A system for passively cooling nuclear fuel in a pressurized water reactor during refueling that employs gravity and alignment of valves using battery reserves or fail in a safe position configurations to maintain the water above the reactor core during reactor disassembly and refueling. A large reserve of water is maintained above the elevation of and in fluid communication with the spent fuel pool and is used to remove decay heat from the reactor core after the reaction within the core has been successfully stopped. Decay heat is removed by boiling this large reserve of water, which will enable the plant to maintain a safe shutdown condition without outside support for many days.