Reliability Assessment of Piping in a Nuclear Power Plant Considering Flaw Detection Probability

This study was performed to clarify the effects of flaw detection probability on piping reliability of a nuclear power plant. Stress-corrosion cracking (SCC) is still sporadically detected in austenitic stainless steel piping in Japanese BWR plants. The suitability for continued service of cracked pipes is basically evaluated by using the “Rules on fitness -for service for nuclear power plants.” Here future inspection rules are employed. However, the possibility of undetection of existing cracks and that of inaccurate measurements cannot be eliminated in UT-based inspection. Thus a probabilistic fracture mechanics (PFM) analysis was carried out referring measured flaw size, and the reliability of piping was evaluated considering the possibility of undetection of existing cracks and that of inaccurate measurements. The results of the analysis indicate that, if the interval and quality of the inspection are maintained at a certain specified level, the possibility of undetection of existing cracks and that of inaccurate measurements less affect failure probability.

Effects of Flaw Sizing Errors on the Reliability of Vessels and Piping

This paper describes probabilistic fracture mechanics calculations that simulate fatigue crack growth, flaw detection, flaw sizing accuracy, and the impacts of flaw acceptance criteria. The numerical implementation of the model is based on a Latin hypercube approach. Calculations have been performed for a range of parameters. For representative values of flaw detection probability, flaw sizing errors, and flaw acceptance criteria, detection capability is the most limiting factor with regard to the ability of the inservice inspections to reduce leak probabilities. However, gross sizing errors or significant relaxations of current flaw acceptance standards could negate the benefits of outstanding probability of detection capabilities.

Cumulative Detection Probability as a Basis for Pressure Vessel Inspection Intervals

This work discusses use of the cumulative flaw detection probability as the basis for establishing pressure vessel inspection intervals. The method is based on the accumulated probability of detecting a flaw over several inspections. It explicitly incorporates a user decision as to the acceptable level of failure risk. A four-step approach is outlined including fracture mechanics flaw growth calculations with probabilistic treatment of detection probability. It is concluded that (a) inspection intervals based on the cumulative probability of detection provide significant advantages over traditional cycle-based methods, (b) pressure vessel recertification inspections should rely on high percentage inspections conducted on a relatively infrequent basis.