Abstract
Since the start of commercial nuclear power operation, the industry has reported a variety of steam generator corrosion damage. Operating practices introduced in the 1980s and 1990s to improve the secondary side chemistry and to reduce the introduction of oxidants (e.g. EPRI water chemistry guidelines) has slowed the rate of secondary side degradation. Even with the enhanced attention to secondary side chemistry and attempts to reduce the amount of corrosion products being deposited in the steam generators, ODSCC continues to be an ongoing issue for Alloy 600MA units and has been recently reported in a domestic Alloy 600TT unit. Tube removal and the subsequent failure analysis continue to be a viable method to understanding the relationships between material condition, operating chemistry, and stress loading. Results of the destructive examination of pulled Alloy 600MA and Alloy 600TT tubes are discussed. In each case, the analytical investigations did not identify a definitive cause for the observed ODSCC. In the case of the freespan ODSCC of Alloy 600MA tubing, the presence of localized longitudinal stringers appeared to be a differentiating factor. For the ODSCC observed in the tube support plate region of Alloy 600TT tubing, the concurrent existence of a less than optimum microstructure and a higher than expected residual stress were identified. Nothing from the analytical investigations clearly identified an “aggressive chemical environment” to explain the observed ODSCC for either the Alloy 600MA or Alloy 600TT tubes.
