A passive safety system for a nuclear power plant (100) cools the plant after shutdown, even when primary water circulation is disabled. The system comprises a source of compressed gas (112, 805) which can be the system's only source of operating energy, a source of external cooling water (106, 500), and interconnection components. If the reactor overheats, the gas is used to force the cooling water into the reactor's core. The gas can be taken from a highly compressed source and decompressed to a lower pressure suitable for forcing the water from the source, in which case the water can first be used to supply heat to the expanding gas to prevent it from freezing its environment. The system can be located underground or can be portable, e.g., carried on railroad cars or other wheeled conveyances. The system can be located above ground, or in a covered trench (705).