Following is a factfile on the International Nuclear Events Scale (INES), following Japan's decision on Tuesday to upgrade the gravity of the Fukushima accident from five to seven on this benchmark.
- The UN's International Atomic Energy Agency (IAEA) describes INES as a tool for "promptly communicating to the public in consistent terms" what a nuclear episode means. It has three factors: radioactivity releases to the public, barriers against radiation at a nuclear site and civil-defence measures.
- Implemented in 1990, it has a zero-to-seven rating of gravity, where seven is the maximum. Levels one to three are categorised as incidents, and levels four to seven as accidents. Each increase in level on the scale indicates a roughly 10-fold increase in severity.
- There has only been one seven-rated accident, the 1986 Chernobyl disaster, and one six-rated accident, at Kyshtym, Russia, in 1957, in an explosion at a waste tank.
- There have been two five-rated accidents, comprising a fire at the Windscale nuclear plant in northwestern England in 1957 and the partial meltdown at Three Mile Island in Pennsylvania in 1979.
- Following are the steps in the INES scale, followed by some of the general criteria for listing an event:
. ZERO: Events known as "deviations" that have no safety significance
. LEVEL ONE: Anomaly
Minor problem with safety components at a nuclear facility, but significant safety margin remaining
. LEVEL TWO: Incident
Radiation levels in an operating area of a nuclear facility of more than 50 millisieverts (mSv) per hour. Exposure of a member of the public to radiation in excess of 10 mSv, exposure of a worker in excess of statutory annual limits.
. LEVEL THREE: Serious Incident
Severe contamination in an area of a facility, with non-lethal injuries such as radiation burns. Low probability of significant public exposure.
. LEVEL FOUR: Accident with local consequences
Partial meltdown or damage to fuel, release of significant quantities of radioactive material within an installation. No counter-measures likely to be needed other than local food controls.
. LEVEL FIVE: Accident with wider consequences
Severe damage to reactor core, large quantities of radioactive material released within a site. Limited release of material to the wider environment, requiring implementation of some planned countermeasures.
. LEVEL SIX: Serious accident
Significant release of radioactive material likely to require implementation of planned countermeasures.
. LEVEL SEVEN: Major accident
Major release of radioactive material with widespread health and environmental effects, requiring implementation of planned and extended countermeasures.