fast reactor

(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.

Japanese Meaning

高速炉（高速中性子炉）：減速材を用いず、速い中性子で制御された核分裂連鎖反応を実現する原子炉。これにより、よりコンパクトな炉心設計や燃料の効率的利用、さらには核分裂性物質の増殖や他の有用な放射性核種の生成が可能となる。ただし、高濃縮ウラン（ウラン‐235が20%以上）等の高濃度の核分裂性物質が必要となる。