The International Nuclear Event Scale

Underlying Logic of the Scale

(Criteria given in matrix are broad indicators only)

                                                     Criteria 
     Level/                                                              Defense-In-Depth
   Descriptor            Off-Site Impact           On-Site Impact          Degradation

       7            Major Release:
 Major Accident
                    Widespread Health and
                    environmental effects.
       6            Significant Release:
Serious Accident
                    Full implementation of
                    local emergency plans.
       5            Limited Release:            Severe core
 Accident with                                  damage.
 Off-site Risks     Partial implementation of
                    local emergency plans.
       4            Minor Release:              Partial core damage.
Accident Mainly                                 Acute health effects.
in Installation     Public exposure on the      to workers.
                    order of prescribed limits.
       3            Very Small Release:         Major contamination.     Near Accident.
Serious Incident                                Overexposure of 
                    Public exposure at a        workers                  Loss of Defense-In-
                    fraction of prescribed                               Depth provisions.
                    limits.
       2                                                                 Incidents with
   Incident                                                              potential safety
                                                                         consequences.
       1                                                                 Deviations from
    Anomaly                                                              authorized
                                                                         functional domains
       0                                                                 No safety
  Below Scale                                                            signification.


Source: International Atomic Energy Agency