This paper describes effects of a defect sizing accuracy on the reliability of a nuclear power plant (NPP) piping when “Rules on Fitness-for-Service (FSS)” is applied. The effect of defect sizing accuracy on failure probability of PLR piping of boiling water reactor (BWR), in which stress corrosion cracking (SCC) frequently observed, was evaluated in this study. The failure probabilities with parameters of defect sizing accuracy and safety factor considered in the crack stability assessment were evaluated using probabilistic fracture mechanics (PFM) analysis. As the results, the defect sizing accuracy has a much effect on leak probability and this effect cannot be covered by the safety factor taken in FFS. In order to maintain reliability of an NPP when FFS is applied, the defect sizing accuracy in non-destructive examinations have to be grasped beforehand, and it is necessary to take it into consideration in the crack growth and the crack stability assessments.

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