The Fukushima Daiichi nuclear disaster was an energy accident at the Fukushima Daiichi Nuclear Power Plant in Ōkuma, Fukushima Prefecture, started 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 ensuing tsunami flooded the emergency generators that were providing power to the pumps that cooled the reactors. The coolant loss led to three nuclear meltdowns, hydrogen-air explosions, and the release of radioactive material in Units 1, 2 and 3 between 12 and 15 March. Coolant loss also raised concerns over the recently loaded spent fuel pool of Reactor 4, which increased in temperature on 15 March due to decay heat from the freshly added spent fuel rods but did not boil down to exposure.