Generic probabilistic safety analysis: application to the comparison of the emergency feedwater system designs of three PWRs and to the modernisation and backfitting of older Swedish NPPs

The relationship between configuration characteristics of safety systems and their unavailabilities was examined on the basis of the results of 25 PSAs for PWR and BWR plants [1 and 2]. The results of the evaluations provide groupings of configuration characteristics and associated ranges of unavailabilities that can be expected for the various groups. The safety functions

• residual heat removal at BWR plants

• coolant make-up at BWR plants

• auxiliary/emergency feedwater supply to steam generators at PWR plants

• emergency core cooling, including containment spray injection/recirculation, at PWR plants

were considered.Two applications are reported:

• evidence of validation of the generic approach, based on the results of a blind prediction of the unavailability of the emergency feedwater function at the plants N4, Sizewell-B and Konvoi, and subsequent comparison with results obtained by conventional reliability analysis;

• outline of the use of the approach in the framework of the modernisation and backfitting programme for Swedish nuclear power plants.

In the early discussions of safety objectives for the modernisation programme, the generic approach provided input to the exploration of possible target values for safety systems. In the process of reviewing proposed technical solutions, Swedish Nuclear Power Inspectorate (SKI) is using the generic approach as one of the tools for assessing safety function unavailabilities.