Fukushima nuclear accident, Japan

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

Types of nuclear accidents: INES Scale - International Scale of Nuclear Events

La escala INES es un instrumento para cuantificar la gravedad de un suceso nuclear y radiológico (ya sean accidentes o incidentes nucleares).

INES son las siglas en inglés de International Nuclear Events Scale (Escala Internacional de Sucesos Nucleares)

The INES scale is used throughout the world to communicate to the public systematic information about the importance of such nuclear and radiological events from the point of view of safety. 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 importance of events derived from a wide range of activities, including industrial and medical use. of radiation sources, the exploitation of nuclear power facilities and the transport of radioactive materials.

Nuclear events can be classified in this INES scale in seven levels. The events of levels 1 to 3 are called "incidents", while in the case of levels 4 to 7 we speak of "accidents". Each level rise in the scale indicates that the severity of the events is approximately ten times higher. When events are not important from the point of view of safety they are called "deviations" and are classified "Below the scale / Level 0".

Description by levels of the INES scale:

Serious Accident - Level 7 of the INES Scale

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

Effects on People and the Environment

Nuclear accidents classified in level 7 of the international scale of nuclear events involve the serious release of radioactive materials with extensive effects on health and the environment. To control such a disaster, far-reaching emergency measures are needed. According to the IAEA standards, an accident at level 7 of INES is classified when the total emissions 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 amounts 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 caused a critical nuclear accident. As a result of the accident there was a powerful explosion that expelled a significant fraction of radioactive material from the nucleus to the environment, which resulted in an eventual death toll.

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

Major Accident - Level 6 of 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.

The events with level 6, like those of level 7, have consequences in human and environmental aspects because the amount of radioactive material released directly influences the living environment.

Effects on People and the Environment

An atomic accident of level 6 on the international scale of nuclear events. It implies a significant release of radioactive materials. It is probable that this release will require the application of the planned countermeasures.

Nuclear Accidents Classified with Level 6

In the history of nuclear accidents we find an accident that has been classified as level 6: the accident with a storage tank at the Mayak nuclear fuel reprocessing plant, near Kyshtym. In the Mayak nuclear accident about 2 · 10 17becquerels of radioactivity were released.

Accident with Far-reaching 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

In the aspect of people and the environment, a nuclear event of level 5 implies the limited release of radioactive materials, which probably requires the application of some of the planned countermeasures.

At level 5, several radiation deaths can already occur.

Barriers and Radiological Controls

These types of nuclear accidents involve serious damage to the core of the nuclear reactor.

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

Nuclear Accidents Classified in Level 5 of 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 (United Kingdom), 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 catch fire, releasing material from radioactive batteries as dust in the environment.

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

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

The nuclear disaster in Goiânia (Brazil) occurred on September 13, 1987. A source of uninsured cesium chloride radiation left in an abandoned hospital was recovered by thieves who did not know their nature and were sold in a scrapyard. 249 people were contaminated and 4 died.

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 scale of nuclear events involves the minor release of radioactive materials, with little likelihood of having to apply the intended countermeasures, except for local food controls.

A nuclear accident of this level involves at least one death by radiation.

Barriers and Radiological Controls

Fusion of fuel or damage to nuclear fuel, which causes a release greater than 0.1% of the core inventory.

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

Nuclear Accidents Classified in Level 4 of the INES Scale

The nuclear accidents classified in level 4 most highlighted so far are the following:

  • Sellafield (United Kingdom) - five incidents from 1955 to 1979.
  • Experimental power plant SL-1 (United States) - 1961
  • Nuclear power station of Saint-Laurent (France) - 1969.
  • Buenos Aires (Argentina) - 1983, criticality accident in the RA-2 research reactor
  • Jaslovské Bohunice (Czechoslovakia) - 1977.
  • The nuclear accident of Tokaimura (Japan) - 1999.
  • Mayapuri (India) - 2010.

Major Incident - Level 3 of the INES Scale

For this level, apply  the  aspects  of human and environmental , the  radiological barriers and controls  and  defense in depth . 

People and the Environment

Exhibition ten times higher than the annual limit established for workers' exposure.

Non-lethal effect of radiation on health (for example, burns).

Barriers and Radiological Controls

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

Severe contamination in an area not foreseen in the design, with a low probability of significant public exposure.

Defense in Depth

Quasi-accident in a nuclear power plant without security provisions pending implementation.

Loss or theft of sealed sources of high radioactivity.

Wrong delivery of sealed sources of high radioactivity, without adequate procedures to manipulate them.

Incident - Level 2 of the INES Scale

People and the Environment

Exhibition of a person from the public above 10 mSv.

Exposure of a worker over the annual regulatory limits.

Barriers and Radiological Controls

Radiation levels higher than 50 mSv / h in an area of ​​operation.

Significant contamination within an installation in an area not foreseen in the design.

Defense in Depth

Major failures in security provisions, although without real consequences.

Finding of an orphaned sealed source, of a device or of a packaging for the transport of high radioactivity, with indication of the safety provisions, without there being any impairment.

Inadequate packaging of a sealed source of high radioactivity.

Anomaly - Level 1 of the INES Scale

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

An anomaly classified in level 1 of the INES scale implies small problems with the security systems with enough redundant systems on the left.

Defense in Depth

Overexposure of a person from the public above the statutory annual limits. Minor problems in security components, with important defensive measures in depth pending implementation. Loss or theft of radioactive sources, devices or packaging for transport of low activity.


IAEA - International Atomic Energy Agency


Published: December 12, 2010
Last review: June 26, 2019