[en] The fuel temperature should also generally be maintained below 570 .deg. C for the short-term off-normal and accident conditions. The decay heat removal system is designed by passive or an active cooling system for the dry storage of the spent fuel. A modular storage system was proposed as part of the effort. A modular storage system is a canister-based spent nuclear fuel dry storage system that consists with three or seven canisters. In this study, thermal performance evaluation carried out with computational fluid dynamics(CFD) code to estimate temperature of concrete overpack, canister structure and air according to NUREG-2152 as additional guidance on the use of CFD. Thermal analyses of ventilation system have been carried out for the determination of the optimum inlet and outlet duct height. The computational fluid dynamics code ANSYS CFX 17 was used for the thermal analysis of modular storage system. In this study, thermal performance evaluation carried out with ANSYS CFX, commercial CFD code, to estimate temperature of concrete overpack, canister structure and air. The purpose of this assessment is to derive the area of inlet and outlet for storing a number of canisters. As a result of thermal analysis, the hydraulic diameter has a significant impact on the heat transfer. When the hydraulic diameter are increased, the heat transfer increases.