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
Fukushima Daiichi nuclear disaster (Unit 1 Reactor) Fuel Pool Cooling (FPC) line. On 10 August, the spent fuel pool was switched from the water-injection system – that functioned some 5 months – to a circulatory cooling system. For the first time since the 11 March disaster, all four damaged reactors at the plant were using circulatory cooling systems with heat-exchangers. Fukushima Daiichi nuclear disaster (Unit 1 Reactor) The was a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. It is the largest nuclear disaster since the Chernobyl