Published on Feb 20, 2016
The Fukushima Daiichi nuclear disaster (福島第一原子力発電所事故 Fukushima Dai-ichi (About this sound pronunciation) genshiryoku hatsudensho jiko?) was an energy accident at the Fukushima I Nuclear Power Plant, initiated primarily by the tsunami of 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 with three nuclear meltdowns and releases of radioactive materials 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 plant comprised 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 earthquake, reactors 4, 5 and 6 were shut down in preparation for re-fueling. However, their spent fuel pools still required cooling. Immediately after the earthquake, the electricity producing reactors 1, 2 and 3 automatically shut down their sustained fission reactions, inserting control rods in what is termed a SCRAM. Following this legally mandated “safety precaution” which ceases the reactors’ normal running conditions, the reactors were unable to generate power to run their own coolant pumps. Emergency diesel generators came online, as designed, to power electronics and coolant systems, all of which operated right up until the tsunami destroyed the generators for reactors 1–5 due to their location in unhardened low-lying areas. The two generators cooling reactor 6 were undamaged and were sufficient to be pressed into service to cool the neighboring reactor 5 along with their own reactor, averting the overheating issues that reactor 4 suffered.