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DEFENCE IN DEPTH
Defence in depth: facilities

SUMMARY FOR 'POTENTIAL' EVENTS

 

What we see here is the approach for potential events. Click in the circle above step 1 for one further piece of guidance.

Step 1:
Assume structure failed or action not taken

Step 2:
Identify what the INES rating would have been

Step 3:
Reduce rating based on likelihood of assumption in step 1

 
 
 
  • If the defect is found by planned inspections, and
  • Is within the size that requires no further action (neither repair nor a special safety case),

the rating should be Level 0 and there is no need to consider the other steps.

Discovery of such a defect is not really an event.

Now read section 6.2.3.2 of the Manual which explains what we have just learned.

6.2.3.2. Potential events (including structural defects)

Some events do not of themselves reduce the number of safety layers but do correspond to an increased likelihood of a reduction. Examples are discovery of structural defects, a leak terminated due to action by operating personnel or faults discovered in process control systems. The approach to rating such events is as follows. First, the significance of the potential event should be evaluated by assuming it had actually occurred and applying the guidance of Section 6.2.3.1, based on the number of safety layers that would have remained. Second, the rating should be reduced, depending on the likelihood that the potential event could have developed from the event that actually occurred. The level to which the rating should be reduced must be based on judgement.

One of the most common examples of potential events is the discovery of structural defects. The surveillance programme is intended to identify structural defects before their size becomes unacceptable. If the defect is within this size, then Below scale/Level 0 would be appropriate.

If the defect is larger than expected under the surveillance programme, rating of the event needs to take account of two factors.

Firstly, the rating of the potential event should be determined by assuming that the defect had led to failure of the component and applying the guidance of Section 6.2.3.1. The rating of the potential event derived in this way should then be adjusted depending on the likelihood that the defect would have led to the potential event, and by consideration of the additional factors discussed in Section 6.2.4.