In open pool type research reactor, spent fuel is stored in service pool. So the pool water is the ultimate heat sink and behaves as a shielding barrier for the spent fuel. In service pool, spent fuel is cooled by natural circulation. When accident occurs, it is needed to prevent to increase temperature of spent fuel above th design limit. In this study, cooling the spent fuel by natural circulation is simulated by CFD. Spent fuel is simulated by conservative method, and the temperature of spent fuel storage rack is calculated.