The Fukushima I nuclear accidents are a series of ongoing equipment failures and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the 9.0 magnitude Tōhoku earthquake and tsunami on 11 March 2011. The plant comprises six separate boiling water reactors maintained by the Tokyo Electric Power Company (TEPCO). This accident is the largest of the 2011 Japanese nuclear accidents arising from the Tōhoku earthquake and tsunami, and experts consider it to be the second largest nuclear accident after the Chernobyl disaster, but more complex as all reactors are involved.

At the time of the quake, reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. The remaining reactors shut down automatically after the earthquake, with emergency generators starting up to run the control electronics and water pumps needed to cool reactors. The plant was protected by a seawall designed to withstand a 5.7 metres (19 ft) tsunami but not the 14-metre (46 ft) maximum wave which arrived 41 – 60 minutes after the earthquake. The entire plant was flooded, including low-lying generators and electrical switchgear in reactor basements and external pumps for supplying cooling seawater. The connection to the electrical grid was broken. All power for cooling was lost and reactors started to overheat, due to natural decay of the fission products created before shutdown. The flooding and earthquake damage hindered external assistance.

Evidence soon arose of partial core meltdown in reactors 1, 2, and 3; hydrogen explosions destroyed the upper cladding of the buildings housing reactors 1, 3, and 4; an explosion damaged the containment inside reactor 2; multiple fires broke out at reactor 4. Despite being initially shutdown, reactors 5 and 6 began to overheat. Fuel rods stored in pools in each reactor building began to overheat as water levels in the pools dropped. Fears of radiation leaks led to a 20-kilometre (12 mi) radius evacuation around the plant while workers suffered radiation exposure and were temporarily evacuated at various times. One generator at unit 6 was restarted on 17 March allowing some cooling at units 5 and 6 which were least damaged. Grid power was restored to parts of the plant on 20 March, but machinery for reactors 1 through 4, damaged by floods, fires and explosions, remained inoperable. Flooding with radioactive water through the basements of units 1-4 continues to prevent access to carry out repairs.

Measurements taken by the Japanese science ministry and education ministry in areas of northern Japan 30–50 km from the plant showedradioactive caesium levels high enough to cause concern. Food grown in the area was banned from sale. It was suggested that worldwide measurements of iodine-131 and caesium-137 indicate that the releases from Fukushima are of the same order of magnitude as the releases of those isotopes from the Chernobyl disaster in 1986; Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants.[16][17] Plutonium contamination has been detected in the soil at two sites in the plant.[18] Two workers hospitalized as a precaution on 25 March had been exposed to between 2000 and 6000 mSv of radiation at their ankles when standing in water in unit 3.

Japanese officials initially assessed the accident as level 4 on the International Nuclear Event Scale (INES) despite the views of other international agencies that it should be higher. The INES level was eventually raised successively to 5 and then the maximum 7. The Japanese government and TEPCO have been criticized for poor communication with the public and improvised cleanup efforts. Experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up the area. On 20 March, the Chief Cabinet SecretaryYukio Edano announced that the plant would be decommissioned once the crisis was over.

[Source Wikipedia.org]