Direct auxiliary cooling system (DRACS) is one candidate for FHR (Fluoride-salt-cooled High temperature reactor) decay heat removal system. DRACS relies on buoyancy as the driving force to form natural circulation to remove the decay heat. As a passive engineered safety feature, some key parameters and models must be validated. In order to study the characteristics of the natural circulation of the molten salts, a high-temperature molten salt natural circulation experiment loop has been designed and constructed by the TMSR (Thorium Molten Salt Reactor) center of the Chinese Academy of Sciences (CAS) with nitrate selected to be coolant. A series of experiments have been scheduled to be conducted on the loop, this loop could be used as a validation facility for DRACS. In this paper, steady-state natural circulation experiment results are shown. The results show that NNCL (nitrate natural circulation loop) was running steady and reliable, and the heat can be removed continuously. The RELAP5-MS code is employed to simulate NNCL behavior, and the simulation results coincide with experiment results. The modified RELAP5-MS can be used for the molten salt natural circulation system analysis. Based on these experiments and simulation results, the DRACS system can be used in the molten salt reactor as the decay heat removal system.