POD and other important parameters influencing risk reduction by in-service inspection

Shipsha, Andrey; Kiwa Inspecta Technology AB; Sweden

Dillström, P; Kiwa Inspecta Technology AB; Sweden
Gunnars, J.; Kiwa Inspecta Technology AB; Sweden
Shipsa, A.; Kiwa Inspecta Technology AB; Sweden

ID: ECNDT-0543-2018
Session: Nuclear Industry 1
Room: H2
Date: 2018-06-13
Time: 11:50 - 12:10

Nuclear piping and components are susceptible for degradation by different mechanisms, e.g. stress corrosion cracking (SCC) and fatigue which may cause leakage or failure. In order to justify continued safe operation of nuclear power plants (NPP) an effective in-service inspection (ISI) is required. Due to a great number of welds and other locations with potential degradation, it is also essential to have a reliable methodology for inspection program planning. For addressing this challenge, different methods for risk-informed planning of in-service inspection (RI-ISI) are under development.
ISI is typically accompanied by non-destructive testing (NDT) for monitoring the extent of potential degradation and/or for assuring the absence of unexpected degradation. It is commonly believed that a choice of suitable NDT method in combination with an appropriate inspection interval provides a significant contribution to lowering the probability for unexpected leakage and failure. There is on-going extensive development of advanced NDT methods that can offer a greater Probability of Detection (POD) and thereby allow for detection of smaller size flaws. Also, the simulation techniques for NDT testing are actively developed.
Development of probabilistic methods and structural reliability models (SRM) has allowed for quantitative assessment of leakage and rupture probabilities taking into account the actual piping dimensions, loads, material properties and damage growth under different degradation mechanisms. This paper presents some results based on probabilistic analyses with different SRM models. These analyses have demonstrated that POD and inspection interval, determining the ISI efficiency, are not the only parameters that govern the reduction of risk for leakage/rupture. The risk reduction also vary depending on the base and weld materials used in a pipe, loads, weld residual stresses (WRS), environment, etc.