The International Nuclear Event Scale
General description of the scale
The International Nuclear Event Scale
(INES) is a tool to promptly and consistently communicate to the
public the safety significance of reported events 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 by the International Atomic
Energy Agency (IAEA) and the Nuclear Energy Agency (NEA) of the
Organisation for Economic Co-operation and Development. The group
was guided in its work by the findings of a series of
international meetings held to discuss general principles
underlying such a scale. 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.
Initially applied for a trial period to classify events at
nuclear power plants, 32 countries participated in the trial and
international agencies and user countries monitored progress. The
Scale operated successfully and now has been made available for
formal adoption by each country. The Scale also has been extended
and adapted to enable it to be applied to all nuclear
installations associated with the civil nuclear industry and to
any events occurring during the transport of radioactive
materials to and from those facilities.
Events are classified on the Scale at seven levels. Their
descriptors and criteria are shown opposite with examples of the
classification of nuclear events which have occurred in the past
at nuclear installations. The lower levels (1-3) are termed
incidents, and the upper levels (4-7) accidents. Events which
have no safety significance are classified as level 0/ below
scale 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. The words used are not intended to be
precise or definitive. Each criterion is defined in detail within
an INES Users' 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 most exposed
person off-site an estimated radiation dose numerically
equivalent to about one-tenth 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 plant damage such as
a core melt).
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.
Using the Scale
- 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.
- If a radiological emergency were to occur in the vicinity
of a nuclear installation or during the transport of
radioactive materials, existing national emergency
planning arrangements would be implemented. The Scale
should not be used as part of the formal emergency
arrangements.
- Although the same scale is used for all installations, it
is physically impossible for events to occur which
involve the release to the environment of considerable
quantities of radioactive material at some types of
installation. 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.
- Industrial accidents or other events which are not
related to nuclear or radiological operations are not
classified and 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 from country to country. The INES has been designed
to take account of this fact.
Examples of classified nuclear events
- The 1986 accident at the Chernobyl nuclear power plant in
the Soviet Union (now in the 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 reprocessing plant in
the United Kingdom (now Sellafield) 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
nonobservance 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 by the victim 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.
(Criteria given in
matrix are broad indicators only)
Detailed definitions are provided in the INES users'
manual
| |
CRITERIA OR
SAFETY ATTRIBUTES |
| OFF-SITE IMPACT |
ON-SITE IMPACT |
DEFENCE IN DEPTH
DEGRADATION |
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 |
2
INCIDENT |
|
SIGNIFICANT SPREAD OF
CONTAMINATION/OVEREXPOSURE OF A WORKER |
INCIDENTS WITH SIGNIFICANT FAILURES I
N SAFETY PROVISIONS |
1
ANOMALY |
|
|
ANOMALY BEYOND THE AUTHORISED
OPERATING REGIME |
0
BELOW SCALE EVENT DEVIATION |
NO SAFETY
SIGNIFICANCE |
|
| OUT OF SCALE EVENT |
NO SAFETY
RELEVANCE |
| LEVEL |
DESCRIPTOR |
CRITERIA |
EXAMPLES |
ACCIDENTS
7 |
MAJOR ACCIDENT |
- 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 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
Ukraine), 1986 |
| 6 |
SERIOUS ACCIDENT |
- 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 |
| 5 |
ACCIDENT WITH OFF-SITE
RISK |
- 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.
- Severe damage to the nuclear
facility. 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.
|
Windscale Pile, UK, 1957
Three Mile Island, USA, 1979
|
| 4 |
ACCIDENT WITHOUT
SIGNIFICANT OFF-SITE RISK |
- External release of radioactivity
resulting in a dose to the most exposed
individual off-site of the order of a few
milisieverts. 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 nuclear
facility. Such an accident might include damage
to nuclear plant leading to major on-site
recovery problems such as partial core melt in a
power reactor and comparable events at
non-reactor installations.
- Irradiation of one or more workers
which result in an overexposure where a high
probability of early death occurs.
|
Windscale Reprocessing
Plant, UK, 1973 Saint-Laurent NPP, France, 1980
Buenos Aires Critical Assembly, Argentina, 1983
|
INCIDENTS
3 |
SERIOUS INCIDENT |
- External release of radioactivity
above authorised limits, resulting in a dose to
the most exposed individual off site of the order
of tenths of millisievert.* With such a release,
off-site protective measures may not be needed.
- 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
|
| 2 |
INCIDENT |
- Incidents with significant failure
in safety provisions but with sufficient defence
in depth remaining to cope with additional
failures.
- 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.
|
|
| 1 |
ANOMALY |
- Anomaly beyond the authorised
operating regime. This may be due to equipment
failure, human error or procedural inadequacies.
(Such anomalies should be distinguished from
situations where operational limits and
conditions are not exceeded and which are
properly managed in accordance with adequate
procedures. These are typically "below
scale").
|
|
| BELOW
SCALE/ZERO |
DEVIATION |
NO SAFETY
SIGNIFICANCE |
The doses are expressed in terms of
effective dose equivalent (whole body dose). Those criteria where
appropriate can also be expressed in terms of corresponding
annual effluent discharge limits authorised by National
authorities.
BACK
TOP