The Fukushima Daiichi nuclear disaster (福島第一原子力発電所事故 Fukushima Dai-ichi (
pronunciation) genshiryoku hatsudensho jiko?) was an energy accident at the Fukushima I Nuclear Power Plant, initiated primarily by the tsunami that was triggered by the Tōhoku earthquake on 11 March 2011. The damage caused by the tsunami produced equipment failures, and without this equipment a loss-of-coolant accident followed, resulting in three nuclear meltdowns and the release of radioactive material beginning on 12 March. It is the largest nuclear disaster since the Chernobyl disaster of 1986 and the second disaster (after Chernobyl) to be given the Level 7 event classification of the International Nuclear Event Scale.
The largest wave in the tsunami arrived 50 minutes after the initial earthquake. The 13 meter tall wave overwhelmed the plant's seawall, which was 10 m high. The moment of impact was recorded by a camera. Water quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded diesel generators failed soon afterwards, resulting in a loss of power to the critical coolant water pumps. These pumps must continuously circulate coolant water through a Generation II reactor for several days to keep the fuel rods from melting, as the fuel rods continued to generate decay heat after the SCRAM event. The fuel rods will become hot enough to melt during the fuel decay time period if an adequate heat sink is not available. After the secondary emergency pumps (run by back-up electrical batteries) ran out, one day after the tsunami, 12 March, the water pumps stopped and the reactors began to overheat. The insufficient cooling eventually led to meltdowns in reactors 1, 2, and 3, where the fuel rods melted through the bottoms of their Reactor Pressure Vessels.
Meanwhile, as workers struggled to supply power to the reactors' coolant systems and restore power to their control rooms, a number of hydrogen-air chemical explosions occurred, the first in Unit 1, on 12 March and the last in Unit 4, on 15 March. It is estimated that the hot zirconium fuel cladding-water reaction in reactors 1-3 produced 800 to 1000 kilograms of hydrogen gas each. The pressurized gas was vented out of the reactor pressure vessel where it mixed with the ambient air, and eventually reached explosive concentration limits in units 1 and 3. Due to piping connections between units 3 and 4, or alternatively from the same reaction occurring in the spent fuel pool in unit 4 itself, unit 4 also filled with hydrogen, resulting in an explosion. In each case, the hydrogen-air explosions occurred at the top of each unit, that is in their upper secondary containment buildings. Drone overflights on 20 March and afterwards captured clear images of the effects of each explosion on the outside structures, while the view inside was largely obscured by shadows and debris.
There have been no fatalities linked to short term overexposure to radiation reported due to the Fukushima accident, while approximately 18,500 people died due to the earthquake and tsunami. One estimate of the total human fatalities which will result from the nuclear accident is up to 10,000, with a maximum cancer mortality and morbidity of 1,500 and 1,800, respectively. In addition, the rates of psychological distress among evacuated people rose fivefold compared to the Japanese average due to the experience of the disaster and evacuation.
