Nuclear reactor which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly when a reactor reaches the critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are "necessary" to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short
Jōyō (nuclear reactor) Jōyō (nuclear reactor) It was made with the purpose of doing tests on and advancing the development of that type of reactor, as an irradiation test facility for construction materials. It also does tests with the nuclear fuel as well as activation experiments. The reactor has gone through 3 different core changes. The current core provides the neutron flux of 4×10 cms for E>0.1 MeV. After an incident in 2007, the reactor is suspended for repairing, recovery works were planned to be completed in 2014. Following the closure of the unsuccessful follow-on fast breeder reactor Monju in 2016, a decision