Abstract: An autonomous, decentralized fast breeder reactor includes a single reactor main vessel which houses a plurality of small-size reactor subsystems each having a small-scale fast breeder reactor core, and a plurality steam generator subsystems. These subsystems function in an autonomous manner and are caused to undergo a heat transfer with one another by a coolant circulating naturally through the interior of the main vessel, thereby constructing a cooperatively operating system. Steam generated by the steam generators is introduced to a turbine system and utilized in generating electricity. The condensate from the turbines is cooled by a heat accumulating pool, and the heat is utilized in a separate system. The entire system is installed underground and use is made of the difference in elevation. Use also is made of solid bedrock to construct a housing facility for the reactor.