INES Scale
5 min read
In the event of an accident or other safety-related event at a nuclear facility or involving radioactive material, the public needs clear and timely information about its significance. For this purpose, the International Nuclear and Radiological Event Scale (INES) was developed by international experts convened by the International Atomic Energy Agency (IAEA) and the OECD Nuclear Energy Agency. Introduced in 1990, the scale was initially applied mainly to events at nuclear power plants, but its scope was later expanded to cover a wide range of nuclear and radiological events, including those involving radioactive sources and the transport of radioactive material.
INES has seven levels, with the safety significance of events increasing from Level 1 to Level 7. Although the scale is logarithmic in some of its quantitative criteria, the levels should not be interpreted simply as one event being a fixed number of times more severe than another.
Levels 1—3 are classified as incidents, with Level 1 termed an anomaly, while Levels 4—7 are classified as accidents. Safety-related events with no significant safety consequences are classified as Below Scale/Level 0 and are termed deviations. Events unrelated to nuclear or radiation safety are not classified on the INES scale.
The INES scale considers three areas of impact:
People and the environment
Evaluates actual radiological consequences for people and the environment, including releases of radioactive material and radiation exposure.
Radiological barriers and controls
Evaluates events involving damage to radiological barriers, contamination or elevated radiation levels within a facility, even when there are no significant consequences for people or the environment.
Defense in depth
Evaluates degradation of the safety provisions intended to prevent accidents or limit their consequences, even when the event has caused no actual radiological consequences.
The event is assigned the highest level resulting from the assessment of these three areas.
An event is rated by the competent authorities of the country in which it occurs. If all the circumstances are not yet known or the event is still developing, a provisional rating may be issued and later revised when more information becomes available.
The purpose of INES is to provide a consistent and easily understandable way of communicating the safety significance of nuclear and radiological events to the public and the media. It helps place events in their proper safety perspective and reduces the risk of misunderstanding or misrepresentation. INES is a communication tool and is not intended to compare the safety performance of individual nuclear facilities, operating organisations or countries.
INES scale pyramid (The International Nuclear and Radiological Event Scale).
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General Description of INES Levels
| INES Level | People and the Environment | Radiological Barriers and Control | Defence-in-Depth |
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Major Accident LEVEL 7 |
Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures. |
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Serious Accident LEVEL 6 |
Significant release of radioactive material likely to require implementation of planned countermeasures. |
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Accident with Wider Consequences LEVEL 5 |
Limited release of radioactive material likely to require implementation of some planned countermeasures. Several deaths from radiation. |
Severe damage to reactor core. Release of large quantities of radioactive material within an installation with a high probability of significant public exposure. This could arise from a major criticality accident or fire. |
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Accident with Local Consequences LEVEL 4 |
Minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls. At least one death from radiation. |
Fuel melt or damage to fuel resulting in more than 0.1% release of core inventory. Release of significant quantities of radioactive material within an installation with a high probability of significant public exposure. |
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Serious Incident LEVEL 3 |
Exposure in excess of ten times the statutory annual limit for workers. Non-lethal deterministic health effect (e.g., burns) from radiation. |
Exposure rates of more than 1 Sv/h in an operating area. Severe contamination in an area not expected by design, with a low probability of significant public exposure. |
Near accident at a nuclear power plant with no safety provisions remaining. Lost or stolen highly radioactive sealed source. Misdelivered highly radioactive sealed source without adequate procedures in place to handle it. |
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Incident LEVEL 2 |
Exposure of a member of the public in excess of 10 mSv. Exposure of a worker in excess of the statutory annual limits. |
Radiation levels in an operating area of more than 50 mSv/h. Significant contamination within the facility into an area not expected by design. |
Significant failures in safety provisions but with no actual consequences. Found highly radioactive sealed orphan source, device or transport package with safety provisions intact. Inadequate packaging of a highly radioactive sealed source. |
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Anomaly LEVEL 1 |
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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. |
