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
Timeline of the Fukushima Daiichi nuclear disaster TEPCO informs the Nuclear and Industrial Safety Agency and the government of Fukushima Prefecture of the results of soil tests for uranium (, and ) carried out around the Fukushima Daiichi plant. The uranium found was considered to be natural, as its isotope ratios were consistent with the natural abundance. At 21:14 a cooling pump at reactor five stops. At 08:12 the next day, work began on a spare pump, and cooling was restored at 12:49. The reactor temperature had risen to 92.2 °C. The cause of the outage is suspected to be motor failure. It is reported that 22