The International Nuclear and Radiological Event Scale INES

Much attention has been paid to the communication of nuclear and radiological events since the Chernobyl accident. INES, the International Nuclear and Radiological Event Scale, has contributed to the communications.


​​​INES severity classes.

The INES scale was developed as an international collaboration between the International Atomic Energy Agency (IAEA) and the Organization for Economic Co-operation and Development (OECD). Experts from several countries have also participated in the work. The scale was formed according to the French model. A similar scale was also used in Japan.

The INES scale was introduced in 1990. The scale was officially adopted in 1992 for nuclear power plant incidents and in 1994 for other nuclear facilities. In the 2000s, the use of the scale was expanded to cover not only nuclear facility incidents but also events related to the transport, storage and use of radioactive substances and radiation sources.

The scale is used by approximately 70 countries.

Exchange of information

The IAEA maintains a network for the exchange of information between participating countries. Events in severity class 2 and above must be reported to the IAEA. The aim is to make an event of this severity class available to the IAEA within 24 hours of the event. The IAEA will also be notified of events in the lower classes if the events have aroused international interest. The IAEA communicates the information it receives to participating countries.

The information is available in the IAEA’s database: News - The Information Channel on Nuclear and Radiological Events. The data in the database cannot be used for cross-country comparisons, as different countries have different practices for reporting events of minor security significance. In addition, the number of events in classes INES 2 and above is too low to draw statistical conclusions.

The severity class is determined in the country of the event. Countries using the severity scale can organize the classification of events as they wish. In Finland, it is standard practice for a power company to submit a proposal for a severity class to the Radiation and Nuclear Safety Authority (STUK) and for STUK to decide on the class.

In Finland, the determination of the INES class is relevant to events that have occurred in the following nuclear facilities or operations:

  • nuclear power plants
  • handling, storage and transport of fresh and spent nuclear fuel
  • waste treatment, storage and disposal facilities
  • radiation sources
  • use of radiation

Events related to the security arrangements of nuclear facilities and nuclear safeguards are not classified on the INES scale in Finland.

Definitions and examples of events

INES 7 – Major accident

Significant release of radioactive material from a large nuclear power plant into the environment. Typical of such a release is that it contains both short-lived and long-lived fission products (iodine-131 equivalents amounting to more than tens of thousands of terabecquerels). Such releases may cause immediate health hazards and subsequent health effects in large areas, even in several countries, as well as long-term environmental impacts.


The strongest earthquake in Japan’s history, on 11 March 2011, and the resulting tsunami badly damaged the Fukushima Dai-ichi nuclear power plant on the east coast of Japan. Radioactive substances were released into the air and into the sea. The accident was classified as class 7 on the basis of its environmental impacts. Read more about Fukushima accident.

A reactor at the Chernobyl nuclear power plant in the Soviet Union (present Ukraine) was destroyed in an explosive manner in 1986. The complete destruction of the reactor caused a massive release of radioactive substances, and more than 30 plant workers died from injuries sustained in the accident. The environmental impacts indicate that the accident was in class 7. Read more about Chernobyl accident.

INES 6 – Serious accident

Release of radioactive substances into the environment (iodine-131 equivalents from thousands to tens of thousands of terabecquerels). Such releases are likely to result in the initiation of measures to the full extent in order to limit serious health detriments.


In 1957, an explosion occurred at a reprocessing plant in the Soviet Union (now Russia) containing a container of highly active liquid waste, which resulted in the release of radioactive materials. Measures, such as evacuating the population of the area, were taken to limit the damage to health. The environmental impact indicates that the accident was in class 6.

INES 5 – Accident with wider consequences

Release of radioactive substances into the environment (iodine-131 equivalents from hundreds to thousands of terabecquerels). Such a release would lead to a partial triggering of measures to reduce the likelihood of health detriments.

Serious damage to the nuclear facility. This may involve extensive damage to the reactor of a nuclear power plant, a large uncontrolled power surge (criticality accident), a fire or an explosion, as a result of which a significant amount of radioactive material is released into the plant premises.


At the Three Mile Island nuclear power plant in the United States, so much cooling water was lost from an open safety valve that the reactor dried up, overheated and partially melted in 1979. A large amount of radioactive substances were spread indoors at the facility, but releases into the environment were low. Based on the impacts inside the plant, the accident falls into class 5.

INES 4 – Accident with local consequences

The release of radioactive substances into the environment causes a radiation dose of more than one millisievert to the most exposed persons living in the vicinity of the plant. Such releases may necessitate some measures outside the plant, such as local food monitoring.

Significant damage to the nuclear facility. An example of such an accident is the partial meltdown of a nuclear power plant reactor or a similar event at another nuclear facility. The accident may cause a long-term interruption in the operation of the plant.

Radiation doses received by one or more workers at the plant that are likely to result in rapid death.


In 1973, the Windscale (now Sellafield) reprocessing plant in the UK released radioactive substances into the plant's premises as a result of a chemical reaction that produced heat in a process tank. Based on the impacts inside the plant, the accident falls into class 4.

In 1980, at the gas-cooled nuclear power plant of Saint Laurent, France, a metal plate detached from the reactor structures and blocked the cooling flow of two fuel rod clusters. This resulted in severe fuel damage. However, no radioactive substances were released into the environment. Based on the impacts inside the plant, the accident falls into class 4.

In 1983, a short-term power surge (criticality accident) occurred at the RA-2 research reactor in Buenos Aires, Argentina. The reason for the accident was that the safety instructions were not followed when changes were made to the reactor core. The accident resulted in the death of a controller who was working 3 to 4 metres away. Based on the impacts inside the plant, the accident falls into class 4.

INES 3 – Serious incident

Discharges of radioactive substances into the environment exceed the discharge limits approved by the authorities. Discharges into the environment cause a radiation dose of less than one millisievert to the most exposed persons living in the vicinity of the plant. No measures are required outside the plant.

An incident that results in radiation doses causing immediate health detriments to workers or a significant amount of radioactive substances (activity of a few thousand terabecquerels) spreading inside the plant in such a way that they can be recovered and stored as waste.

An incident in which a single additional fault in the safety system could lead to an accident or the necessary safety systems would be unable to prevent an accident as a result of a malfunction. The barriers to the dispersion of radioactive substances have weakened significantly.

Example: The Vandellòs nuclear power plant in Spain had a fire in 1989. The incident did not cause radioactive substances to be released, nor did it cause fuel damage or the contamination of the plant premises. Several safety assurance systems were damaged in the fire, which makes the event a class 3 incident.

INES 2 – Incident affecting safety

An event in which there is a significant defect in the factors affecting safety, but where safety is still ensured despite a possible additional defect.

An incident that causes a radiation dose exceeding the dose limit for workers. An incident that results in a significant release of radioactive substances indoors in areas of the plant to which they are not supposed to spread. Contaminated facilities require cleaning before being put back into service.


In Sweden, unit 1 of the Forsmark nuclear power plant experienced an electrical failure in 2006, which revealed design errors in the power plant's reserve power systems. Due to this, some of the power supplies with battery backup that are important for safety (so-called UPS) went out, and only half of the reserve power generators were working. As a result, half of the display devices in the control room went dark for 22 minutes, among other things. The plant's controllers restored sufficient power to the plant by connecting it to the national grid via an alternative route.

A worker was exposed to radiation from a strong cobalt-60 radiation source used in industrial imaging in Kotka in 2010. The incident occurred when the radiation source used in the imaging was accidentally not returned to its protective container before the operator returned to its vicinity. The worker had a personal radiation dose meter with which the radiation dose received by the worker was confirmed to be approximately 58 millisieverts. Based on the radiation dose, the event is classified in class 2.

In 1991, there was a fire in the main distribution building of Olkiluoto 2, as a result of which the plant unit lost its connections to the external power grid. For 7.5 hours, the unit depended on electricity generated by four emergency diesel generators. The event showed shortcomings in securing the external power supply. Based on this, the event is classified in class 2.

In Loviisa 2, the feed water pipeline of the secondary circuit was interrupted in 1993 when the plant unit was running at full capacity. The reason for the interruption was the erosion corrosion of a pipe. During the incident, the reactor controller's actions were correct and quick, and the leak was over in nine minutes. In Loviisa 1, the feed water pipeline had been broken in a similar manner in 1991. After the Loviisa 1 incident, the condition monitoring of the pipelines was enhanced. Despite the measures taken, erosion corrosion in Loviisa 2 led to the rupture of the pipeline. The event was classified in class 2. The classification was based on an increase allowed by the classification rules due to the recurrence of the event.

INES 1 – Anomaly affecting safety

Class 1 includes events that materially deviate from the requirements for safe operation, which may be the result of a device failure, an operating error or inadequate procedures. However, the deviation does not significantly compromise safety.

INES 0 – No safety significance

Rating class 0 includes events whose safety significance is so limited that they cannot be placed on the actual scale.

Class 0 includes, for example, the fast shutdown of a reactor (reactor trip) when all the plant's systems operate as planned during the situation.