FACTBOX: The International Nuclear Event Scale
(Reuters) - More than 20 years after the Chernobyl disaster, public fear of nuclear power remains strong.
But nuclear accidents are very rare and the industry is one of the most tightly regulated, with a global system of measuring the threat posed to public safety.
The International Nuclear Event Scale (INES) was designed by the International Atomic Energy Agency (IAEA) and the Organization for Economic Co-operation and Development (OECD).
The scale, which operates in over 60 countries, can be applied to any event associated with radioactive material or radiation and events during transport of radioactive material.
Events are classified at seven levels: levels one to three are considered "incidents," while levels four to seven are termed "accidents" and those without safety significance are termed "deviations" and are classified below scale at level 0.
Chernobyl is the only level seven accident to date and the vast majority of events are classified below Level 3.
The following is a brief explanation of what constitutes each level on the scale and examples:
ACCIDENTS
LEVEL 7 - MAJOR ACCIDENT - External release of a large part of the radioactive material in a large facility like a power reactor, threatening serious health effects; delayed health problems over a wide area, possibly involving several countries; long-term environmental consequences.
Example: The 1986 Chernobyl nuclear power plant disaster in the Soviet Union, now in Ukraine, caused widespread environmental and human health problems.
LEVEL 6 - SERIOUS ACCIDENT - Large external release of radioactive material, likely to result in full use of emergency measures to limit serious health problems.
Example: The 1957 accident at the Kyshtym reprocessing plant in the Soviet Union, now Russia, led to a large off-site release. Emergency measures including evacuation of the population were taken.
LEVEL 5 - ACCIDENT WITH OFF SITE RISK - Smaller external release of radioactive material, likely resulting in partial use of emergency plans to lessen the threat to public health.
Severe damage to the installation, possibly involving a large part of the core of a power reactor, or a big fire or explosion releasing large quantities of radioactivity inside the installation.
Examples: The 1979 accident at Three Mile Island in the United States resulting in a badly damaged reactor core. The off-site release of radioactivity was limited but the incident is classified as Level 5 because of the on-site impact.
A 1957 accident at the reactor pile at Windscale, now called Sellafield, in the UK released radioactive materials into the atmosphere and is classified Level 5 because of it.
LEVEL 4 - ACCIDENT WITHOUT SIGNIFICANT OFF SITE RISK - Small external release of radioactivity not likely to require off-site protective actions, except possible local food control.
Significant damage to the installation, possibly including damage causing major on-site recovery problems such as partial reactor core melt and comparable events at other installations.
Irradiation of one or more workers resulting in an overexposure causing a high probability of early death.
Examples: A 1973 accident, again at the UK's Windscale reprocessing plant, involved a release of radioactive material into a plant operating area.
A 1980 accident at the Saint-Laurent power plant in France resulted in partial damage to the reactor but no external radioactivity. It was Level 4, based on the on-site impact.
An accident at the RA-2 critical assembly plant in Buenos Aires, Argentina in 1983, resulted in the death of an engineer who was a few meters away. The event is classified as Level 4, based on the on-site impact.
INCIDENTS
LEVEL 3 - SERIOUS INCIDENT - Smaller external releases of radioactivity where off-site protective measures may not be needed.
On-site events resulting in doses to workers large enough to cause acute health problems or a severe spread of contamination in a secondary containment vessel but where the material can be returned to safe storage.
Incidents in which a further failure of safety systems could lead to accident conditions, or where safety systems could be unable to prevent another accident.
Example: A fire at Spain's Vandellos nuclear power plant in 1989 did not cause an external release of radiation and did not damage the reactor core or cause site contamination but it did damage the plant's safety systems.
LEVEL 2 - INCIDENT - Failure in safety provisions but with sufficient defenses remaining to cope with additional failures. Includes events where the failures themselves would be rated Level 1 but which show significant additional organizational inadequacies or safety failures.
An event resulting in a dose to a worker exceeding an annual dose limit and/or leading to significant radioactivity in the installation in areas which are not designed for radiation.
LEVEL 1 - ANOMALY - Anomaly beyond the authorized regime but with significant defenses still in place. May be due to equipment failure, human error or procedural faults.
LEVEL 0 - BELOW SCALE - Deviations where operational limits and conditions are not exceeded and which are properly managed in accordance with adequate procedures.
For example: a single random failure in a redundant system discovered during periodic tests, a planned reactor trip proceeding normally, spurious initiation of protection systems without significant consequences, leakages within operational limits, minor spreads of contamination within controlled areas.
Source: IAEA, OECD.
