Fukushima Daiichi is a multi-reactor nuclear power site in the Fukushima Prefecture of Japan. A nuclear disaster occurred there after a 9.0 magnitude earthquake and subsequent tsunami on 11 March 2011. The earthquake triggered a scram shut down of the three active reactors, and the ensuing tsunami crippled the site, stopped the backup diesel generators, and caused a station blackout. The subsequent lack of cooling led to explosions and meltdowns, with problems at three of the six reactors and in one of the six spent-fuel pools.

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of 2022, the International Atomic Energy Agency reports there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.