[en] Since 1997, Korea Atomic Energy Research Institute(KAERI) has been developing the technology related to a deep geological disposal as a long term spent fuel management strategy. As a result, KAERI proposed a Korean Reference high level waste disposal System(KRS) which is based on the concept of a permanent disposal. Since the recent trend is considering the retrieval of spent fuels, KAERI is currently developing the A-KRS(Advanced KRS) which enables the retrieval of spent fuels and enhances the densification factors of spent fuels. Hence, the design concept of the canisters is modified by corresponding to the A-KRS. Basically, KRS consists of an engineered barrier and a natural barrier. The main components of the engineered barrier are the canister and buffer. Since a thermal performance is directly related to diverse aspects of the KRS, it should meet the requirement that the maximum temperature at the interface between the buffer and the canisters is below 100 .deg. C to maintain the physical and chemical properties of the buffer that acts as an essential barrier of the disposal system. In this research, disposal canisters for KRS and AKRS are used and the maximum buffer temperature is calculated by varying the disposal tunnel spacing and disposal hole pitch for thermal integrity and optimized layout of the KRS