Under severe accident conditions, the most crucial action for recovery from the accident state is to cool the core debris and prevent or terminate attack on the remaining fission product barriers. One means of preventing attack on the containment structures is to retain the core debris within the reactor vessel. The Three Mile Island Unit 2 (TMI-2) accident demonstrated that this could be accomplished by water resident within the reactor vessel combined with injection on a continual basis to quench the debris and remove decay heat over the long term. Some accident situations could result in the transport of molten core debris to the lower plenum, as occurred to some extent (∼20 tonnes) during the TMI-2 accident, boiloff of water in the lower plenum, and an inability to add water to the reactor coolant system (RCS). In this extreme set of circumstances, sufficient external reactor pressure vessel (RPV) cooling may be available to prevent failure of the RPV lower head and, thereby, retain the core debris within the vessel. Containment configurations like Zion would result in substantial accumulation of water around the lower parts of the reactor vessel for most accident sequences. For some pressurized water reactor containments, there could be substantial water accumulation around the reactor vessel and the hot and cold legs before core damage and drainage of debris to the lower plenum. If this water could directly contact the carbon steel vessel surface and RCS piping, substantial energy could be removed from the primary system and in particular the RPV lower head. The experiments, which were performed in support of the Commonwealth Edison individual plant examination and accident management programs, are heat transfer tests designed to demonstrate that nucleate boiling is the dominant heat removal process from the outer surface of a simulated RPV lower head surrounded by typical reflective insulation used in nuclear power plants. With this heat removal mechanism on the outer surface, the heat flux is limited by thermal conduction through the carbon steel head, both for the experiments and for a reactor system. Experiments were performed in which the reactor vessel lower head was simulated with a 0.32-m (12.75-in.)-o.d. pipe cap. Wall thicknesses of 1.75 cm (0.688 in.) and 3.3 cm (1.312 in.) were used to provide substantially different heat fluxes to the outer surface. The heat source was molten iron thermite at a temperature of ∼2400 K, which was poured onto the dry inner surface of the lower head. Water provided cooling on the outer surface. Both uninsulated and insulated configurations were investigated. The measured heat fluxes were essentially the same for these two different cases. This clearly demonstrates that the water flow rate through the insulation is sufficient to supply cooling water to the RPV outer surface under such accident conditions. In addition, the measured heat fluxes are well in excess of those that can be attributed to film boiling. Hence, the vessel outer surface was cooled by nucleate boiling during the entire transient.