A hypothetical severe accident was investigated when nuclear fuels are melted. Experiments were performed for natural convection heat transfer of the melted core at the bottom of the reactor vessel. To achieve highly buoyant force, mass transfer experiment was adopted by using copper electroplating system based on the analogy concept. Mean and local Nusselt numbers were measured and compared for 2 different configurations of the melted core: 2-layer and 3-layer oxide pools. The phenomenological analysis was carried out comparing with 2 different oxide pool configurations of the present works and the existing studies. Heat ratio of the lower head to the top plate was affected by Prandtl number and geometrical difference. Compared with the existing studies, the Nu of lower head were about 40% lower, and those of the top plate were about 60% higher. The local Nusselt number ratio of the lower head gradually increased along the upper region and did not become affected by thermal conditions of the top plate. The local top plate Nusselt numbers for the 2D test rig were uniform, while those for the 3D test rig had steep slope with the highest Nusselt number at the center.