Abstract: Experiments regarding air natural circulation and hydrogen distribution inside the fuel building under loss of coolant accidents of the spent fuel pool and spent fuel uncovered were conducted, and the test data of gas temperature field and concentration distribution were acquired. It is found that heating rate will slow down while temperature rising because of heat removal effect of air circulation. Natural cross ventilation between the building and environment evidently delays heating process, and after 24 h the temperature of heaters is below 490 ℃. Inside the spray zone air temperature decreases rapidly, and average air temperature decreases by 100 ℃ within 5 min. Hydrogen stratifies in air, and the stratification is broken down by spray within 2 min. Regarding the hydrogen/steam atmosphere, as steam condensing and air inflowing the mixed gas becomes combustible, however spray makes no positive effect. On the basis of this work, both active and passive mitigation strategies are proposed.