Scale to enable communication of safety information in nuclear accidents
The International Nuclear and Radiological Event Scale (INES) was introduced in 1990[1] by the
International Atomic Energy Agency (IAEA) in order to enable prompt communication of
safety significant information in case of
nuclear accidents.
The scale is intended to be
logarithmic, similar to the
moment magnitude scale that is used to describe the comparative magnitude of earthquakes. Each increasing level represents an accident approximately ten times as severe as the previous level. Compared to earthquakes, where the event intensity can be quantitatively evaluated, the level of severity of a
human-made disaster, such as a nuclear accident, is more subject to interpretation. Because of this subjectivity, the INES level of an incident is assigned well after the fact. The scale is therefore intended to assist in disaster-aid deployment.
Details
A number of criteria and indicators are defined to assure coherent reporting of
nuclear events by different official authorities. There are seven nonzero levels on the INES scale: three incident-levels and four 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
defense in depth degradation.
Level
Classification
Description
Examples
7
Major accident
Impact on people and environment:
Major release of
radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.
To date, there have been two Level 7 accidents:
Chernobyl disaster, 26 April 1986. Unsafe conditions during a test procedure resulted in a powerful steam explosion and fire that released a significant fraction of core material into the environment, resulting in an eventual death toll of 4,000–27,000.[2][3][4][5][6] As a result of the plumes of
radioisotopes, a 30 km (19 mi)
exclusion zone around the reactor was established.
Fukushima nuclear disaster, a series of events beginning on 11 March 2011. Major damage to the backup power and containment systems caused by the
2011 Tōhoku earthquake and tsunami resulted in overheating and leaking from some of the Fukushima I nuclear plant's reactors.[7] A temporary exclusion zone of 20 km (12 mi) was established around the plant.[8][9]
6
Serious accident
Impact on people and environment:
Significant release of radioactive material likely to require implementation of planned countermeasures.
To date, there has been one Level 6 accident:
Kyshtym disaster at
Mayak Chemical Combine (MCC) Soviet Union, 29 September 1957. A failed cooling system at a military
nuclear waste reprocessing facility caused an explosion with a force equivalent to 70–100 tons of TNT.[10] About 70 to 80 metric tons of highly radioactive material were carried into the surrounding environment. At least 22 villages were evacuated.[11]
5
Accident with wider consequences
Impact on people and environment:
Limited release of radioactive material likely to require implementation of some planned countermeasures.
Several deaths from radiation.
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.
Windscale fire at
Sellafield (
United Kingdom), 10 October 1957.[14]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. 100 to 240 cancer deaths were caused by the incident.[15][16][17]
Three Mile Island accident near
Harrisburg, Pennsylvania (
United States), 28 March 1979.[18] A combination of design and operator errors caused a gradual
loss of coolant, leading to a partial
meltdown. The amounts of radioactive gases released into the atmosphere are still not known, so injuries and illnesses that have been attributed to this accident can only be deduced from epidemiological studies.
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.[11]
4
Accident with local consequences
Impact on people and 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.
Sellafield (United Kingdom) – five incidents from 1955 to 1979.[19]
Andreev Bay nuclear accident (Soviet Union) – 1982, a spent nuclear fuel storage facility was damaged and caused approximately 700,000 tonnes (770,000 tons) of highly radioactive water to leak into the Barents Sea.[20][21]
Davis–Besse Nuclear Power Station (United States), 2002; negligent inspections resulted in corrosion through 6 in (150 mm) of the carbon steel reactor head leaving only 3⁄8-inch (9.5 mm) of stainless steel cladding holding back the high-pressure reactor coolant.
Sellafield (United Kingdom) 2017; confirmed exposure to radiation of individuals which exceed or are expected to exceed, the dose limits (2 incidents in this year).[24]
Sellafield Magnox Swarf Storage Silo (United Kingdom) 2019; confirmed silo liquor imbalance caused by a leak in the legacy storage facility leading to contamination below ground level.[25]
1
Anomaly
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.)
Tricastin (
Drôme, France), July 2008; leak of 18,000 L (4,000 imp gal; 4,800 US gal) of water containing 75 kg (165 lb) of unenriched
uranium into the environment.[26]
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.[27]
Penly (
Seine-Maritime, France) 5 April 2012; an abnormal leak on the primary circuit of the reactor 2 was found in the evening of 5 April 2012 after a fire in reactor 2 around noon was extinguished.[28]
Sellafield (Cumbria, United Kingdom) 1 March 2018; Due to cold weather, a pipe failed causing water from the contaminated basement to flow into a concrete compound, which was subsequently discharged into the
Irish Sea.[29]
Hunterston B nuclear power station (Ayrshire, United Kingdom) 2 May 2018; Cracks of the graphite bricks in Advanced Gas-cooled Reactor 3 were found during an inspection. About 370 fractures were discovered, above the operational limit of 350.[30]
Sellafield Legacy Ponds sump tank (United Kingdom) 2019; detected liquid levels in a concrete sump tank have fallen.[31]
Sellafield 15 May 2016; Loss of active ventilation within the Magnox Swarf Storage Silo. Extract fans were switched off for 16 hours in order to undertake some improvements to the ventilation system, but when it was restarted the system indicated zero flow. [32]
17 November 2002, Natural Uranium Oxide Fuel Plant at the
Nuclear Fuel Complex in Hyderabad, India: A chemical explosion at a fuel fabrication facility.[42]
Criticism
Deficiencies in the existing INES have emerged through comparisons between the 1986
Chernobyl disaster, which had severe and widespread consequences to humans and the environment, and the 2011
Fukushima nuclear disaster, which caused one fatality and comparatively small (10%) release of radiological material into the environment. The Fukushima Daiichi nuclear accident was originally rated as INES 5, but then upgraded to INES 7 (the highest level) when the events of units 1, 2 and 3 were combined into a single event and the combined release of radiological material was the determining factor for the INES rating.[43]
One study found that the INES scale of the IAEA is highly inconsistent, and the scores provided by the IAEA incomplete, with many events not having an INES rating. Further, the actual accident damage values do not reflect the INES scores. A quantifiable, continuous scale might be preferable to the INES.[44]
The following arguments have been proposed: firstly, the scale is essentially a discrete qualitative ranking, not defined beyond event level 7. Secondly, it was designed as a public relations tool, not an objective scientific scale. Thirdly, its most serious shortcoming is that it conflates magnitude and intensity. An alternative nuclear accident magnitude scale (NAMS) was proposed by British nuclear safety expert
David Smythe to address these issues.[45]
Alternatives
Nuclear Accident Magnitude Scale
The Nuclear Accident Magnitude Scale (NAMS) is an alternative to INES, proposed by David Smythe in 2011 as a response to the
Fukushima Daiichi nuclear disaster. There were some concerns that INES was used in a confusing manner, and NAMS was intended to address the perceived INES shortcomings.
As Smythe pointed out, the INES scale ends at 7; a more severe accident than Fukushima in 2011 or
Chernobyl in 1986 would also be measured as INES category 7. In addition, it is not continuous, not allowing a fine-grained comparison of nuclear incidents and accidents. But then, the most pressing item identified by Smythe is that INES conflates magnitude with intensity; a distinction long made by
seismologists to describe
earthquakes. In that area,
magnitude describes the physical energy released by an earthquake, while the
intensity focuses on the effects of the earthquake. In analogy, a nuclear incident with a high magnitude (e.g. a core meltdown) may not result in an intense
radioactive contamination, as the incident at the Swiss
research reactor in Lucens shows – but yet it resides in INES category 4, together with the
Windscale fire of 1957, which has caused significant contamination outside of the facility.
Definition
The definition of the NAMS scale is:
NAMS = log10(20 × R)
with R being the radioactivity being released in
terabecquerels, calculated as the equivalent dose of
iodine-131. Furthermore, only the atmospheric release affecting the area outside the nuclear facility is considered for calculating the NAMS, giving a NAMS score of 0 to all incidents which do not affect the outside. The factor of 20 assures that both the INES and the NAMS scales reside in a similar range, aiding a comparison between accidents. An atmospheric release of any radioactivity will only occur in the INES categories 4 to 7, while NAMS does not have such a limitation.
An estimation of its magnitude seems to be related to the problematic definition of a radiological equivalence between different type of involved
isotopes and the variety of
paths by which activity might eventually be ingested,[46] e.g. eating fish or through the
food chain.
Smythe lists the following incidents; Chernobyl, Russia 1986 (M = 8.0), Three Mile Island, USA (M = 7.9), Fukushima-Daiichi, Japan 2011 (M = 7.5), Kyshtym, former USSR 1957 (M = 7.3).[47]
^Spiegelberg-Planer, Rejane.
"A Matter of Degree"(PDF). IAEA Bulletin. IAEA. Retrieved 24 May 2016.
^Webb, G A M; Anderson, R W; Gaffney, M J S (2006). "Classification of events with an off-site radiological impact at the Sellafield site between 1950 and 2000, using the International Nuclear Event Scale". Journal of Radiological Protection. 26 (1). IOP: 33–49.
Bibcode:
2006JRP....26...33W.
doi:
10.1088/0952-4746/26/1/002.
PMID16522943.
S2CID37975977.
^David Smythe (12 December 2011). "An objective nuclear accident magnitude scale for quantification of severe and catastrophic events". Physics Today.
doi:
10.1063/PT.4.0509.
S2CID126728258.