Accident rating of the Fukushima Daiichi nuclear disaster

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 a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on March 11, 2011. It is the largest nuclear disaster since the Chernobyl disaster of 1986.
The plant comprises six separate boiling water reactors originally designed by General Electric (GE), and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. Immediately after the earthquake, the remaining reactors 1-3 shut down automatically, and emergency generators came online to control electronics and coolant systems. However the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency...