Officials of the Nuclear and Industrial Safety Agency (NISA) initially did not believe that events at the Fukushima No. 1 nuclear power plant after the March 11 Great East Japan Earthquake were serious enough to be categorized as an accident.
Officials of the Nuclear and Industrial Safety Agency (NISA) initially did not believe that events at the Fukushima No. 1 nuclear power plant after the March 11 Great East Japan Earthquake were serious enough to be categorized as an accident.
Events rated at a level 4 or above on the International Nuclear and Radiological Event Scale (INES) are considered an "accident." However, NISA officials at first viewed events at the plant as a "serious incident," ranking it as a level 3 on the scale.
On the night of March 12, NISA officials announced that the situation at the Fukushima plant was severe enough to be classified as a level 4 on the scale, or an "accident with local consequences."
The level 3 assessment was made at 12:30 a.m. on March 12, about 10 hours after the magnitude-9.0 earthquake struck. The quake cut non-emergency power sources, and the subsequent tsunami then flooded the emergency generators necessary to cool the No. 1 to No. 3 reactors at the Fukushima No. 1 plant, as well as the No. 1, No. 2 and No. 4 reactors at the Fukushima No. 2 plant.
About two and a half hours later, an announcement was made that radioactive steam was to be vented from one of the reactors at the Fukushima No. 1 plant. It is believed that the quake and tsunami had by then damaged piping to the reactors and that radioactive materials were already leaking into the outside atmosphere.
At 6 a.m. on March 12, radiation levels at the central control rooms of the reactors were found to be about 1,000 times above normal levels.
On March 18, NISA officials upgraded the assessment to a level 5, placing the Fukushima accident on the same level as the Three Mile Island accident in 1979. On April 12, NISA officials again revised their assessment upward to a level 7, placing it at the same level as the Chernobyl nuclear accident of 1986.
The INES has eight levels to assess incidents and the general rule is that a provisional assessment should be reported to the International Atomic Energy Agency within 24 hours.
While Kenji Sumita, professor emeritus of nuclear engineering at Osaka University, admitted it was difficult to assess a rapidly changing situation, he said, "In hindsight, their assessment of the situation was faulty."