International nuclear event scale

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The International Nuclear and Radiological Event Scale, or INES, is a worldwide tool for communicating to the public in a consistent way the safety significance of nuclear and radiological events.

The INES Scale explains the significance of events from a range of activities, including industrial and medical use of radiation sources, operations at nuclear facilities and transport of radioactive material.

Events are classified on the scale at seven levels: Levels 1–3 are called "incidents" and Levels 4–7 "accidents." The following are the INES levels and events from the IAEA.

INES Level   General Description    Accidents 
Major Accident Level 7  

• Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures   

Chernobyl, Soviet Union (now Ukraine), 1986 

Serious Accident Level 6


• 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.

Accident with Wider consequences Level 5

• Limited release of radioactive material likely to require implementation of some planned countermeasures.


Three Mile Island, USA, 1979 – Severe damage to reactor core.

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.

Accident with Local Consequences Level 4  • Minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls. 

Tokaimura, Japan, 1999 – Fatal overexposures of workers following a criticality event at a nuclear facility.

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.

Yanango, Peru, 1999 — Incident with radiography source resulting in severe radiation burns.


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. 



Level 1


• Overexposure of a member of the public in excess of statutory annual limits. 



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