Fukushima Daiichi nuclear disaster on the International Nuclear Event Scale (INES). This scale runs from 0, indicating an abnormal situation with no safety consequences, to 7, indicating an accident causing widespread contamination with serious health and environmental effects. Prior to Fukushima, the Chernobyl disaster was the only level 7 event on record, while the Three Mile Island accident was rated as level 5. A 2012 analysis of the intermediate and long-lived radioactivity released found about 10–20% of that released from the Chernobyl disaster. Approximately 15 PBq of caesium-137 was released, compared with approximately 85 PBq of caesium-137 at Chernobyl, indicating the release of of
Radiation effects from the Fukushima Daiichi nuclear disaster radioactive caesium. In addition to the large releases of contaminated water (520 tons and 4.7 PBq) believed to have leaked from unit 2 from mid-March until early April, another release of radioactive water is believed to have contaminated the sea from unit 3, because on 16 May TEPCO announced seawater measurements of 200 Bq per cubic centimeter of caesium-134, 220 Bq per cubic centimeter of caesium-137, and unspecified high levels of iodine shortly after discovering a unit-3 leak. At two locations 20 kilometers north and south and 3 kilometers from the coast, TEPCO found strontium-89 and strontium-90 in the seabed