The Fukushima Daiichi nuclear disaster was an energy accident at the Fukushima Nuclear Power Plant in Fukushima, Japan, initiated primarily by the tsunami following the Tōhoku earthquake on 11 March 2011.
Immediately after the earthquake, the active reactors automatically shut down their sustained fission reactions. However, the tsunami disabled the emergency generators that would have provided power to control and operate the pumps necessary to cool the reactors. The insufficient cooling led to three nuclear meltdowns, hydrogen-air chemical explosions, and the release of radioactive material.
No deaths followed short term radiation exposure, though there were a number of deaths in the evacuation of the nearby population,while 15,884 died (as of 10 February 2014 due to the earthquake and tsunami).
Radioactive material was released from the containment vessels for several reasons: deliberate venting to reduce gas pressure, deliberate discharge of coolant water into the sea, and uncontrolled events. Concerns about the possibility of a large scale release led to a 20-kilometre (12 mi) exclusion zone around the power plant and recommendations that people within the surrounding 20–30 km zone stay indoors.
Later, the UK, France and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading contamination. In 2015, the tap water contamination was still higher in Tokyo compared to other cities in Japan. Trace amounts of radioactivity, including iodine-131, caesium-134 and caesium-137, were widely observed.