The experimental fast reactor JOYO at the O-arai Engineering Center of the Japan Nuclear Cycle Development Institute is the first liquid sodium fast reactor in Japan. The purpose of constructing JOYO was to obtain technical information about liquid-metal fast breeder reactors (LMFBRs) through experience with their design, construction, and operation and to use the reactor as a fast neutron irradiation facility for the development of fuels, materials, and other components required for the LMFBR program. Through design, construction, testing, operation, and maintenance experience, JOYO has contributed much to the LMFBR development program. In addition to providing operating experience, many kinds of irradiation tests have been conducted for the development of fuels and materials under the conditions of higher fast neutron flux and temperature than those in light water reactors. JOYO has been operated successfully for a quarter-century without any serious problem, and this operation demonstrated the safety and reliability of the sodium-cooled fast reactor.

The reactor has just been upgraded to the MK-III core to increase irradiation capability for playing a greater role in providing an irradiation field as a fast reactor. Given the worldwide trend of fast reactor shutdowns, JOYO is an increasingly valuable world resource for current and future reactor development.