The International
Nuclear and Radiological Event Scale(INES) was introduced in 1990 by
the International Atomic Energy Agency (IAEA) in order to enable prompt
communication of safety significance information in case of nuclear accidents.
The
scale is intended to be logarithmic, similar to the Richter scale that is used
to describe the comparative magnitude of earthquakes. Each increasing level
represents an accident approximately ten times more severe than the previous
level. Compared to earthquakes, where the event intensity can be quantitatively
evaluated, the level of severity of a man-made disaster, such as a nuclear
accident, is more subject to interpretation. Because of the difficulty of
interpreting, the INES level of an incident is assigned well after the incident
occurs. Therefore, the scale has a very limited ability to assist in
disaster-aid deployment.
Commonly,
the organisation where the nuclear incident occurs assigns a first provisional
INES rating to an incident, after it is being reviewed and possibly revised by
the designated national radiation authority.
A
number of criteria and indicators are defined to assure coherent reporting of
nuclear events by different official authorities. There are 7 levels on the
INES scale; 3 incident-levels and 4 accident-levels.
There is also a level 0.
The
level on the scale is determined by the highest of three scores: off-site
effects, on-site effects, and defence in depth degradation.
Impact on People and Environment
Major release of radioactive material
with widespread health and environmental effects requiring implementation of
planned and extended countermeasures
The
only accident:
● Chernobyl disaster, 26 April 1986. A power surge
during a test procedure resulted in a criticality accident, leading to a
powerful steam explosion and fire that released a significant fraction of core
material into the environment, resulting in a death toll of 56 as well as
estimated 4 000 additional cancer fatalities among people exposed to elevated
doses of radiation and a permanent loss of large areas of habitable land. The
disaster is the only Level 7 Event that has ever occurred.
Impact on People and Environment
Significant release of radioactive material
likely to require implementation of planned countermeasures.
Example
● Kyshtym disaster at Mayak, Soviet Union, 29 September
1957. A failed cooling system at a military nuclear waste reprocessing facility
caused a steam explosion that released 70–80 tons of highly radioactive
material into the environment. Impact on local population is not fully known.
Impact on People and Environment
Limited release of radioactive material
likely to require implementation of some planned countermeasures.
Several deaths from radiation.
Example
● Windscale fire (United Kingdom), 10 October 1957.
Annealing of graphite moderator at a military air-cooled reactor caused the
graphite and the metallic uranium fuel to catch fire, releasing radioactive
pile material as dust into the environment.
Impact on Radiological Barriers and
Control
Severe damage to reactor core.
Release of large quantities of radioactive
material within an installation with a high probability of significant public
exposure. This could arise from a major criticality accident or fire.
Example:
Three Mile Island accident (Harrisburg, United States), 28 March 1979. A
combination of design and operator errors caused a gradual loss of coolant,
leading to a partial meltdown. Radioactive gases were released into the
atmosphere.
Other
examples:
● First Chalk River Accident Chalk River, Ontario,
Canada, 12 December 1952. Reactor core damaged.
● Goiânia accident (Brazil), 13 September 1987. An
unsecured caesium chloride radiation source left in an abandoned hospital was
recovered by scavenger thieves unaware of its nature and sold at a scrapyard.
249 people were contaminated and 4 died.
Impact on People and the Environment
Minor release of radioactive material
unlikely to result in implementation of planned countermeasures other than
local food controls.
At least one death from radiation.
Impact on Radiological Barriers and
Control
Fuel melt or damage to fuel resulting in
more than 0.1% release of core inventory.
Release of significant quantities of
radioactive material within an installation with a high probability of
significant public exposure.
Examples:
● Sellafield (United Kingdom) – 5 incidents 1955 to 1979
● SL-1 Experimental Power Station (United States) –
1961, reactor reached prompt criticality, killing three operators.
● Saint-Laurent Nuclear Power Plant (France) – 1980,
partial core meltdown.
● Buenos Aires (Argentina) – 1983, criticality accident
during fuel rod rearrangement killed one operator and injured 2 others.
● Jaslovské Bohunice (Czechoslovakia) – 1977,
contamination of reactor building.
● Tokaimura nuclear accident (Japan) – 1999, three
inexperienced operators at a reprocessing facility caused a criticality
accident; two of them died.
● Fukushima I Nuclear Power Plant (Japan) – 2011,
reactor shutdown after the 2011 Sendai earthquake and tsunami, failure of
emergency cooling caused an explosion.
Impact on People and Environment
Exposure in excess of ten times the
statutory annual limit for workers.
Non-lethal deterministic health effect
(e.g., burns) from radiation.
Impact on Radiological Barriers and
Control
Exposure rates of more than 1 Sv/h in an
operating area.
Severe contamination in an area not
expected by design, with a low probability of significant public exposure.
Impact on Defence-in-Depth
Near accident at a nuclear power plant with
no safety provisions remaining.
Lost or stolen highly radioactive sealed
source.
Misdelivered highly radioactive sealed
source without adequate procedures in place to handle it.
Examples:
● THORP plant Sellafield (United Kingdom) – 2005.
● Paks Nuclear Power Plant (fuel rod damage in cleaning
tank) (Hungary) – 2003.
● Vandellos Nuclear Power Plant, Spain (A fire destroyed
many control systems; the reactor was shut down) – 1989.
Impact on People and Environment
Exposure of a member of the public in
excess of 10 mSv.
Exposure of a worker in excess of the
statutory annual limits.
Impact on Radiological Barriers and
Control
Radiation levels in an operating area of
more than 50 mSv/h.
Significant contamination within the
facility into an area not expected by design.
Impact on Defence-in-Depth
Significant failures in safety provisions
but with no actual consequences.
Found highly radioactive sealed orphan
source, device or transport package with safety provisions intact.
Inadequate packaging of a highly
radioactive sealed source.
Examples:
● Ascó Nuclear Power Plant, (Catalonia, Spain) April
2008; radioactive contamination
● Forsmark Nuclear Power Plant (Sweden); backup
generator failure.
Impact on Defence-in-Depth
Overexposure of a member of the public in
excess of statutory annual limits.
Minor problems with safety components with
significant defence-in-depth remaining.
Low activity lost or stolen radioactive
source, device or transport package.
(Arrangements
for reporting minor events to the public differ from country to country. It is
difficult to ensure precise consistency in rating events between INES Level-1
and Below scale/Level-0)
Examples:
● Gravelines (Nord, France), 8 August 2009; during the
annual fuel bundle exchange in reactor #1, a fuel bundle snagged on to the
internal structure. Operations were stopped, the reactor building was evacuated
and isolated in accordance with operating procedures.
● TNPC (Drôme, France), July 2008; leak of 6,000 litres
(1,300 imp gal; 1,600 US gal) of water containing 75 kilograms (170 lb) of
Uranium into the environment.
No
safety significance.
Examples:
● 4 June 2008: Krško, Slovenia: Leakage from the primary
cooling circuit
● 17 December 2006, Atucha, Argentina: Reactor shutdown
due to Tritium increase in reactor compartment
● 13 February 2006: Fire in Nuclear Waste Volume
Reduction Facilities of the Japanese Atomic Energy Agency (JAEA) in Tokaimura
There
are also events of no safety relevance, characterized as "out of
scale".
Examples:
● 17 November 2002, Natural Uranium Oxide Fuel Plant at
the Nuclear Fuel Complex in Hyderabad, India: A chemical explosion at a fuel
fabrication facility
● 4 November 1999: H.B. Robinson, United States: A
Tornado sighting within the protected area of the NPP.
● 15 April 1999: San Onofre, United States: Discovery of
suspicious item in nuclear power plant