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Following the accidents at Three Mile Island in 1979 and Chernobyl in 1986, the IAEA decided to create an INES. This scale was implemented worldwide in 1991. It has eight severity levels rated from 0 to 7. For quantifiable events of a comparable nature, the scale is logarithmic, the change from one level to another corresponding to a factor of 10.

Since 1991, the INES has enabled the establishment of a common language for the assessment of an incident or accident in the nuclear sector. The OECD, then the IAEA, drew heavily on a nuclear event severity scale set up by France in 1987 to design the INES. This common international reference makes it easier to understand public opinion. Information on an event is communicated via the IAEA to all countries that have adopted the INES [LEC 04].

The INES is based on several criteria that are taken into account to define the nuclear event’s severity level. The reported events are analyzed according to their consequences at three levels: (1) wider impacts on people or property (worker and/or public health); (2) on-site impacts; and (3) defense-in-depth impacts (presence of several containment barriers). This approach is detailed in Table 1.1.

Table 1.1. The severity levels of a nuclear event. The INES

Type	INES	Wider impact	On-site impact	Damage to defense-in-depth
Major accident	7	Major release: widespread effects on health and the environment
Serious accident	6	Significant release likely to require full implementation of planned countermeasures
Accident (resulting in wider consequences)	5	Limited release likely to require partial application of planned countermeasures	Serious damage to the reactor or radiological barriers
Accident (not resulting in a significantly wider risk)	4	Minor release: public exposure within statutory limits	Significant damage to the reactor or radiological barriers, or lethal exposure of a worker	Loss of defenses and contamination
Serious incident	3	Very small release: public exposure represents a fraction of the statutory limits	Serious contamination or acute effects on a worker’s health	Accident narrowly avoided. Loss of defense lines
Incident	2	No consequences	Significant contamination or overexposure of a worker	Incident with significant failure of safety provisions
Anomaly	1	No consequences	No consequences	Anomaly not included in the authorized modus operandi
Deviation	0	No consequences	No consequences	Insignificant anomaly from a safety point of view

The transition from incident (levels 1–3) to accident (levels 4–7) is characterized by environmental contamination that can damage public health.

Events that occurred before the INES was created were rated retrospectively. In the end, and without being exhaustive, we can note that only two accidents received a rating of 7 (major accident). These are the accidents at Chernobyl (Ukraine) on April 26, 1986 and Fukushima (Japan) on March 11, 2011. One accident was classified as a level 6 accident (serious accident), the Kyshtym disaster in the USSR (Mayak nuclear complex) in 1957. Four events were considered to be level 5 (accident). These were the accident at the Chalk River Laboratories in Canada in 1952, the fire at Windscale (now Sellafield) in the United Kingdom in 1957, the Three Mile Island nuclear accident in the United States in 1979 and the Goiânia nuclear accident in Brazil in 1987 (Table 1.4). This table lists the 10 accidents considered to be the most serious.