Degradation of steam generator tubes in Krsko NPP: recent status and trends

In recent years, much data and experience concerning the degradation of SG tubes accumulated. Because of regular full length inspection of all active tubes in steam generators, a huge amount of inspection records is now available. A computerized database was developed by Reactor Engineering division to accelerate the management of about 100.000 records. Initially, the data base was designed to support the development and decisions related to the plugging criteria for damaged tubes. In this paper, two prevailing groups of data are statistically analyzed: (1) the axial cracks in expansion transitions and (2) ODSCC at tube support plates. Both of them caused a vast majority of repaired tubes (e.g., plugs and sleeves). The main results of the analysis include the trends in number of defects, distribution of defect sizes, defect progression (growth) and expected life of a detected defect. Based on the results of statistical analysis, some predictions are given concerning the remaining lifetime of the steam generators. Those predictions may contribute to the optimum performance of the plant until the replacement of steam generators. (author)