CRACK GROWTH MODEL FOR THE PROBABILISTIC ASSESSMENT OF INSPECTION STRATEGIES FOR STEAM GENERATOR TUBES

Assessment of the conditional probabilities of tube failures, leak rates, and ultimately risk of exceeding licensing dose limits as an approach used to steam generator tube fitness-for-service assessment has begun to be used increasingly in recent years throughout the nuclear power industry. One of the important topics in the assessment is the crack growth model that predicts the growth of crack size with time. The plot of the log of change in crack length per cycle, against the log of stress intensity factor range shows three distinct regions where in Regions I and III the change in crack length per cycle increases faster with stress intensity factor. On the other hand Region II shows linear variation. While effective for predicting Region II of fatigue crack growth, "Paris law" is often not properly used in making life predictions because a majority of the fatigue life of a specimen occurs in Region I. Physical models for Region growth have not yet been successful. Recently substantial effort is made in modeling crack growth model with experimental data for Regions I and II. The present work reviews recent work in predicting the fatigue crack growth rate in Region I and II The characteristics of such models are examined and their limitations are discussed in context with steam generator tube crack growth. Recommendations are made on crack growth models for region I and II.