The INES scale of nuclear incidents and accidents

 

PRESENTATION AND OBJECTIVES

In the same way as for natural phenomena such as earthquakes, wind and avalanches, France in 1987 set up a scale of severity for nuclear events, which the IAEA used extensively as a basis for the INES scale (International Nuclear Event Scale).

This scale, which was implemented internationally in 1991, is based partly on objective and partly on subjective criteria. It is used by about sixty countries and its purpose is to facilitate media and public perception of the scale of any nuclear incidents. It is not an assessment tool and cannot in any circumstances be used as a basis for international comparison. There is in particular no strict correlation between the number of non-serious incidents declared and the probability of a serious accident occurring in a facility.

NATURE OF THE EVENTS RATED ON THE INES SCALE

The INES scale is designed to cover events occurring in all civil nuclear installations, including those classified as secret, and during nuclear material transports. These events are rated by the ASN on 8 levels from 0 to 7, depending on their severity.

Application of the INES scale to nuclear installations is based on three rating criteria (columns 2, 3 and 4 of the following table):

off-site impact, assessed in terms of radioactive releases that can affect the public and the environment;
on-site impact, which can affect workers and the conditions of the installations;
degradation of the lines of defence in depth of the installation, consisting of successive barriers (safety systems, procedures, technical or administrative checks, etc.) placed between the radioactive products and the environment.

For transportation of radioactive materials on the public highway, only the off-site impact and degradation of the defence in depth criteria are used for application of the INES scale.

EXAMPLES OF EVENTS RATED ON THE INES SCALE

Level 0. In France: several hundred events are rated level 0 every year. They concern deviations from the normal operation of the facilities or the normal transportation of radioactive materials, but which have no impact in terms of safety.
Level 1. In France: about a hundred events are rated level 1 every year. These comprise anomalies, deviations from facility normal operating conditions or from normal transportation operations, owing to equipment failure, human error, or inadequate compliance with procedures.

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