Statistical analysis and model prediction for steam generator degradation

Steam Generator tubes are susceptible to different types of mechanical and corrosion-induced damage. Among the causes underlying tube deterioration, special mention may be made of the phenomenon known as stress corrosion cracking, which is classified as PWSCC when occurring on the primary side and ODSCC when appearing on the secondary. In view of the progressive deterioration of the steam generator tubes experienced in certain Spanish plants, it is advisable that a tool be developed allowing the evolution of different types of degradation, and consequent tube plugging, to be estimated, in order to clarify the maintenance tasks to be performed during each plant outage, gain insight into the remaining lifetime of these components and, finally, incorporate the information obtained into a steam generator replacement strategy. For this reason, a study of the two aforementioned types of defects has been performed at the ALMARAZ 1 and 2 and ASCO 1 and 2 plants. This study consisted of creating an advanced prediction model capable of estimating both the number of affected tubes identified in subsequent refueling outages and the magnitude of the possible indications found, be they previously existing indications undergoing evolution or newly developed indications. In this way, and following establishment of plugging limits, it is possible to determine the number of tubes to be plugged or repaired. Long-term predictions make it possible to draw up a strategy for subsequent refueling outages, up to steam generator replacement