The scale is inspired on the Richter scale for earthquakes, each increasing level representing an accident ten times more severe than the previous level. However, compared to earthquakes, where ground elevation is relatively easy to assert, the level of severity of a Man made disaster, such as a nuclear accident, is more subject to interpretation, and often the INES level of an incident is only assigned well past the incident occurrence. Therefore the scale cannot aid immediate appropriate disaster-control deployment.
A number of criteria and indicators are defined to assure coherent reporting of nuclear events by different official authorities. There are 7 levels on the INES scale; 3 incident-levels and 4 accident-levels
7 – Major Accident6 – Serious Accident5 – Accident With Wider Consequences4 – Accident With Local Consequences2 – Incident1 – Anomaly
The level on the scale is determined by the highest of three scores: Off site effects, on site effects, and Defence in depth degradation.
A large off-site impact with widespread health and environmental effects. So far, the Chernobyl disaster is the only accident to reach level 7. An example of a non nuclear accident of roughly the same magnitude would be the Bhopal disaster which resulted in thousands of off-site deaths.
Significant off-site release, likely to require full implementation of planned countermeasures. Example: Mayak accident (former Soviet Union) - 1957.
Minor off-site impact resulting in public exposure of the order of the prescribed limits.
Significant damage to a reactor core/radiological barriers or the fatal exposure of a worker.
A very small off-site impact, public exposure at levels below the prescribed limits.
Severe spread of contamination on-site and/or acute health effects to one or more workers.
It is a "near accident" event, when no safety layers are remaining.
This is an incident with no off-site impact, a significant spread of contamination on-site may have occurred.
Overexposure of a worker.
Incidents with significant failures in safety provisions.
This is an anomaly beyond the authorized operating regime.
This is a "below-scale event" of no safety significance.
There are also events of no safety relevance, characterized as "out of scale".
SPEECH BY MR. CHIAKI TAKAHASHI, STATE SECRETARY FOR FOREIGN AFFAIRS OF JAPAN, AT KYIV SUMMIT ON SAFE AND INNOVATIVE USE OF NUCLEAR ENERGY.
Apr 19, 2011; TOKYO, Japan -- The following information was released by the Ministry of Foreign Affairs of Japan: His Excellency President...