Meltdown

damage at Fukushima Daiichi power plantTwo of the damaged containment buildings at the Fukushima Daiichi nuclear power plant, northeastern Fukushima prefecture, Japan, several days after the March 11, 2011, earthquake and tsunami that crippled the installation.

Meltdown, is the release of thermal energy and radiation because of inadequate cooling 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 (usually water), 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 accident of 1986, and Fukushima accident of 2011.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Erik Gregersen.
Citation Information
Article Title: Meltdown
Website Name: Encyclopaedia Britannica
Publisher: Encyclopaedia Britannica, Inc.
Date Published: 29 October 2026
Access Date: November 13, 2026