Nuclear Event Scale

Nico Corti
May 24, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017

What Is INES?

Fig. 1: Graphic depiction of INES Scale. (Source: Wikimedia Commons)

The International Nuclear Event Scale (INES) is a communication tool used to quickly and effectively inform the public about the safety significance of events related with sources of ionizing radiation. INES was developed in 1990 by international experts convened jointly by the IAEA and the Nuclear Energy Agency of the Organization for Economic Co-operation and Development Initially the scale was of a more limited scope. [1] It was only applied to classify events at nuclear power plants. Later, its scope was expanded so that the scale could be applied to all installations associated with the civil nuclear industry. More recently, it has been further expanded in order to meet the growing need for communication of the significance of all events associated with the use, storage and transport of radioactive material and radiation sources. [1]

What's Its Purpose?

The primary purpose of INES is to facilitate communication and understanding between the technical community, the media and the public on the safety significance of events. [1] It aims to mitigate any kind of confusion surrounding these types of events. This confusion can be extremely dangerous when dealing with such potentially harmful materials. So the aim of the scale is to keep the public as well as nuclear authorities accurately informed on the occurrence and consequences of reported events. [1]

When Is INES Used?

The INES scale applies to any event associated with the use, storage,and transport of radioactive material and radiation sources. It does not matter if the event occurred at a nuclear facility or not. These types of events include ones involving the loss or theft of radioactive sources or packages and the discovery of orphan sources, such as sources being discovered in scrap metal. The scale is also used for the rating of events resulting in actual exposure of workers and the public in medical, research and educational institutions. [1] Avoiding confusion is especially important in these contexts. The scale is a threat level assessment and warning system, it does not cover the actual or potential medical consequences for any member of the public that may have been exposed to dangerous material. The scale does not cover the actual or potential consequences for patients exposed as part of a medical procedure. [1] INES assessments do not include military applications of these types of materials. Finally, it is never appropriate to use the scale to compare and contrast the relative safety of countries, organizations, or even facilities because the scale assesses particular vents instead of places or entities. [1]

What Is the Actual Scaling System?

INES uses a numerical assessment rating to explain the significance of events associated with sources of ionizing radiation. Events are rated at seven levels as seen in Fig. 1. The first three levels are classified as incidents, while levels four through seven are classified as accidents. The scale is designed such that the severity of an event is approximately ten times greater for each increase in level of the scale. [1] These levels consider these three main areas of impact: (1) People and the environment, (2) Radiological barriers and control, and (3) Defense in depth. Some incidents do not qualify in severity and receive a rating of Below Scale/Level 0. [1]

Real Life Application

The following are three events, of varying severity, that were classified on the International Nuclear Event Scale: (1) the Chernobyl Nuclear Accident, (2) the Three Mile Island Accident, and (3) the Blayais Nuclear Power Plant flood. The Chernobyl Nuclear Accident occurred in 1986. A power surge during a test procedure resulted in an explosion which eventually lead to the release of a significant amount of core material to be released into the environment. The immediate death toll was 56 but there is another projected 4,000 casualties due to cancer stemming from radiation contact. [2] This event is classified as a level 7 major accident. The highest possible threat level. The Three Mile Island Accident occurred in Pennsylvania in 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 released into the atmosphereas a result of the meltdown. As a result of the unknown quantity of the leak, resulting injuries and illnesses that have been attributed to this accident can be deduced from epidemiological studies but can never be proven definitively. [3] This event was classified as a level 5 accident with wider consequences.

The Blayais Nuclear Power Plant flood occurred in France in 1999. This event was a flood caused by a combination of the tide and high winds from a storm which led to the seawalls of the power plant being overwhelmed. The event resulted in the loss of the plant's off-site power supply and knocked out several safety-related systems. This event was classified as a level 2 incident on the International Nuclear Event Scale.

© Nico Corti. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] D. Smythe, "An Objective Nuclear Accident Magnitude Scale For Quantification of Severe and Catastrophic Events," Physics Today, 12 Dec 11.

[2] D. Williams, "Cancer After Nuclear Fallout: Lessons from the Chernobyl Accident," Nat. Rev. Cancer 2, 543 (2002).

[3] M. C. Hatch et al., "Cancer Rates After the Three Mile Island Nuclear Accident and Proximity of Residence to the Plant," Am. J. Public Health 81, 719 (1991).