High-level waste (HLW) from nuclear power plants is very dangerous to humans, and it must be permanently isolated. A deep geological disposal method disposes the HLW by placing the waste in an engineered barrier, at depths of more than 500m below the ground. The canister containing the HLW is surrounded by a buffer, backfill, and near-field rock, thus serving as a repository system. The key barrier component, buffer, should prevent the potential radionuclide from leaking out of the damaged canister. The aim of this study is to present an optimum initial repository condition for the thermal stability of buffer materials through a numerical model. The Thermal-Hydraulic-Mechanical (THM) correlation in the repository system was considered and a Firefly algorithm was used for the optimization. Five parameters, such as initial dry density and initial water content of the buffer and those of the backfill and permeability of rock, were selected to be considered for the initial design parameters. A metamodel was constructed and applied to the Firefly algorithm to determine the optimal initial design condition. The minimum temperature developed in the buffer material and its optimal initial condition were given for a typical Korean repository system in which a Korean bentonite was used for the buffer material.