Editing Nuclear chain reaction (section)

== Nuclear power plants and control of chain reactions ==
{{main|Chicago Pile-1|Nuclear reactor physics|Nuclear criticality safety}}

Chain reactions naturally give rise to reaction rates that grow (or shrink) [[Exponential growth|exponentially]], whereas a nuclear power reactor needs to be able to hold the reaction rate reasonably constant. To maintain this control, the chain reaction criticality must have a slow enough time scale to permit intervention by additional effects (e.g., mechanical control rods or thermal expansion). Consequently, all nuclear power reactors (even [[fast-neutron reactor]]s) rely on delayed neutrons for their criticality. An operating nuclear power reactor fluctuates between being slightly subcritical and slightly delayed-supercritical, but must always remain below prompt-critical.

It is impossible for a nuclear power plant to undergo a nuclear chain reaction that results in an explosion of power comparable with a nuclear weapon, but even low-powered explosions from uncontrolled chain reactions (that would be considered "fizzles" in a bomb) may still cause considerable damage and [[Nuclear meltdown|meltdown in a reactor]]. For example, the [[Chernobyl disaster]] involved a runaway chain reaction, but the result was a low-powered steam explosion from the relatively small release of heat, as compared with a bomb. However, the reactor complex was destroyed by the heat, as well as by ordinary burning of the graphite exposed to air.<ref name=Lamarsh>{{cite book |last=Lamarsh |first=John |author2=Baratta, Anthony  |title=Introduction to Nuclear Engineering |year=2001 |publisher=Prentice Hall |isbn=978-0-201-82498-8 }}</ref> Such steam explosions would be typical of the very diffuse assembly of materials in a nuclear reactor, even under the worst conditions.

In addition, other steps can be taken for safety. For example, power plants licensed in the United States require a negative [[void coefficient]] of reactivity (this means that if [[loss of coolant accident|coolant is removed]] from the reactor core, the nuclear reaction will tend to shut down, not increase). This eliminates the possibility of the type of accident that occurred at Chernobyl (which was caused by a positive void coefficient). However, nuclear reactors are still capable of causing smaller chemical explosions even after complete shutdown, such as was the case of the [[Fukushima Daiichi nuclear disaster]]. In such cases, residual [[decay heat]] from the core may cause high temperatures if there is loss of coolant flow, even a day after the chain reaction has been shut down (see [[SCRAM]]). This may cause a chemical reaction between water and fuel that produces hydrogen gas, which can explode after mixing with air, with severe contamination consequences, since fuel rod material may still be exposed to the atmosphere from this process. However, such explosions do not happen during a chain reaction, but rather as a result of energy from radioactive beta decay, after the fission chain reaction has been stopped.