堆内熔融物滞留（in-vessel retention, IVR）是第三代反应堆广泛采用的一种严重事故缓解策略，通过向反应堆压力容器（reactor pressure vessel, RPV）与保温层的间隙注水来解决堆芯熔融物冷却的问题。在目前的压水堆安全分析中，IVR策略的成功准则是堆内熔池导出至RPV壁面的局部热通量不超过下封头冷却水侧的临界热通量（critical heat flux, CHF）。在堆内熔池形成的过程中，不相容成分在熔化后会发生动态分层过程，进而影响熔池的温度场和速度场以及RPV钢壳上热负荷的分布。为了探讨熔池分层过程对IVR策略有效性的影响，搭建一个数值模拟模型，模拟从混合态单层熔池到轻金属，重金属和氧化物相互分离的动态分层过程，在此基础上研究了熔池内的热工现象和RPV下封头的完整性。模拟结果表明，在RPV外壁面冷却条件良好的情况下，下封头的热负荷明显小于相应的CHF限值，证明了IVR策略的有效性。

In-vessel retention (IVR) strategy is a solution widely adopted by Generation Ⅲ reactors to deal with the corium coolability problem by flooding the reactor pit (cavity) during a serve accident of pressurized water reactor (PWR). The stratification of different melted core internals will influence the temperature field and velocity field of the corium pool, thereby affecting the distribution of heat load on the RPV steel shell. In the current safety analysis of PWR, the success criteria for IVR strategy is the local heat flux imposed from the corium pool to the reactor pressure vessel (RPV) wall should not exceed the critical heat flux (CHF) of boiling on the lower head. CFD model was built to simulate multiform dynamic stratification processes of core melt from a mixed state to three layers of light metal, heavy metal and oxide, provides a basis for the further study of thermal hydraulic phenomenons in the corium pool and the integrity of RPV lower head. The simulation results shows that, with the good cooling condition on the outer wall of RPV, the thermal loads on the lower head is significantly less than the corresponding CHF limits under different stratification process, and the integrity of lower head can be maintained during a severe accident.