Deep geological disposal is widely accepted to be the safest and most efficient form of disposal for high-level radioactive waste [1]. In the deep geological disposal, highlevel radioactive waste such as spent nuclear fuels is typically disposed into the bedrock at a depth of about 500 m, eg, in Swedish SKB concept [2]. The bedrock isolates the waste from the human environment as a natural barrier. The deposition holes are filled with bentonite buffer around emplaced canisters, and disposal tunnels are backfilled. Potential rock types suitable for the host rocks of a geological repository include crystalline rock, clay, and rock salt [3]. It is necessary to ensure the stability of the disposal system for tens of thousands of years considering the half-lives of the wastes. In South Korea, the KAERI Reference Disposal System for Spent Nuclear Fuel (KRS) was developed in 2007 [4] based on the Swedish KBS-3 concept. Subsequently, KRS for High Burnup Spent Nuclear Fuel (KRSHB) and the Improved Korean Reference Disposal System (KRS+) were suggested by [5] and [6], respectively. Coupled thermo-hydraulic-mechanical (THM) processes are the key processes that ensure the long-term performance of the deep disposal of radioactive waste. The heat released from the high-level radioactive waste is the source of heat transfer by conduction and convection, fluid flow and phases, and thermal stress in the geological repository. The increase in temperature can change the saturation and generate thermal stress in the buffer. The changes in stress and water saturation can influence the material properties, such as permeability and thermal conductivity as well as thermal, hydraulic, and mechanical processes at the nearfield rock mass [7].

Field experiments and numerical analyses have been conducted to study the coupled THM processes at the near-field rock mass of deep disposal systems over the last several decades [8]. For example, the Drift Scale Test at Yucca Mountain, USA was carried out with a heating system of canister heaters and wing heaters from 1997–2006.