How does the International Nuclear and Radiological Event Scale classify nuclear accidents?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

The International Nuclear and Radiological Event Scale (INES) is a global system used to classify events associated with nuclear facilities, as well as the transport and storage of radiation sources. The scale consists of seven levels, determined by criteria involving defense-in-depth, radiological controls, and the impact on people and the environment.1

The levels are categorized as follows:

Levels 1–3 (Incidents):
- 1. Anomaly: Minor safety component issues with significant defense-in-depth remaining.1
- 2. Incident: Significant internal contamination and worker exposure exceeding annual limits.1
- 3. Serious incident: Severe internal contamination resulting in nonlethal health effects, such as radiation burns.1
Levels 4–7 (Accidents):
- 4. Accident with local consequences: Fuel damage with significant internal release and potential public exposure.1
- 5. Accident with wider consequences: Severe core damage and large internal releases with a high probability of significant public exposure.1
- 6. Serious accident: Significant release requiring planned countermeasures.1
- 7. Major accident: Major release with widespread health and environmental impacts requiring extended countermeasures.1