International Nuclear Event Scale  

Chernobyl is often referred to as a level 7 nuclear event, but what exactly does that mean?

The level on the scale is determined by the highest of three scores, off-site effects, on-site effects and defence in depth degradation. (Example if one fire suppression system was too fail, there will be another to back it up. Meaning that single layer of defence is exclusively relied upon no matter how robust it is deemed to be) 

Level 0 – Deviation  

No safety significance.  

Examples:  

  • Krško (Slovenia) – 4th June 2008. Leakage from the primary cooling circuit.
  • Atucha (Argentina) – 17th December 2006. Reactor shutdown due to tritium increase in reactor compartment.  
  • Tokaimura (Japan) – 13th February 2006. Fire in Nuclear Waste Volume Reduction Facilities of the Japanese Atomic Energy Agency (JAEA).  

Level 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.  

Examples:  

  • Sellafield (Cumbria, UK) – 1st March 2018. Cold weather caused a pipe to crack causing water from the contaminated basement into a concrete compound, which was subsequently discharged into the Irish Sea.  
  • Hunterston B (Ayrshire, Scotland) – 2nd May 2018. Cracks of the graphite bricks in the Advanced Gas-Cooled Reactor 3 were found during inspection. Approximately 370 fractures where detected, above the operational limit of 350.  
  • Penly (Seine-Maritime, France) – 5th April 2012. An abnormal leak on the primary circuit of reactor 2 was found in the evening after a fire in reactor 2 around noon was extinguished.  
  • Gravelines (Nord, France) – 8th August 2009. During the annual fuel bundle exchange in reactor no. 1, a fuel bundle snagged on to the internal structure. Operations were suspended, the reactor building was evacuated and isolated in accordance with standard operating procedure.  
  • TNPC (Drôme, France) – July 2008. Lleak of 18,000 litres of water containing 75 kilograms of unenriched uranium into the environment.  

Level 2 – Incident  

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.  
  • Significant contamination within the facility into an area no expected by design.  

Impact on defence in depth:  

  • Significant failure 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 source.  

Examples:  

  • Blaynais (France) – December 1999. Flood  
  • Ascó (Spain) – April 2008. Radioactive contamination.  
  • Forsmark (Sweden) – July 2006. Backup generator failure. Two were online but the fault could have caused all four to fail.  
  • Gundremmingen (Germany) – 1977. Weather caused a short-circuit of the high-tension power lines and rapid shutdown of reactor.  
  • Hunterson B (Ayrshire, Scotland) – 1998. Emergency diesel generators for reactor cooling pumps, failed to start after multiple grid failures during the Boxing Day Storm of 1998.  
  • Shika (Japan) – 1999. Criticality incident caused by dropped control rods, covered up until 2007.  
  • Sellafield Magnox Reprocessing Facility (Cumbria, UK) – 2017. Confirmed exposure to radiation of individuals which exceed or are expected to exceed, the dose limits (2 incidents in this year).  

Level 3 – Serious Incident  

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 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:  

  • Sellafield, THORP plant (Cumbria, UK) – 2005. A very large leak of a highly radioactive solution held within containment.  
  • Paks (Hungary) – 2003. Fuel rod damage in cleaning tank.  
  • Vandellòs I (Spain) – 1989. Fire destroyed many control systems; the reactor was shut down.  
  • Davis-Besse (United States) – 2002. Negligent inspections resulted in corrosion through 15.24 cm of the carbon steel reactor head leaving only 9.5 mm of stainless-steel cladding holding back the high-pressure (2500 psi) reactor coolant.  

Level 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 controls:  

  • 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 (Cumbria, UK) – Five incidents from 1955 to 1979.  
  • SL-1 Experimental Power Station (United States) – 1961. Reactor reached criticality, killing three operators.  
  • Saint-Laurent (France) – 1969. Partial core meltdown  
  • Saint-Laurent (France) – 1980. Graphite overheating.  
  • Buenos Aires (Argentina) – 1983. Criticality accident on research reactor RA-2 during fuel rod rearrangement killed one operator and injured two others.  
  • Jaslovské Bohunice (Czechoslovakia) – 1977. Contamination of reactor building.  
  • Tokaimura (Japan) – 1999. Three inexperienced operators at a reprocessing facility caused a criticality accident; two of them died.  
  • Mayapuri (India) – 2010. A university irradiator was sold for scrap and dismantled by dealers unaware of the hazardous materials.  

Level 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 controls:  

  • 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 critical accident or fire.  

Examples:  

  • Windscale (aka Sellafield) (Cumbria, UK) – 10th 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.  
  • Three Mile Island (Harrisburg, Pennsylvania, United States) – 28th March 1979. A combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown. An unknown amount of radioactive gases were released into the atmosphere, so injuries and illnesses that have been attributed to this accident can be deduced from epidemiological studies but can never be proven.  
  • First Chalk River (Ontario, Canada) – 12th December 1952. Reactor core damaged.  
  • Goiânia (Brazil) – 13th 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.  

Level 6 – Serious accident  

Impact on people and environment:  

  • Significant release of radioactive material likely to require implementation of planned countermeasures.  

Examples:  

To date, there has been one Level 6 Serious accident:  

  • Kyshtym, Mayak Chemical Combine (USSR) – 29th September 1957. A failed cooling system at a military nuclear waste reprocessing facility caused an explosion with a force equivalent to 70 to 100 tons of TNT. About 70 to 80 metric tons of highly radioactive material were carried into the surrounding environment. The impact on the local population is not fully known, however reports of a unique condition known as chronic radiation syndrome is reported due to the moderately high dose rates that 66 locals were continually exposed to. At least 22 villages were evacuated.  

Level 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.  

Examples:  

To date (30th May 2019), there have been two Level 7 Major accidents:  

  • Chernobyl (Ukraine, USSR) – 26th April 1986. Unsafe conditions 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 an eventual death toll of 56. As a result of the plumes of radioisotopes, the city of Chernobyl (population 14,000) was largely abandoned, the larger city of Pripyat (population 49,400) was completely abandoned, and a 30 kilometres (19 miles) exclusion zone around the reactor was established.  
  • Fukushima Daiichi (Japan) – 11th 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. A temporary exclusion zone of 20 kilometres (12 miles) was established around the plant. Officials considered evacuating Tokyo, Japan’s capital and the world’s most populous metropolitan area, 225 kilometres (140 mi) away.