Nuclear reactor in which nuclear fissions are caused by fast neutrons because little or no moderator is used WordNet 3.0 & Open English WordNet

A fast-neutron reactor 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, as opposed to slow thermal neutrons used in thermal-neutron reactors. Wikipedia

Nuclear reactor in which the fission chain reaction is sustained by fast neutrons Wikidata

Nuclear reactor which produces more fissile material than it consumes, using fast-moving neutrons and making plutonium-239 from uranium-238, thereby increasing the reactor's efficiency. OmegaWiki