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IMPACT ON PEOPLE AND THE ENVIRONMENT
Release

THE BASIS OF THE DEFINITION (2)

 

The second factor that influenced the basis of the definition was that

early estimates of releases from large accidents can only be approximated

The use of precise boundaries within INES would imply inappropriate precision. For this reason phrases such as

of the order of thousands to tens of thousands

are used in the definitions

Having determined what are criteria for the event rated at the highest level, criteria for lower levels can now be defined. The difference between levels is a factor of ten, reflecting the logarithmic nature of INES.

So the definitions are:
Level 7 - radiologically equivalent to the order of more than several tens of thousands of TBq of 131I.
Level 6 - radiologically equivalent to the order of thousands to tens of thousands of TBq of 131I.
Level 5 - radiologically equivalent to the order of hundreds to thousands of TBq of 131I.
Level 4 - radiologically equivalent to the order of tens to hundreds of TBq of 131I.

When INES was being designed, it was difficult to accommodate the need for broad figures reflecting uncertainty, and the need for consistent interpretation of a definition. See footnote, page 17 of the Manual for how this issue was resolved.

These criteria relate to accidents where early estimates of the size of release can only be approximated. For this reason, it is inappropriate to use precise numerical values in the definitions of the levels. However, in order to help ensure consistent interpretation of these criteria internationally, it is suggested that the boundaries between the levels are about 500, 5 000 and 50 000 TBq 131I, respectively.

NOTE this refers to the definition of accidents, so this applies to INES Levels 4 to 7.