an improved shut - down heat removal system for a liquid metal nuclear reactor of the type having a vessel for holding hot and cold pools of liquid sodium is disclosed herein . generally , the improved system comprises a redan or barrier within the reactor vessel which allows an auxiliary heat exchanger to become immersed in liquid sodium from the hot pool whenever the reactor pump fails to generate a metal - circulating pressure differential between the hot and cold pools of sodium . this redan also defines an alternative circulation path between the hot and cold pools of sodium in order to equilibrate the distribution of the decay heat from the reactor core . the invention may take the form of a redan or barrier that circumscribes the inner wall of the reactor vessel , thereby defining an annular space therebetween . in this embodiment , the bottom of the annular space communicates with the cold pool of sodium , and the auxiliary heat exchanger is placed in this annular space just above the drawn - down level that the liquid sodium assumes during normal operating conditions . alternatively , the redan of the invention may include a pair of vertically oriented , concentrically disposed standpipes having a piston member disposed between them that operates somewhat like a pressure - sensitive valve . in both embodiments , the cessation of the pressure differential that is normally created by the reactor pump causes the auxiliary heat exchanger to be immersed in liquid sodium from the hot pool . additionally , the redan in both embodiments forms a circulation flow path between the hot and cold pools so that the decay heat from the nuclear core is uniformly distributed within the vessel .