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
Nuclear power of the fission reactor power-level, principally the nuclear engineering reliance on a negative Void coefficient of reactivity, a form of Passive nuclear safety. The chain reaction can also be manually stopped completely by inserting control rods into the reactor core. While again depending on the reactor design, a mixture of both passive and active Nuclear reactor safety systems, the most well known being the Essential service coolant system, operate to remove decay heat. Alongside this, a number of further Defense in depth (nuclear engineering) principles are relied upon, such as physical containment barriers. Some serious nuclear and radiation accidents have