Types of Nuclear Accidents: INES Scale - International Nuclear Event Scale

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The INES scale is an instrument to quantify the severity of a nuclear and radiological event (either nuclear accidents or incidents).

INES stands for International Nuclear Events Scale.

The INES scale is used worldwide to communicate to the public systematic information about the safety significance of such nuclear and radiological events. In the same way that Richter scales are used to quantify the intensity of an earthquake or the Celsius scale to measure temperature, the INES scale indicates the significance of events arising from a wide range of activities, including the industrial and medical use of radiation sources, the operation of nuclear power facilities and the transport of radioactive materials.

Nuclear events can be classified on this INES scale into seven levels. Events in levels 1 to 3 are referred to as "incidents", while those in levels 4 to 7 are referred to as "accidents". Each level up on the scale indicates that the severity of the events is approximately ten times higher. When occurrences are of no safety significance, they are referred to as "deviations" and are classified as "Below Scale / Level 0".

Anomaly - Level 1 on the INES scale

This is a small or very small nuclear incident. At this level only the defense in depth aspect. It is then the exposure of one or more citizens to radiation doses above the allowable limits.

An anomaly classified at level 1 on the INES scale implies small problems with safety systems with sufficient redundant systems left.

Defense in depth

Overexposure of a member of the public above annual regulatory limits. Minor problems in safety components, with major defense in depth measures pending implementation. Loss or theft of radioactive sources, devices or low activity transport packaging.

Incident - INES Scale Level 2

People and environment

Exposure of a member of the public above 10 mSv.

Exposure of a worker above the annual regulatory limits.

Radiological barriers and controls

Radiation levels above 50 mSv/h in an area of operation.

Significant contamination within a facility in an area not foreseen in the design.

Defense in depth

Significant failures in safety provisions, although without actual consequences.

Finding of an orphan sealed source, device or packaging for transport of high radioactivity, with indication of safety provisions, without impairment.

Inadequate packaging of a sealed source of high radioactivity.

Major incident - Level 3 on the INES scale

For this level, human and environmental aspects, radiological barriers and controls, and defense in depth are applied.

People and environment

Exposure ten times the annual limit established for worker exposure.

Non-lethal health effects of radiation (e.g., burns).

Radiological barriers and controls

Exposure rates greater than 1 Sv/h in an area of operation.

Severe contamination in an area not anticipated in the design, with little likelihood of significant public exposure.

Defense in depth

Near miss at a nuclear power plant with no pending safety provisions.

Loss or theft of high radioactivity sealed sources.

Misdelivery of high radioactivity sealed sources without adequate procedures in place to handle them.

Accident with local consequences - Level 4 of the INES scale.

Level 4 is also subdivided into two aspects, people and environment and radiological barriers and controls.

People and the environment

A level 4 nuclear accident on the international nuclear event scale involves a minor release of radioactive materials, with little likelihood of having to apply planned countermeasures other than local food controls.

A nuclear accident at this level involves at least one radiation fatality.

Radiological barriers and controls

Fuel meltdown or damage to nuclear fuel, resulting in a release greater than 0.1% of the core inventory.

Release of significant quantities of radioactive materials within a facility, with a high probability of significant public exposure.

Nuclear accidents classified at level 4 of the INES Scale

The most prominent Tier 4 nuclear accidents to date are as follows:

  • Sellafield (United Kingdom) - five incidents from 1955 to 1979.

  • SL-1 experimental power plant (United States) - 1961.

  • Saint-Laurent nuclear power plant (France) - 1969.

  • Buenos Aires (Argentina) - 1983, criticality accident at the RA-2 research reactor.

  • Jaslovské Bohunice (Czechoslovakia) - 1977.

  • Tokaimura nuclear accident (Japan) - 1999.

  • Mayapuri (India) - 2010.

Accident with major consequences - Level 5 of the INES scale

Accidents at level 5 of the INES scale can be divided into two aspects: people and the environment and radiological barriers and controls.

People and the environment

On the people and environment side, a level 5 nuclear event involves a limited release of radioactive materials, which probably requires the application of some of the planned countermeasures.

Several radiation fatalities may already occur at level 5.

Radiological barriers and controls

This type of nuclear accident involves severe damage to the nuclear reactor core.

Release of large quantities of radioactive materials within a facility, with a high probability of exposure of the population; possibly caused by a fire or a severe criticality accident.

Nuclear accidents classified at level 5 on the INES scale

In the history of nuclear energy we find 4 accidents that have been classified at this level of the INES scale: Windscale fire, Three Mile Island, Chalk River, and Goiânia.

The Windscale fire nuclear accident, also known as Sellafield (UK), occurred on October 10, 1957. The annealing of the graphite moderator in an air-cooled military reactor caused the graphite and uranium metal fuel to ignite, releasing radioactive battery material as dust into the environment.

The Three Mile Island accident near Harrisburg, Pennsylvania (USA) occurred on March 28, 1979. The disaster was caused by a combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown of the reactor core. An unknown amount of radioactive gases were released into the atmosphere, so injuries and illnesses that have been attributed to this accident can only be inferred from epidemiological studies.

The first nuclear accident at Chalk River, Ontario (Canada), was classified at level 5 on the INES scale. This atomic disaster occurred on December 12, 1952. The reactor core was damaged.

Major accident - Level 6 on the INES scale

In the case of a level 6 accident on the INES scale, a considerable amount of radioactive material is released, and it is likely that the planned emergency measures will be implemented.

Level 6 events, as well as level 7 events, have consequences on human and environmental aspects because the amount of radioactive material released directly influences the living environment.

Effects on people and the environment

A level 6 atomic accident on the international scale of nuclear events. It involves a significant release of radioactive materials. This release is likely to require the application of planned countermeasures.

Nuclear accidents classified as level 6

The Kyshtym accident was a nuclear accident that took place on September 29, 1957 at Mayak. Mayak is the name of a nuclear fuel reprocessing plant in the Soviet Union.

The Mayak nuclear accident had a magnitude of level 6 on the INES Scale. It is currently the third worst nuclear disaster in history. It ranks behind the Chernobyl and Fukushima Daiichi nuclear disasters (both level 7 on the international nuclear accident scale). The disaster was named Kyshtym after the nearest town.

Serious accident - Level 7 on the INES Scale

Level 7 of the INES scale is the highest level at which an event can be classified. The most severe nuclear accidents are classified at this level. Due to the large-scale nature of this level, Level 7 events fall under human and environmental aspects.

Effects on people and the environment

Nuclear accidents classified at level 7 of the international scale of nuclear events involve the serious release of radioactive materials with wide-ranging effects on health and the environment. To control such a disaster, far-reaching emergency measures are needed. According to IAEA standards, an accident at INES level 7 is classified when total releases correspond to a few tens of thousands of terabecquerel (TBq).

Nuclear accidents classified with level 7 of the INES scale

So far there have been two level 7 accidents: the Chernobyl nuclear disaster and the Fukushima nuclear disaster. In both cases, large quantities of radioactive material were released, which meant that large areas had to be evacuated.

The Chernobyl disaster, April 26, 1986. Unsafe conditions during a test procedure led to a critical nuclear accident. The accident resulted in a powerful explosion that expelled a significant fraction of radioactive material from the core into the environment, resulting in an eventual death toll.

The Fukushima Daiichi nuclear disaster was caused by a series of events that began on March 11, 2011. Major damage to backup power and containment systems caused by the 2011 Tōhoku earthquake and tsunami caused some of the nuclear reactors at the Fukushima I nuclear power plant to overheat and leak.


and so far this article about the INES scale of nuclear accidents for my next article I will be talking about nuclear waste all this for my nuclear week all about and among other curious facts about this very dangerous but interesting resource.

ee next article

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