中文摘要：

严重事故下熔融物与下封头间球形窄缝通道的存在对于下封头结构的完整性有一定的积极意义。本工作通过理论分析,在汽液两相间逆向对流限制机理的基础上提出了球形窄缝通道内的CHF机理模型和预测关系式,预测结果与实验数据符合较好,验证了所建模型的正确性,并进一步分析了系统压力、熔融物半径、间隙尺寸等关键参数对临界热流密度的影响规律。利用本工作的预测模型对三哩岛（TMI-2）事故后堆芯熔融物特性进行了计算分析,结果表明,熔融物与下封头内壁面间的球形窄缝可有效带走堆芯余热,保证了下封头的完整性。

英文摘要：

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 critieal 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.