Abstract: The molten pool oxide-layer heat transfer (MORN) experiment was built to investigate the heat transfer characteristics of the oxide layer in a three-dimension molten pool with water and nitrate simulants. The results indicate that different bottom cooling boundary conditions can affect the molten pool temperature and the power split ratio. Under forced water cooling condition, there is a great fluctuation of the heat flux of molten pool wall, and the maximum qlocal/qmean is 6.5-7.9 times of the minimum qlocal/qmean. When the top and the bottom cooling boundary conditions of the reactor pressure vessel (RPV) lower head are the same, the power split ratio is approximately 100%. The power split ratio depends not only on the type of top and bottom cooling boundary conditions, but also on whether the top and bottom cooling boundaries are sufficiently cooled, i.e., the power split ratio is not always 100%. The correlation of MORN-Nitrate wall heat flux distribution is applied to the calculation of heat flux at the RPV lower head of AP1000, and it is found that the lower head would not fail during the core degradation accident, and IVR strategy is effective.