In a severe accident of light water reactors, the loss of coolant and heat sink can cause core degradation. The molten fuel pellet (UO2), molten cladding material (Zr-ZrO2), and molten structural materials (stainless steel) form molten corium, and finally relocate in the lower head of the reactor pressure vessel (RPV). In case of coolant loss, the corium molten pool form in the lower plenum which will threaten the integrity of RPV. To prevent the RPV failure, the ex-reactor vessel cooling (ERVC) system, which floods the RPV with cooling water, is applied to ensure the survivability of RPV and eventually achieve the goal of fission product retention. Therefore, the molten pool’s behavior is well studied by a large number of experiments, such as BALI, LIVE-2D, COPRA. Natural heat convection, which is related to the latent heat of fission products and the cooling of ex-vessel, will induce heat flux alone the RPV lower plenum. The peak value of heat flux is estimated at around 70°. However, there is still uncertainty to predict the Nuup/Nudn ratio especially when the condition of the prototype molten pool (1017) to be extrapolated. This research aims at analyzing the scale effects based on Buckinhum’s PI theory. The molten pool phenomenon related to dimensionless parameters is firstly calculated. After rigorous calculation, the influence of each dimensionless parameter is evaluated according to the behavior of the corium pool. Finally, the experimental results of BALI and COPRA are obtained, in order to evaluate the uncertainty between the chosen simulant from those two experiments.