THE INTERNATIONAL NUCLEAR EVENT SCALE

The objective of the scale is to promote self-assessment of the significance of an event at a nuclear facility, and to promote open communications between the nuclear community, the media and the public.

Development of the Scale

Events at nuclear facilities are classified in most countries according to the categories and terminology of the International Nuclear Event Scale (INES).

The INES was developed under the auspices of the International Atomic Energy Agency (IAEA), an arm of the United Nations, and the Nuclear Energy Agency of the Organization for Economic Cooperation and Development (OECD). The scale was field tested in 1991 and formally adopted in 1992.

INES was developed as a communication tool, not an emergency response tool. It is used to explain the safety significance of events at nuclear plants to the public and the news media.

The United States did not participate in the pilot stage of INES. However, in January 1993, the U.S. Nuclear Regulatory Commission (NRC) began a two-year limited participation in the use of INES. During the trial period, the NRC agreed to communicate, as part of a post-classification activity, the severity levels of U.S. reactor events to IAEA using the international scale. At the same time, the NRC continued to use the four-level response scale that categorizes the severity of an event for emergency planning purposes.

In May 1995, after the trial period ended, the NRC decided to continue the policy of limited participation for power reactors.

Fundamental differences in the U.S. and the INES systems for classifying events prevent the conversion of one classification system to the other.

• The INES is a tool to assess the severity of the safety consequences of events that occur at nuclear power plants. It relies on a sound technical basis--the probabilistic safety assessment--and employs user friendly procedures that are now computerized.
• The INES scale is primarily used to communicate information to the public. It has seven levels, ranging from Level 1, which has no safety significance, to Level 7, which involves the release of large of amounts of radiation.
• The INES Information Service has been joined by 59 countries that are committed to the prompt communication of event consequences when they are significant for safety or for the public interest.
• The NRC scale categorizes the severity of an event to determine the appropriate emergency response, such as activation of response organizations and facilities. It has four levels: unusual event, alert, site area emergency and general emergency.

There is one other important difference. INES is a voluntary system of reporting information to IAEA. The U.S. system is required by federal regulation.

See a brief description of the INES categories and criteria used for classification. See also: the underlying logic of the scale.

Classifying Nuclear Events with the INES

Had the INES existed at the time, these nuclear events would have been classified as follows:

Chernobyl. The 1986 accident in Ukraine involved wide environmental and health effects and would have been classified as a Level 7 "Major Accident."

Three Mile Island. The 1979 accident that seriously damaged the core of Unit 2 at this nuclear power plant in Pennsylvania involved the release of very small amounts of radioactivity outside the plant and would have been classified a Level 5 "Accident With Off-site Risks."

January 1996

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