The working principle of the INES scale is identical to the principle of scales for measuring the seriousness of some natural phenomena (earthquakes, winds…). It aims at making easier the perception of the significance of incidents or accidents in nuclear installations.
Only events having an impact (even potential) on nuclear safety of the installations are included in this scale.
Events are classified on a scale at seven levels (from 1 to 7), according to their importance.
Events, which have no safety significance are termed “deviations” and are classified below scale, or level “0”.
Events, which have no safety relevance are termed “out-of-scale”.
3 Serious Incident
4 ACCIDENT without significant off-site risk
5 ACCIDENT with off-site risk
6 Serious accident
7 Major accident