• Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.

• 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 an eventual death toll of 56 at the time of the UNSCEAR report, as well as a projected 4,000 additional cancer fatalities (official WHO estimate) that will eventually occur among the people exposed to the highest doses of radiation. As a result of the plumes of radioisotopes, the city of Chernobyl (pop. 14,000) was largely abandoned, the larger city of Pripyat (pop. 49,400) was completely abandoned, and a {{convert|30|km}} exclusion zone around the reactor was established.

• Fukushima Daiichi 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.^[2] A temporary exclusion zone of {{convert|20|km}} was established around the plant,^[3] and officials considered evacuating Tokyo, Japan's capital and the world's most populous metropolitan area, {{convert|225|km}} away.^[4] The evacuation process is believed to have led to some 1,600 deaths.^[5]

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

• 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.^[6] 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.^[7]

• 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 aka Sellafield (United Kingdom), 10 October 1957.^[8] 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 accident near Harrisburg, Pennsylvania (United States), 28 March 1979.^[9] 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 accident,^[10][11] 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.^[7]

• 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.^[12]
• SL-1 Experimental Power Station (United States) – 1961, reactor reached prompt criticality, killing three operators.
• Saint-Laurent Nuclear Power Plant (France) – 1969, partial core meltdown; 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 nuclear accident (Japan) – 1999, three inexperienced operators at a reprocessing facility caused a criticality accident; two of them died.^[7]
• Mayapuri (India) - 2010, a university irradiator was sold for scrap and dismantled by dealers unaware of the hazardous materials.

• 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.

• THORP plant, Sellafield (United Kingdom), 2005; very large leak of a highly radioactive solution held within containment.
• Paks Nuclear Power Plant (Hungary), 2003; fuel rod damage in cleaning tank.
• Vandellòs I Nuclear Incident in Vandellòs (Catalonia, Spain), 1989; fire destroyed many control systems; the reactor was shut down.
• Davis-Besse Nuclear Power Station (United States), 2002; negligent inspections resulted in corrosion through 6 inches (15.24 cm) of the carbon steel reactor head leaving only 3⁄8 inch (9.5 mm) of stainless steel cladding holding back the high-pressure (~2500 psi, 17 MPa) reactor coolant.

• 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.

• Blayais Nuclear Power Plant flood (France) December 1999
• Ascó Nuclear Power Plant (Spain) April 2008; radioactive contamination.
• Forsmark Nuclear Power Plant (Sweden) July 2006; backup generator failure; two were online but fault could have caused all four to fail.
• Gundremmingen Nuclear Power Plant (Germany) 1977; weather caused short-circuit of high-tension power lines and rapid shutdown of reactor
• Shika Nuclear Power Plant (Japan) 1999; criticality incident caused by dropped control rods, covered up until 2007.^[13]

• 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)

• Penly (Seine-Maritime, France) 5 April 2012; an abnormal leak on the primary circuit of the reactor n°2 was found in the evening of 5 April 2012 after a fire in reactor n°2 around noon was extinguished.^[14]
• 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.^[15]
• TNPC (Drôme, France), July 2008; leak of {{convert|18000|L}} of water containing {{convert|75|kg}} of unenriched uranium into the environment.^[16]

• 4 June 2008: Krško, Slovenia: Leakage from the primary cooling circuit.^[17]
• 17 December 2006, Atucha, Argentina: Reactor shutdown due to tritium increase in reactor compartment.^[18]
• 13 February 2006: Fire in Nuclear Waste Volume Reduction Facilities of the Japanese Atomic Energy Agency (JAEA) in Tokaimura.^[19]