Nuclear reactor with a chain reaction maintained by fast neutrons and generating more fissile material than it consumes. The reactor core is composed of ‘fissile’ U-235 enriched uranium or Pu-239 fuel which is surrounded by a ‘breeder’ blanket of natural uranium containing ‘fertile’ U238. This ‘fertile’ U-238 is converted into the fissile Pu-239 by capture of neutrons (escaping from the core) and two subsequent beta decays. The reactor core is designed to obtain a high ‘breeding’ ratio, practically exclusively with fast neutrons. Since the neutrons should be decelerated as little as possible, water is excluded as a coolant due to its decelerating effect. For technical reasons, sodium, which is liquid above temperatures of 97.8°C, is particularly well suited. By converting the natural uranium in the ‘breeder’ blanket into Pu239 which can then be recycled into fuel, the fast breeder can utilize uranium up to 60 times better than by thermal reactors such as PWRs.