A nuclear reactor using fast neutrons for fission, producing in breeding envelope more fissionable fuel than it consumes. A mixture of uranium and plutonium serves as fuel for fast neutron reactors. They do not use a moderator and use molten sodium as a coolant. The main advantage of fast neutrons is that, after collision with depleted uranium, they can produce fissile plutonium. The core of such reactors is surrounded by a breeder blanket, where fission is almost nonexistent. The resulting plutonium thus has the optimum isotope ratio for further use as fuel. If the reactor is set up to produce more plutonium than it consumes, we speak of a breeder. Fast breeder reactors can also fission actinides, the main component of high-level waste.