Abstract
This paper applies probabilistic fracture mechanics calculations to determine the effects of inspection on leak 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, operating stresses, and materials relevant to 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 1 to 10 years, and both preservice and inservice inspections.
| Original language | English |
|---|---|
| Pages (from-to) | 207-210 |
| Number of pages | 4 |
| Journal | American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP |
| Volume | 346 |
| State | Published - 1997 |
| Externally published | Yes |
Funding
The work presented in this paper is part of ongoing research supported by the US Nuclear Regulatory Commission (NRC) at Pacific Northwest National Laboratory (PNNL) that is developing improved methods for predicting the reliability of nuclear piping systems and the effect of inservice inspections towards reducing piping failure probabilities. Studies have addressed the potential benefits of ultrasonic inspections of welds for reducing failure probabilities associated with fatigue crack growth ( Khaleel and Simonen, 1994 ) and stress corrosion cracking ( Khaleel and Simonen, 1997 ). Other work has focused on improved methods of probabilistic fracture mechanics modeling, including the initiation of fatigue cracks ( Khaleel and Simonen, 1998a ), the effects of leak detection ( Simonen et al., 1998 ), the significance of subsurface versus buried flaws ( Simonen and Khaleel, 1998b ) and errors in measuring the sizes of detected flaws ( Simonen and Khaleel, 1997 ). Studies have also addressed critical inputs to fracture mechanics calculations such as the parameters that characterize the number and sizes of fabrication flaws in piping welds ( Chapman and Simonen, 1998; Khaleel et al., 1999; Simonen and Chapman, 1999 ). Uncertainties associated with calculated failure probabilities have also been estimated ( Simonen and Khaleel, 1998a; Khaleel and Simonen, 1999b ). Results of research have supported the development of NRC guidance for implementation of risk-informed inservice inspection of piping ( USNRC, 1997 ). The work reported in this paper was supported by the US Nuclear Regulatory Commission under Contract DE-AC06-76RLO 1830; NRC JCN W6275; Deborah Jackson Program Monitor.