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Defence in depth: reactors



This page just summarizes again the approach. 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 5.1.5 of the Manual, which explains what we have just learned.

5.1.5. Potential events (including structural defects)

Some events do not of themselves result in an initiator or a degraded safety system operability but do correspond to an increased likelihood of such an event. Examples are discovery of structural defects or a leak terminated by operating personnel. The general approach to rating these events is as follows. First, the significance of the potential event should be evaluated by assuming it had actually occurred and applying Section 5.1.3 or 5.1.4, based on the operability of safety provisions that existed at the time. The choice of section depends on whether the potential event was an initiator or a degradation of a safety system. Secondly, 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 event is the discovery of a defect 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 Section 5.1.3 or 5.1.4. If the defect is in a safety system, applying Section 5.1.4 will give the basic rating of the potential event. The possibility of common mode failure may need to be considered. If failure of the component containing the defect could have led to an initiator, then applying Section 5.1.3 will give the basic rating of the potential event. Although the defect may have been found during shutdown, its significance must be considered over the time during which it is likely to have existed.

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 component failure, and by consideration of the additional factors discussed in Section 5.2.