Statistical treatment of non destructive testing of steam generator tubes to modelize their future behaviour

Many Belgian steam generators are affected by Primary Water Stress Corrosion Cracking (PWSCC) in the tube mechanical roll transitions. The length propagation of these (often through wall) defects has been monitored since 1984 by Eddy Current Testing, using a Rotating Pancake Coil; since 1988, 100% of the tubebundle affected area is inspected at each outage. The size and accuracy of this inspection data base allowed a statistical analysis and identification of the parameters governing the degradation process. The resulting predictive model is used for: - estimating the number of tubes to plug or repair during the next unit outage (predictive maintenance) - evaluation of long term repair versus replacement strategy - probabilistic assessment of steam generator (primary to secondary) leakrate under accidental conditions. The last application considers probabilistic distributions for the various parameters affecting the critical length of a crack and uses a leakage model which is a function of the ratio of actual to critical length of a through wall crack; a software program (LABOLEAK) has been developed to implement this methodology on a personal computer. These various models are detailed in the paper and illustrated by typical results