meltdown

meltdown, occurrence in which a huge amount of thermal energy and radiation is released as a result of an uncontrolled chain reaction in a nuclear power reactor. The chain reaction that occurs in the reactor’s core must be carefully regulated by control rods, which absorb neutrons, and a moderator, which reduces their energy. If the core becomes too hot, it can melt, releasing large amounts of radiation. See also Three Mile Island accident of 1979, Chernobyl accidentof 1986, and Fukushima accident of 2011.