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
Anti-nuclear movement in the United States do better to contemplate a radical redesign, or if that was not possible, to abandon such technology entirely. Nuclear power plants are a complex energy system. and opponents of nuclear power have criticized the sophistication and complexity of the technology. Helen Caldicott has said: "... in essence, a nuclear reactor is just a very sophisticated and dangerous way to boil water – analogous to cutting a pound of butter with a chain saw." These critics of nuclear power advocate the use of energy conservation, efficient energy use, and appropriate renewable energy technologies to create our energy future. Amory Lovins, from