LMFBR (Liquid Metal Fast Breeder Reactor) is a type of nuclear reactor that uses liquid sodium as its coolant and U-238 to absorb neutrons, which in turn will breed Pu-239, an element which can be used in nuclear fissions. U-238 does not produce a nuclear fission, so it is not used as the primary fuel source. If it absorbs a stray neutron from the nuclear fission, it will simply absorb it, which is why it is not used in other types of reactors. But it will decay to Pu-239 which is fissile. This is the primary fuel source in LMFBR. An example of this would be the "Super-Phonix" reactor in France.
