中文摘要：

采用MELCOR程序,对小型动力堆全部电源丧失严重事故下,下封头失效后＂堆坑＂不同熔融物分层模型进行计算,并对熔融物分层对事故后果的影响进行了研究。结果表明：不同模型下熔融物的总厚度及其变化趋势基本一致;堆腔底板材料为混凝土时,堆芯熔融物的分层较为复杂,而金属材料相对简单。小型动力堆不会发生堆芯熔融物与混凝土相互作用（MCCI）。不同模型的计算对放射性后果基本无影响,但对＂堆坑＂熔穿进程有影响,强迫混合模型熔穿时间最快、机理计算模型熔穿时间最慢。从安全分析的角度,选择强迫混合模型较为保守。

英文摘要：

Using MELCOR code,the different models of molten material stratification in reactor cavity for small power reactor blackout severe accident after pressure vessel lower head failure were calculated,and the effect of molten material stratification on accident consequence was researched.The results show that the total thickness and the change trend of molten material in different models are basically consistent.When choosing concrete as the base material of reactor cavity,the stratification of core molten material is relatively complex,and choosing metal material as the base material of reactor cavity,the stratification of core molten material is relatively simple.The molten coreconcrete interactions（MCCI）will not occur for small power reactor.The calculation of the different models has no influence on radioactive consequence,but has some influence on melting process of the reactor cavity.The melting time of enforced mixing model isthe fastest,and the melting time of mechanistic calculation model is the slowest.For safety analysis,choosing enforced mixing model is more conservative.