Fast breeder reactors (FBR) are nuclear reactors into which research is currently being carried out at Monju by Japan Atomic Energy Agency, a national research and development agency. They use plutonium-uranium mixed-oxide (MOX) as fuel while non-fissionable uranium-238 is converted into plutonium to produce more plutonium than the actual amount of fuel used.

 The primary coolant used by fast breeder reactors is liquid sodium, which has excellent levels of thermal transfer and helps prevent the deceleration of neutrons. The heat generated within the nuclear reactor is transferred from the primary coolant across to the secondary liquid sodium coolant in an intermediate heat exchanger. Following this, water is turned into steam in the steam generator, which is then used to turn the turbine generator and produce electricity.