Abstract: The existence of the hemispherical narrow gap between the molten corium and the lower head during severe accidents is very significative to the integrity of the lower head. Based on the counter current flow limitation between the vapour phase and liquid phase, a mechanism model and a predicting formula of CHF in hemispherical narrow gap were proposed. The established model was validated by comparing the predicted results with the test data, which showed good agreement between them. Furthermore, the effects of key parameters including the system pressure, the radius of molten corium and the gap size on the critical heat flux were also analyzed. Besides, the accident of TMI-2 was also calculated by using the predicting formula. The results indicate that the narrow gap between the molten corium and the lower head can take away the afterheat of the reactor core effectively and ensure the integrity of the lower head.