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Defence in depth

What should affect the rating?


If defence in depth is about providing multiple layers of defence, what should affect the rating?

Obviously, how close the event was to an accident—what has failed and what was still left out of the defence in depth provisions.

But equally important—what kind of accident would it be if everything failed? No one would expect to rate an event where someone nearly got 20 mSv at the same level as one where nearly all cooling to a reactor was lost!

There are three aspects of the event that need to be considered:

Finally, it's necessary to recognize that the concept of defence in depth relies on human performance—training, following procedures, etc., and on safety management—maintenance regime, attitude to safety, updating procedures, etc.

Each of these aspects are considered in a defence in depth rating. The first is very technology dependent so it is covered in the later modules on defence in depth for specific areas. The second and third are addressed in this section.