Abstract: A nuclear reactor system including a heat transport path around which a coolant is pumped by main circulation pumps to transport heat from the reactor core to a steam generator. A further decay heat removal path is connected in parallel with the steam generator and main pumps and has a heat exchange component located at an elevation such that a natural convection flow will occur in the decay heat removal path when the main pumps are shutdown. Means are provided to mainain a small flow in the decay heat removal path during normal operation of the main pump. That small flow is in the intended direction of the natural convection circulation flow and maintains a temperature difference within the decay heat removal path that provides a buoyancy force to immediately start a natural circulation flow if the main pumps are shutdown. The main pumps are to be shutdown, as is the reactor, on loss of the steam generator as a heat sink.