Abstract: A nuclear reactor, especially a sodium-cooled fast reactor in which the coolant is confined at least partially within a vertical primary tank containing the reactor core and surrounded by a main tank containing vertical heat exchangers and pumps for heat removal and continuous circulation of coolant from one tank to the other. Each heat exchanger is provided with inlet and outlet windows respectively at the upper and lower end for the admission and discharge of coolant derived from the primary tank and is surrounded by an outer shell which penetrates together with the heat exchanger into the region located between the primary tank and the main tank.The outer shell delimits with the heat exchanger an annular space which is joined at the level of the inlet windows to one end of at least one elbowed duct, the other end of which extends into the coolant within the primary tank.