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DEFENCE IN DEPTH
Defence in depth: reactors

STRUCTURAL DEFECTS

 

These are all events involving structural defects, sometimes with some leakage. Some of the defects are more significant than others and there are two relevant factors to consider. How big, and where, the defect is.

In one plant, defects were found in the non-isolated part of the safety injection pipework, i.e. their failure would lead to a small LOCA. The defects were small and not likely to lead to pipework failure.

In another plant, defects were in the isolatable part of the system and similar defects were found in all safety injection lines. Had safety injection been initiated, it is likely that they would have resulted in failure of the pipework.

In a third plant, a large leak occured in the main feedwater pipework. There was local flooding and the reactor was shutdown. No post trip cooling systems were affected.

  • In the first example, the defects were small
  • In the second, they were sufficient to cause failure if safety injection had been initiated
  • In the second case, they may have affected all the safety trains
  • In the third case, they did not affect any safety trains.

The next slides will explain how these factors are taken into account in rating events.