A nuclear reactor containment system with passive cooling capabilities. In one embodiment, the system includes an inner containment vessel for housing a nuclear steam supply system and an outer containment enclosure structure. An annular water-filled reservoir may be provided between the containment vessel and containment enclosure structure which provides a heat sink for dissipating thermal energy, in the event of a thermal energy release incident inside the containment vessel, the reactor containment system provides passive water and air cooling systems operable to regulate the heat of the containment vessel and the equipment inside. In one embodiment, cooling water makeup to the system is not required to maintain containment vessel and reactor temperatures within acceptable margins.