A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV, on average), as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is comparatively rich in fissile material.
The fast spectrum is key to breeder reactors, which convert highly abundant uranium-238 into fissile plutonium-239, without requiring enrichment. It also leads to high burnup: many transuranic isotopes, such as of americium and curium, accumulate in thermal reactor spent fuel; in fast reactors they undergo fast fission, reducing total nuclear waste. As a strong fast-spectrum neutron source, they can also be used to transmute existing nuclear waste into manageable or non-radioactive isotopes.