On detecting the earthquake, the active reactors automatically shut down their normal power-generating fission reactions. Because of these shutdowns and other electrical grid supply problems, the reactors' electricity supply failed, and their emergency diesel generators automatically started. Critically, these were required to provide electrical power to the pumps that circulated coolant through the reactors' cores. This continued circulation was vital to remove residual decay heat, which continues to be produced after fission has ceased.[12] However, the earthquake had also generated a tsunami 14 metres (46 ft) high that arrived shortly afterwards, swept over the plant's seawall and then flooded the lower parts of the reactor buildings at units 1–4. This flooding caused the failure of the emergency generators and loss of power to the circulating pumps
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