Probabilistic modelling of steam generator tube pitting corrosion

Pitting corrosion is a serious form of degradation in steam generator (SG) tubing of some nuclear stations. The nature and extent of the pitting process is assessed through inspection programs, typically using various eddy current (EC) techniques, while the impact of pitting is minimized through deposit removal maintenance activities such as water lancing and chemical cleaning of SGs. This paper presents a probabilistic model of SG tube pitting corrosion that incorporates trends observed from a large EC inspection database from a nuclear generating station. The pitting occurrence process is modelled as a stochastic Poisson process and the pit size is treated as a random variable. The model is statistically calibrated with the available EC inspection data. The model is applied to estimate the probability of tube leakage, forced outage rate and the distribution of the number of tubes plugged per SG in a given operating interval. The proposed model is useful in optimizing strategies for the life-cycle management of SGs.