2011 was a year very unfortunate for humanity full of natural disasters, especially for Japan where a series of earthquakes and tsunamis occurred and caused mass destruction. Fukushima nuclear plant was not an excpetion which only made things much worse. A series of equipment failures, nuclear meltdowns, and releases of radioactive materials have occured which caused panic and chaos everywhere across Japan: cities and towns were evacuated. It was the largest nuclear disaster since the Chernobyl disaster of 1986, and only the second disaster (along with Chernobyl) to measure Level 7 on the International Nuclear Event Scale.

When the earthquake occurred the Reactor 4 had been defueled. Immediately after the earthquake, the remaining reactors 1-3 shut down automatically, and emergency generators came online to power electronics and coolant systems. Even though the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through a nuclear reactor for several days in order to keep it from melting down after being shut down. As the pumps stopped, the reactors overheated and stopped working. As the water went away the reactor fuel rods began to overheat rapidly. In the hours and days that followed, Reactors 1, 2 and 3 experienced full meltdown.

The Japanese government concluded that the total amount of radiation released into the atmosphere was approximately one-tenth which was released during the Chernobyl disaster. Significant amounts of radioactive material have also been released into ground and ocean waters around Japan.

Sources used:

http://www.bbc.co.uk/news/world-asia-18718486
http://www.houseoffoust.com/fukushima/blueprint.html
http://www.nature.com/news/specials/japanquake/index.html