As part of joint U.S.-Korean International Nuclear Engineering Research Initiative (INERI) investigating methods to enhance external cooling of advanced reactor vessel under severe accident conditions, a scaling analysis has been performed to study the phenomena of external cooling of an advanced reactor vessel under severe accident conditions. Five key transfer processes have been considered and the characteristic time for each of these processes has been determined and compared with the residence time for external reactor vessel cooling (ERVC) in the flow channel. To complement the scaling analysis, an ERVC upward co-current two-phase flow model has been developed to predict the total mass flow rate induced in the annular channel by the process of downward facing boiling on the vessel outer surface. The model takes into account the wall heat flux level, the geometry of the vessel/insulation system, the local variation of the cross-sectional flow area, and the pressure drops through various segments of the channel. Based on the results of the ERVC flow calculations and the scaling analysis, criteria for experimental simulation have been established to assure that the ERVC phenomena simulated in laboratory-scale experiments would have the same effects as those anticipated for the full-scale reactor system.