In-vessel retention-external reactor vessel cooling (IVR-ERVC ) is a key severe accident management strategy , and the present assessment method on the effectiveness of IVR is mainly based upon the lumped parameter model . With the enlargement of core mass and the RPV size ,the increase of Rayleigh number was found in the calculation of nature convection heat transfer for advanced large size PWR .T he applicability of several natural convection heat transfer correlations was studied and compared ,and natural convection heat transfer under the high Rayleigh number was simulated by using lumped parameter analysis code . A two-layered stratified molten pool configuration was assumed to be developed in the reactor vessel lower head .The heat flow distribution at the outer boundary of RPV wall was simulated ,and the code was applied to assess the feasibility of the IVR-ERVC strategy under a selected severe accident for advanced large size PWR .Recommendation for the designs of reactor core internals was proposed .%熔融物堆内滞留-压力容器外部冷却（IVR-ERVC）是一种重要的核电厂严重事故缓解措施。当前针对IV R有效性评价的方法主要是基于集总参数模型对下封头熔池换热进行分析。在大功率先进压水堆熔池集总参数法计算中，堆芯重量变大、压力容器尺寸增加会导致熔池自然对流换热中的瑞利数Ra′增大。通过使用集总参数分析程序，对比研究熔池氧化层各换热模型的适用范围，计算大功率先进压水堆高瑞利数条件下稳态熔池的自然对流换热，模拟两层稳态熔池模型中压力容器外壁面的热流密度分布，对其进行选定严重事故序列下的IVR-ERVC有效性评价，并对堆内构件设计提出建议。
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