These reactors contribute to almost every field of science, including physics, chemistry, biology, medicine, geology, archeology, and environmental science. A breeder reactor is defined as a reactor that consumes and produces fissile fuel material. Breeder reactors generally produce more fuel than they consume. Farming is the process by which new fissile material is created by capturing neutrons from fissions in fertile materials. Fast breeder reactors are reactors in which the fission reaction is supported by fast neutrons. Fast breeder reactors do not require a moderator, allowing for a variety of working fluids. Two types of fast breeder reactors are gas-cooled fast breeder reactors (GCBR), often cooled by pressurized helium, and liquid metal fast breeder reactors (LMFBR), which are cooled by molten sodium. The reactor core, in the center, has concentrations of ~20% Pu-239 and 80%. The surrounding fuel rods are 100%. The reactor has a high concentration of fissile material inside it, allowing it to sustain a chain reaction even with fast neutrons, despite the lower probability that fast neutrons cause fissions compared to slow ones.