Abstract: A double tank type fast breeder reactor including a reactor vessel, and a tank of a secondary cooling system located outside the reactor vessel. The reactor vessel contains a core and installations of a primary cooling system, and the tank contains circulation pumps of the secondary cooling system and steam generators alternately located in the tank, and a body of sodium serving as a secondary coolant located at outer peripheries of the circulation pumps and steam generators. The tank includes an annular portion defined in the tank between walls of the tank and reactor vessel serving as a path of circulation of the secondary coolant.