Passive indirect shutdown cooling system for nuclear reactors

A nuclear reactor system is described comprising: a reactor core and a main heat transport path containing a main heat removal component; at least one main coolant pump and a first coolant wherein, during normal operation, the first coolant is pumped by the main coolant pump through the reactor core to the main heat removal component and back to the reactor core to transport heat generated in the reactor core to the main heat removal component; the main heat transport path including a heat exchanger located in that path after the main heat removal component's outlet with the secondary side of the heat exchanger being included in a decay heat removal loop containing a second liquid coolant and having a vapor separator connected to an outlet of the secondary side of the heat exchanger; the separator's outlet being connected to an inlet of a further heat exchanger located in a reservoir containing a third liquid coolant which forms a heat sink; the further heat exchanger's outlet being connected to an inlet to the secondary side of the heat exchanger; and wherein the further heat exchanger is located at a higher elevation than the heat exchanger whereby a natural convection flow can occur in the decay heat removal path, the vapor/liquid interface in the loop normally being at a higher elevation than the heat sink which restricts any natural convection flow until boiling of the second liquid coolant occurs in the heat exchanger