Abstract: The corium in the lower plenum mainly containing U-Zr-O-Fe may separate into two immiscible liquids. However, there are significant differences between existing 3-layer models. The configurations of the pool in the lower plenum for AP1000 is assessed by three well-accepted 3-layer models developed by Esmaili & Khatib-Rahbar, Seiler, Salay & Fichot respectively. In addition, the heat transfer in the pool for these configurations is calculated in present study. The results show that the configuration which has the higher heat flux on the vessel contacted with the light metallic layer is obtained by the model developed by Seiler compared with Esmaili & Khatib-Rahbar model. The 3-layer model implemented in MAAP5 code is based on the Salay & Fichot model to treat the compositions in oxide and heavy metallic layers, but assumes that the light metallic layer forms automatically after the corium falls down, which should be developed further.