A study on the effect of alternate inspection schedules on reliability of Indian 540 MWe nuclear power plant piping

In-Service Inspection (ISI) is one of the important key factors for the safety, reliability and economy of the Nuclear Power Plant (NPP). Methodology based on Probabilistic Fracture Mechanics (PFM) is finding increasing acceptability in demonstrating safety of NPP piping. This paper presents a study on the effect of ISI scheduling on the safety of the Indian 540 MWe NPP piping. PFM is used for studying effect of alternate ISI schedules on the piping reliability. Monte Carlo simulation with stratified sampling is used for PFM analysis. The model assumes a circumferential part-through surface crack before the start of plant operation. The crack growth during the lifetime of the plant due to the fatigue loading owing to operational transients is considered in this analysis. This part-through crack having escaped hydro-test, pre-service inspection and different ISI results in either a through wall flaw (that is leak) or leads to the sudden rupture of the piping. The probabilities of both these events are calculated separately. Input parameters like initial flaw size, fracture and tensile properties of material are considered as random variables. The effect of ISI is modeled using Probability of Detection (POD) curves. The rupture is estimated using R6 method. Effect of various alternative inspections schedules recommended in ASME Boiler and Pressure Vessel (B and PV) code section XI on piping reliability is presented. Effect of different flaw size distributions on piping reliability is also studied. The study shows that an early inspection is effective in lowering failure probabilities (leak/break) as the cracks are detected before they grow to critical size. The study also reveals that, if fatigue is the only damaging mechanism, subsequent inspections do not have appreciable effect on the failure probabilities