1.NounA nuclear reactor with no moderator, using fast neutrons directly in a controlled fission chain reaction without slowing them down, allowing for a much more compact reactor which uses fissile material much more efficiently and generates larger quantities of high-energy neutrons which can be used for breeding more fissile material or producing other useful radioactive nuclides, but requiring the use of highly-enriched uranium (at least 20% uranium-235) or other fuel containing a similarly high concentration of fissile material.