The International Nuclear Event Scale
For prompt communication of safety significance
7
MAJOR ACCIDENT

6
SERIOUS ACCIDENT

5

ACCIDENT

ACCIDENT WITH OFF-SITE RISK

4
ACCIDENT WITHOUT SIGNIFICANT
OFF-SITE RISK

3
SERIOUS INCIDENT

INCIDENT
2
INCIDENT

1
ANOMALY

0

DEVIATION

NO SAFETY SIGNIFICANCE

General Description of the Scale
The International Nuclear Event Scale (INES) is a means for promptly
communicating to the public in consistent terms the safety significance of
events reported at nuclear installations. By putting events into proper perspective, the Scale can ease common understanding among the nuclear
community, the media, and the public. It was designed by an international
group of experts convened jointly in 1989 by the International Atomic Energy
Agency (IAEA) and the Nuclear Energy Agency (NEA) of the Organisation
for Economic Co-operation and Development. The Scale also reflects the
experience gained from the use of similar scales in France and Japan as
well as from consideration of possible scales in several other countries.
The Scale was initially applied for a trial period to classify events at
nuclear power plants and then extended and adapted to enable it to be
applied to any event associated with radioactive material and/or radiation
and to any event occurring during transport of radioactive material. It is now
operating successfully in over 60 countries.
The INES Information Service, the communication network built up on
request receives from and disseminates to the INES National Officers of
60 Member States, Event Rating Forms that provide authoritative information related to nuclear events. Event Rating Forms are circulated when
events are significant for:

• operational safety (INES level 2 and above)
• public interest (INES level 1 and below)
The communication process has therefore led each participating country to set up a structure which ensures that all events are promptly rated
using the INES rating procedure to facilitate communication whenever they
have to be reported outside.
Events are classified on the Scale at 7 levels; the upper levels (4-7)
are termed accidents and the lower levels (1-3) incidents. Events which
have no safety significance are classified below scale at level 0 and are
termed “deviations”. Events which have no safety relevance are termed “out
of scale”. The structure of the Scale is shown opposite, in the form of a matrix
with key words. Each level is defined in detail within the INES User’s Manual.
Events are considered in terms of three safety attributes or criteria represented by each of the columns: off-site impact, on-site impact, and defence
in depth degradation.
The second column in the matrix relates to events resulting in off-site
releases of radioactivity. Since this is the only consequence having a direct
effect on the public, such releases are understandably of particular concern.
Thus, the lowest point in this column represents a release giving the critical
group an estimated radiation dose numerically equivalent to about onetenth of the annual dose limit for the public; this is classified as level 3. Such
a dose is also typically about one-tenth of the average annual dose
received from natural background radiation. The highest level is a major
nuclear accident with widespread health and environmental consequences.
The third column considers the on-site impact of the event. This category covers a range from level 2 (contamination and/or overexposure of a
worker) to level 5 (severe damage to the reactor core or radiological barriers).
All nuclear facilities are designed so that a succession of safety layers
act to prevent major on-site or off-site impact and the extent of the safety
layers provided generally will be commensurate with the potential for on- and
off-site impact. These safety layers must all fail before substantial off-site or
on-site consequences occur. The provision of these safety layers is termed
“defence in depth”. The fourth column of the matrix relates to incidents at
nuclear installations or during the transportation of radioactive materials in
which these defence in depth provisions have been degraded. This column
spans the incident levels 1-3.
An event which has characteristics represented by more than one criterion is always classified at the highest level according to any one criterion.
Events which do not reach the threshold of any of the criteria are rated
below scale at level 0.
The back page of this leaflet gives typical descriptions of events at
each level together with examples of the classification of nuclear events
which have occurred in the past at nuclear installations.

Using the Scale
• The detailed rating procedures are provided in the INES User’s Manual.
This leaflet should not be used as the basis for rating events as it only
provides examples of events at each level, rather than actual definitions.

• Although the Scale is designed for prompt use following an event, there
will be occasions when a longer time-scale is required to understand and
rate the consequences of an event. In these rare circumstances, a provisional rating will be given with confirmation at a later date. It is also possible
that as a result of further information, an event may require reclassification.
• The Scale does not replace the criteria already adopted nationally and
internationally for the technical analysis and reporting of events to Safety
Authorities. Neither does it form a part of the formal emergency arrangements that exist in each country to deal with radiological accidents.
• Although the same Scale is used for all installations, it is physically
impossible at some types of installation for events to occur which involve
the release to the environment of considerable quantities of radioactive
material. For these installations, the upper levels of the Scale would not be
applicable. These include research reactors, unirradiated nuclear fuel treatment facilities, and waste storage sites.
• The Scale does not classify industrial accidents or other events which
are not related to nuclear or radiological operations. Such events are termed
“out of scale”. For example, although events associated with a turbine or
generator can affect safety related equipment, faults affecting only the availability of a turbine or generator would be classified as out of scale. Similarly,
events such as fires are to be considered out of scale when they do not
involve any possible radiological hazard and do not affect the safety layers.
• The Scale is not appropriate as the basis for selecting events for feedback of operational experience, as important lessons can often be learnt
from events of relatively minor significance.
• It is not appropriate to use the Scale to compare safety performance
among countries. Each country has different arrangements for reporting
minor events to the public, and it is difficult to ensure precise international
consistency in rating events at the boundary between level 0 and level 1.
The statistically small number of such events, with variability from year to
year, makes it difficult to provide meaningful international comparisons.
• Although broadly comparable, nuclear and radiological safety criteria
and the terminology used to describe them vary form country to country.
The INES has been designed to take account of this fact.

Examples of Rated Nuclear Events
• The 1986 accident at the Chernobyl nuclear power plant in the Soviet
Union (now in Ukraine) had widespread environmental and human health
effects. It is thus classified as Level 7.
• The 1957 accident at the Kyshtym reprocessing plant in the Soviet
Union (now in Russia) led to a large off-site release. Emergency measures
including evacuation of the population were taken to limit serious health
effects. Based on the off-site impact of this event it is classified as Level 6.
• The 1957 accident at the air-cooled graphite reactor pile at Windscale
(now Sellafield) facility in the United Kingdom involved an external release
of radioactive fission products. Based on the off-site impact, it is classified
as Level 5.
• The 1979 accident at Three Mile Island in the United States resulted
in a severely damaged reactor core. The off-site release of radioactivity was
very limited. The event is classified as Level 5, based on the on-site impact.
• The 1973 accident at the Windscale (now Sellafield) reprocessing
plant in the United Kingdom involved a release of radioactive material into
a plant operating area as a result of an exothermic reaction in a process
vessel. It is classified as Level 4, based on the on-site impact.
• The 1980 accident at the Saint-Laurent nuclear power plant in France
resulted in partial damage to the reactor core, but there was no external
release of radioactivity. It is classified as Level 4, based on the on-site impact.
• The 1983 accident at the RA-2 critical assembly in Buenos Aires,
Argentina, an accidental power excursion due to non-observance of safety
rules during a core modification sequence, resulted in the death of the operator, who was probably 3 or 4 metres away. Assessments of the doses
absorbed indicate 21 Gy for the gamma dose together with 22 Gy for the
neutron dose. The event is classified as Level 4, based on the on-site impact.
• The 1989 incident at the Vandellos nuclear power plant in Spain did not
result in an external release of radioactivity, nor was there damage to the
reactor core or contamination on site. However, the damage to the plant’s
safety systems due to fire degraded the defence in depth significantly. The
event is classified as Level 3, based on the defence in depth criterion.
• The vast majority of reported events are found to be below Level 3.
Although no examples of these events are given here, countries using the
Scale may individually wish to provide examples of events at these lower
levels.

Basic Structure of the Scale
(Criteria given in matrix are broad indicators only)
Detailed definitions are provided in the INES User’s Manual

CRITERIA OR SAFETY ATTRIBUTES
OFF-SITE IMPACT

DEFENCE IN DEPTH
DEGRADATION

ON-SITE IMPACT

7
MAJOR ACCIDENT

MAJOR RELEASE:
WIDESPREAD HEALTH
AND ENVIRONMENTAL
EFFECTS

6
SERIOUS ACCIDENT

SIGNIFICANT RELEASE:
LIKELY TO REQUIRE FULL
IMPLEMENTATION OF
PLANNED
COUNTERMEASURES

5
ACCIDENT WITH
OFF-SITE RISK

LIMITED RELEASE:
LIKELY TO REQUIRE
PARTIAL IMPLEMENTATION
OF PLANNED
COUNTERMEASURES

SEVERE DAMAGE
TO REACTOR
CORE/RADIOLOGICAL
BARRIERS

4
ACCIDENT WITHOUT
SIGNIFICANT
OFF-SITE RISK

MINOR RELEASE:
PUBLIC EXPOSURE OF
THE ORDER OF
PRESCRIBED LIMITS

SIGNIFICANT DAMAGE
TO REACTOR
CORE/RADIOLOGICAL
BARRIERS/FATAL
EXPOSURE OF A WORKER

3
SERIOUS INCIDENT

VERY SMALL RELEASE:
PUBLIC EXPOSURE
AT A FRACTION OF
PRESCRIBED LIMITS

SEVERE SPREAD OF
CONTAMINATION/ACUTE
HEALTH EFFECTS TO A
WORKER

NEAR ACCIDENT
NO SAFETY LAYERS
REMAINING

SIGNIFICANT SPREAD OF
CONTAMINATION/
OVEREXPOSURE OF A
WORKER

INCIDENTS WITH
SIGNIFICANT FAILURES
IN SAFETY PROVISIONS

2
INCIDENT

ANOMALY BEYOND THE
AUTHORIZED
OPERATING REGIME

1
ANOMALY

0
DEVIATION

OUT OF SCALE EVENT

NO

SAFETY

N O S A F E T Y R E L E VA N C E

SIGNIFICANCE

The International Nuclear Event Scale
For prompt communication of safety significance
LEVEL/
DESCRIPTOR

ACCIDENTS
7
MAJOR
ACCIDENT
6
SERIOUS
ACCIDENT

5
ACCIDENT WITH
OFF-SITE RISK

4
ACCIDENT
WITHOUT
SIGNIFICANT
OFF-SITE RISK

NATURE OF THE EVENTS

EXAMPLES

• External release of a large fraction of the radioactive material in a large facility (e.g. the
core of a power reactor). This would typically involve a mixture of short and long-lived
radioactive fission products (in quantities radiologically equivalent to more than tens of
thousands of terabecquerels of iodine-131). Such a release would result in the possibility of
acute health effects; delayed health effects over a wide area, possibly involving more than
one country; long-term environmental consequences.

Chernobyl NPP, USSR
(now in Ukraine), 1986

• External release of radioactive material (in quantities radiologically equivalent to the order
of thousands to tens of thousands of terabecquerels of iodine-131). Such a release would
be likely to result in full implementation of countermeasures covered by local emergency
plans to limit serious health effects.

Kyshtym Reprocessing
Plant, USSR
(now in Russia), 1957

• External release of radioactive material (in quantities radiologically equivalent to the order
of hundreds to thousands of terabecquerels of iodine-131). Such a release would be likely
to result in partial implementation of countermeasures covered by emergency plans to
lessen the likelihood of health effects.

Windscale Pile, UK, 1957

• Severe damage to the installation. This may involve severe damage to a large fraction of
the core of a power reactor, a major criticality accident or a major fire or explosion releasing
large quantities of radioactivity within the installation.

Three Mile Island, NPP,
USA, 1979

• External release of radioactivity resulting in a dose to the critical group of the order of a
few millisieverts.* With such a release the need for off-site protective actions would be
generally unlikely except possibly for local food control.
Significant damage to the installation. Such an accident might include damage leading to
major on-site recovery problems such as partial core melt in a power reactor and comparable
events at non-reactor installations.

Windscale Reprocessing
Plant, UK, 1973
Saint-Laurent NPP, France,
1980

• Irradiation of one or more workers resulting in an overexposure where a high probability of
early death occurs.

Buenos Aires Critical
Assembly, Argentina, 1983

•

• External release of radioactivity resulting in a dose to the critical group of the order of
tenths of millisievert.* With such a release, off-site protective measures may not be needed.
INCIDENTS
3
SERIOUS
INCIDENT

2
INCIDENT

1
ANOMALY

DEVIATIONS
0
BELOW SCALE

• On-site events resulting in doses to workers sufficient to cause acute health effects
and/or an event resulting in a severe spread of contamination for example a few thousand
terabecquerels of activity released in a secondary containment where the material can be
returned to a satisfactory storage area.
• Incidents in which a further failure of safety systems could lead to accident conditions, or
a situation in which safety systems would be unable to prevent an accident if certain initiators
were to occur.

Vandellos NPP, Spain,
1989

• Incidents with significant failure in safety provisions but with sufficient defence in depth
remaining to cope with additional failures. These include events where the actual failures
would be rated at level 1 but which reveal significant additional organisational inadequacies
or safety culture deficiencies.
• An event resulting in a dose to a worker exceeding a statutory annual dose limit and/or
an event which leads to the presence of significant quantities of radioactivity in the installation in areas not expected by design and which require corrective action.
• Anomaly beyond the authorised regime but with significant defence in depth remaining.
This may be due to equipment failure, human error or procedural inadequacies and may
occur in any area covered by the scale, e.g. plant operation, transport of radioactive material,
fuel handling, waste storage. Examples include: breaches of technical specifications or
transport regulations, incidents without direct safety consequences that reveal inadequacies
in the organisational system or safety culture, minor defects in pipework beyond the expectations of the surveillance programme.
• Deviations where operational limits and conditions are not exceeded and which are
properly managed in accordance with adequate procedures. Examples include: a single
random failure in a redundant system discovered during periodic inspections or tests, a
planned reactor trip proceeding normally, spurious initiation of protection systems without
significant consequences, leakages within the operational limits, minor spreads of contamination within controlled areas without wider implications for safety culture.

NO
SAFETY SIGNIFICANCE

* The doses are expressed in terms of effective dose equivalent (whole dose body). Those criteria where appropriate can also be expressed in terms of corresponding
annual effluent discharge limits authorized by National authorities.

99-00305/FS-05

International Atomic Energy Agency
Wagramerstrasse 5
A-1400 Vienna, Austria

OECD Nuclear Energy Agency
Le Seine Saint-Germain-12
Boulevard des Iles
92130 Issy-les-Moulineaux, France