This week the Japanese nuclear authorities announced that the partial meltdowns at four of Fukushima I's reactors would be raised to a 'category 5' incident. I had never come across this scale before and, whilst it clearly relates to the seriousness of the incident, I didn't know what it was out of, or what the criteria were.
The scale is the International Nuclear and Radiological Event Scale (INES). It was introduced in 1990 by the International Atomic Energy Agency (IAEA) and is designed to enable clear communication on nuclear incidents. The scale has eight levels from level 0 to level 7. There has only been one level 7 incident (Chernobyl) and one level 6 incident (Kyshtym) (both in the former Soviet Union).
Fukushima joins Three Mile Island (USA), Windscale (UK) and Chalk River (Canada) on the list of level 5 incidents at nuclear power plants.
Japan in 'battle against time' says IAEA