Japan Tuesday raised the severity level of its nuclear crisis to put it on par with the Chernobyl accident 25 years ago, the worst atomic power in history.
But what does that mean?
The International Atomic Energy Agency (IAEA) -- an inter-governmental organization for scientific co-operation in the nuclear field -- said it uses the scale to communicate to the public in a consistent way the safety significance of nuclear and radiological events.
The International Nuclear and Radiological Event Scale, or INES, ranges from one to seven. The most serious level is a seven, which refers to a "major accident," while a one is an "anomaly." The scale is designed so the severity of an event is about 10 times greater for each increase in level.
The following are some examples of accidents according to their INES level from the IAEA, see here
LEVEL 7 - MAJOR ACCIDENT
A major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.
* CHERNOBYL, Soviet Union (now Ukraine), 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, Soviet Union (now Russia), 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.
* THREE MILE ISLAND, USA, 1979 - Severe damage to reactor
core. This event galvanized opposition to a growing core
anti-nuclear power movement in the United States. After
this event, energy companies did not start the construction
of any new reactors in the United States for over 30 years
and stopped work on several reactors that were already under
* WINDSCALE PILE, UK, 1957 - A release of radioactive
material following a fire in a reactor core
* GOIANIA, Brazil, 1987 - Four people died and six people
received high doses of radiation.
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 percent release of core inventory, and the release of significant quantities of radioactive material within an installation with a high probability of significant public exposure.
* TOKAIMURA, Japan, 1999 - Fatal overexposure of workers
following a criticality event at a nuclear facility.
* SAINT-LAURENT-DES-EAUX, France, 1980 - Melting of one
channel of fuel in the reactor with no release outside
* FLEURUS, Belgium, 2006 - Severe health effects for
worker at a commercial irradiation facility as a result of
high doses of radiation.