(nuclear physics, engineering) A nuclear reactor with no moderator, using fast neutrons directly in a controlled fission chain reaction without slowing them down; this allows for a much more compact reactor, and causes the reactor to use fissile material much more efficiently, and to generate larger quantities of high-energy neutrons which can be used for breeding more fissile material or producing other useful radioactive nuclides, but requires the use of highly-enriched uranium (at least 20% uranium-235) or other fuel containing a similarly high concentration of fissile material.