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 safety in the United States designs: The proposed AP1000 has a maximum core damage frequency of per plant per year. The European Pressurized Reactor (EPR) has a maximum core damage frequency of per plant per year. According to the Nuclear Regulatory Commission, 20 states in the USA have requested stocks of potassium iodide which the NRC suggests should be available for those living within of a nuclear power plant in the unlikely event of a severe accident. Radioactive iodine (radioiodine) is one of the products that can be released in a serious nuclear power plant accident. Potassium iodide (KI) is a non radioactive form of