A nuclear reactor using fast neutrons for the fission of uranium and plutonium fuel. Thanks to neutron capture, this type of reactor converts non-fissile uranium ²³⁸U into fissile plutonium ²³⁹Pu, and the same principle can be used to eliminate actinides, an abundant component of high-level radioactive waste. The use of fast neutrons offers improved energy yields from natural uranium, significantly improved nuclear fuel utilization, and the disposal of problematic radioactive waste components.

FBRs do not contain a moderator and mainly use liquid metals (sodium, lead, or lead-bismuth) or helium for cooling. Some have such a configuration of core and breeding envelope that they produce more fissile material than they consume; these are called breeders.