Japan has raised the severity of its nuclear crisis from level 5 to the highest level (level 7), describing it at a "major accident" and putting it on a par with the Chernobyl disaster. Whilst reporting this, The Telegraph (click to view full article) has also included details of previous incidents at different levels - it makes interesting reading.
The International Nuclear Event Scale is intended to be logarithmic therefore each increasing level represents an accident approximately ten times more severe than the previous level. Due to the complexity of evaluating the severity of a nuclear accident it is often some time after the accident that the level will be set and in this case the level was increased after another major aftershock in the east of Japan.
LEVEL 7: MAJOR ACCIDENT
A major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.
CHERNOBYL, 26 April 1986 - An explosion and fire released large quantities of radioactive contamination into the atmosphere, which spread over much of Western Russia and Europe.
LEVEL 6: SERIOUS ACCIDENT:
A significant release of radioactive material likely to require implementation of planned countermeasures.
KYSHTYM, 29 September 1957 - Significant release of radioactive material to the environment from explosion of high activity waste tank.
LEVEL 5: ACCIDENT WITH WIDER CONSEQUENCES:
A limited release of radioactive material likely to require implementation of some planned countermeasures and several deaths from radiation.
WINDSCALE PILE, 10 October 1957 - A release of radioactive .material following a fire in a reactor core.
THREE MILE ISLAND, 28 March 1979 - A combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown. Radioactive gases were released into the atmosphere.
LEVEL 4: ACCIDENT WITH LOCAL CONSEQUENCES:
A minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls and fuel melt, or damage to fuel resulting in more than 0.1 per cent release of core inventory, and the release of significant quantities of radioactive material within an installation with a high probability of significant public exposure.
SAINT-LAURENT-DES-EAUX, France, 1980 - Melting of one channel of fuel in the reactor with no release outside the site.
TOKAIMURA, Japan, 1999 - Three inexperienced operators at a reprocessing facility caused a criticality accident; two of them died.
FLEURUS, Belgium, 2006 - Severe health effects for worker at a commercial irradiation facility as a result of high doses of radiation.
LEVEL 3: SERIOUS INCIDENT
Exposure in excess of ten times the statutory annual limit for workers. Non-lethal deterministic health effect (e.g., burns) from radiation.
LEVEL 2: INCIDENT
Exposure of a member of the public in excess of 10 mSv.
Exposure of a worker in excess of the statutory annual limits.
LEVEL 1: ANOMALY
Overexposure of a member of the public in excess of statutory annual limits. Minor problems with safety components with significant defence-in-depth remaining. Low activity lost or stolen radioactive source, device or transport package.