The effects of initial flaw sizes and inservice inspection on piping reliability

The Pacific Northwest Laboratory (PNL) is conducting research for the US Nuclear Regulatory Commission (NRC) to establish risk-based inservice inspection (ISI) guidelines for nuclear power plant components. Part of this program involves probabilistic fracture mechanics calculations to determine the effects of inspection on leak and rupture probabilities for piping. The approach has been to perform calculations in a structured parametric format, with the parameters selected to cover the range of pipe sizes, degradation mechanisms, relevant operating stresses, and materials of the piping systems of interest. In this paper, the calculations were intended to be generally applicable to mechanical and thermal fatigue of stainless steel piping. Specific areas of uncertainty addressed by the probabilistic calculations of this paper are the numbers of initial flaws, the distributions of flaw sizes, the crack growth rates for these initial flaws, and the probability of detection curves and inspection schedules that describe inservice inspections that are performed to detect these growing flaws. The effectiveness of an inspection strategy is quantified by the parameter ''Factor of Improvement,'' which is the relative increase in piping reliability due to a given inspection strategy as compared to the strategy of performing no inspection. The results of a systematic set of calculations are presented in this paper that address inspection effectiveness for operating stresses giving crack growth rates ranging from very low to very high. Inspection strategies are described that address three reference levels of ultrasonic inspection reliability, intervals between inspections ranging from 2 to 10 years, and both preservice and inservice inspections