Model for predicting vessel failure probabilities due to fatigue crack growth

A numerical approach has been developed to predict the probability that a fabrication flaw in a reactor pressure vessel will extend by fatigue crack growth mechanisms and become a through-wall flaw. The fracture mechanics model treats the size of the flaw, the location of the flaw, and the parameters governing the fatigue crack growth rates as stochastic variables that are described by histograms that represent their statistical distributions. A Latin Hypercube approach forms the basis for efficient numerical calculations of vessel failure probabilities, in particular for those cases having very low probabilities that are not readily calculated by use of more conventional Monte-Carlo simulations. A second aspect of the vessel failure model evaluates the benefits of inservice inspections at prescribed inspection time intervals and with prescribed nondestructive examination capabilities (probability of detection as a function of flaw size). Following a description of the probabilistic model, this paper presents the results of calculations that evaluate the effects of assumptions on flaw location (at inner surface of the vessel versus a random location within the vessel wall) and evaluate the benefits of alternative inservice inspection programs (inspection frequency and NDE method).