e-learning view The Manual here
Defence in depth

Factor 1: How severe


A near miss = where there are minimal or no consequences to people and the environment (no contamination, etc.) and the accident was only just avoided.

How should we rate a near miss accident depending on the size of the potential accident?

In a nuclear power plant, an event can cause maximum potential consequences and be rated at Level 7. Considering defence in depth criteria, that same event cannot exceed a Level 3 rating.

But what about other facilities or sources

reprocessing facility fuel fabrication facility medical isotope manufacturing facility
research reactor radioactive waste storage lost source

For example, if a low activity source has been lost, there is no defence in depth left because we have no controls over what happens, but even in the worst case there will not be major consequences.