When the tsunami overtopped the 5.7-m (19-ft) seawall protecting the Fukushima Daiichi nuclear power plant, it flooded the diesel-powered emergency generators responsible for circulating water to cool the plant’s nuclear reactors. With the local electrical grid knocked out by the earthquake and the backup generators off-line, the nuclear fuel in the cores of the three active reactors at the plant began to overheat.
The water that normally kept the nuclear fuel submerged within the reactor cores boiled off, exposing the nuclear material to the air and further elevating temperatures inside the cores. As the overheated nuclear fuel melted (an event called a nuclear meltdown), chemical reactions within the reactors generated hydrogen gas, which set off explosions in each of the three reactor buildings, releasing radioactive material into the air. To prevent a full-blown catastrophe that could render large portions of their nation uninhabitable, Japanese authorities flooded the reactor cores with seawater pumped in from the ocean.
The 1–2–3 punch of the earthquake–tsunami–nuclear accident left 18,000 people dead and caused hundreds of billions of dollars in material damage. Around 340,000 people were displaced from their homes, and a 20-km (12-mi) area around the Fukushima Daiichi plant has been permanently evacuated due to unsafe levels of radioactive fallout in the soil. Some of the greatest concerns center on contaminated food and water, so crops and seafood from the region will require testing for radiation for many years to come.