050s. The preliminary selection area for the geological disposal of HLW in China has achieved a series of results, and the top-level design work of the scientific research management platform of Beishan Underground Laboratory has been completed [8]. In the deep geological disposal of HLW, there are complex mass, energy and momentum exchange relationships between geotechnical media and the environment, showing a complex thermo-hydro-mechanical (THM) coupling process [9]. The safety performance of the repository barrier system can only be accurately assessed by numerically analyzing the THM multi-field coupling of the repository and investigating the influence law of each parameter on the multi-field coupling. For the study of the THM coupling process in HLW disposal, many scholars have carried out a great deal of work from experimental and theoretical aspects, including indoor tests, in situ tests and numerical simulations [10; 11; 12; 13; 14]. Because of the long period and complex interactions of THM multi-field coupling tests, more theoretical models have been developed for the studies [15; 16]. Coupled THM processes are commonly simulated using the theories of porous media. In the past three decades, many achievements have been made in the multi-field coupling research of geotechnical THM, and a series of mathematical models and numerical simulation methods have been established [17; 18; 19; 20]. In an effort to assess the long-term stability of HLW repositories, several major international cooperation projects have been carried out, including the DECOVALEX project, the FEBEX project and the China Mock-up project, which have made important contributions to promoting the evaluation of the safety of the geological disposal of HLW [21; 22; 23; 24; 25]. During the storage of HLW, the buffer material is under THM coupling.