March 12, 2011 / 7:47 PM / in 7 years

Factbox: International nuclear event scale explained

NEW YORK (Reuters) - The Japanese nuclear safety agency rated the damage at a nuclear power plant at Fukushima at a four on a scale of one to seven, which is not quite as bad as the Three Mile Island accident in the United States in 1979, which registered a five. But what does that mean?

The International Atomic Energy Agency -- 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 with the most serious being a seven referred to as 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


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



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



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

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 construction

* 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



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 overexposures 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

the site

* FLEURUS, Belgium, 2006 - Severe health effects for worker

at a commercial irradiation facility as a result of high

doses of radiation

Reporting by Bernie Woodall in Detroit and Scott DiSavino in New York

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