Under normal refueling and emergency full-core offload condition, the fuel assembly is removed to the Spent Fuel Pool (SFP). Decay heat produced by the spent fuel is carried out by the cooling system. Active cooling method is adopted by the traditional PWR nuclear power plants, which means decay heat is taken away depending on forced circulation of the pump. However, the spent fuel pool, under accident condition, will lost the forced circulating cooling capacity, which will be a threat of for the fuel building safety. To study the thermal-hydraulic characteristics in the SFP missing the forced cooling, through CFX methodology and experiment, change of temperature and heat transfer coefficient of the wall of the heating tube at different heights were discussed, meanwhile the streamline chart and temperature contour were obtained as well. The present result indicated that under different power conditions, different height of water temperature increased at first and then trend to stable at saturation temperature. For a single 9*9 spent fuel assembly, water temperature at the higher height is higher than the lower at the same time, and water temperature at higher location reached a stable value more quickly. In addition, power value had a significant impact on the time of reaching saturation temperature, for example, 7000s is needed to reach saturation under 8.68KW condition while only 3000s under 16.12KW, which illustrates that fuel unload power is crucial to the SFP safety. Based on the experiment data and single phase calculation, heat transfer coefficient at different height of the heating tube decreased slowly at first, and then increased. Especially, heat transfer coefficient at the highest test point rapidly decreased at one point because of boiling crisis.

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