The nuclear accident in 2011 at at the Fukushima Daiichi ("Number One") plant in northern Japan, the second worst nuclear accident in history. The facility, operated by the Tokyo Electric and Power Company (TEPCO), was made up of six boiling-water reactors constructed between 1971 and 1979. At the time of the accident, only reactors 1–3 were operational, and reactor 4 served as temporary storage for spent fuel rods.
On March 11, 2011, a magnitude-9.0 earthquake struck at 2:46 PM off the northeastern coast of Honshu, Japan’s main island. Although all three of the reactors that were operating were successfully shut down, however the ensuing tsunami waves generated by the earthquake damaged the backup generators. At 9 PM, an evacuation order is issued for people within 1.9 miles of the plant. The loss of power caused cooling systems to fail in each of them within the first few days of the disaster.
Rising residual heat within each reactor’s core caused the fuel rods in reactors 1, 2, and 3 to overheat and partially melt down, leading at times to the release of radiation. Melted material fell to the bottom of the containment vessels in reactors 1 and 2 and bored sizable holes in the floor of each vessel—a fact that emerged in late May 2011. Those holes partially exposed the nuclear material in the cores. Explosions resulting from the buildup of pressurized hydrogen gas occurred in the outer containment buildings enclosing reactors 1 and 3 on March 12 and March 14, respectively. Workers sought to cool and stabilize the three cores by pumping seawater and boric acid into them.
Due to conerns about radiation exposure, the Japanese government evacuated an area of 20 kilometers (12.5 miles) from plant. However, A third explosion occurred on March 15 in the building surrounding reactor 2. This prompted officials to expand the evacutation zone to a radius of 30 kilometers.
On April 12 nuclear regulators elevated the severity level of the nuclear emergency from 5 to 7—the highest level on the scale created by the International Atomic Energy Agency—placing it in the same category as the Chernobyl accident.
Finally in the middle of December 2011, the Japanese government declared the facility stable, after the cold shutdown of the reactors was completed. Over time, some residents were allowed back into their homes, however, there is a region known as the "difficult-to-return" zone) continued to remain off-limits due to high radiation levels. By March 2017 all evacuation orders in the areas outside the difficult-to-return zone had been lifted.