Characterization of the secondary side deposits of pulled steam generator tubes

Several Belgian pressurized water reactor (PWR) plants have presented secondary side corrosion on steam generator tubes at different locations and replacements of steam generators are foreseen for this reason. In order to understand and to mitigate secondary side corrosion it is important to characterize the deposits generated during operation and the oxide layers at the tube surface. Also for the choice of tube materials for the new steam generators it is necessary to have a better understanding of the local conditions leading to secondary side degradations. An overview is given of the Belgian experience based on investigations of pulled tubes. Different analytical techniques were used, including elemental as well as structural analysis and molecular identification of organic species. The protective oxide layers at the metal surface may be rather thick and are in most cases characterized by a high ratio of Cr to Ni which points to an acidic environment. A comparison is made with tubes tested in a medium presumed to exist in the service conditions, including model boiler tests