Nuclear chain reaction 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 due to uncontrolled chain reactions, that would be considered "fizzles" in a bomb, may still cause considerable damage and 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
Fukushima Daiichi nuclear disaster (Unit 1 Reactor) Fuel Pool Cooling (FPC) line. On 10 August, the spent fuel pool was switched from the water-injection system – that functioned some 5 months – to a circulatory cooling system. For the first time since the 11 March disaster, all four damaged reactors at the plant were using circulatory cooling systems with heat-exchangers. Fukushima Daiichi nuclear disaster (Unit 1 Reactor) The was a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. It is the largest nuclear disaster since the Chernobyl