Experience with steam generators of the KWU-concept

Eleven plants equipped with KWU pressurized water reactors and 34 steam generators are currently in power operation. Eddy-current tests are conducted at regular intervals to check the condition of the steam generator tubes. The routine inspection techniques are complemented by analysing techniques characterized by rotating ultrasonic high-frequency probes and rotating eddy-current narrow-field-probes. The two steam generators of the Obrigheim plant equipped with Inconel 600 tubing experienced severe intergranular stress corrosion. These steam generators have to be replaced. The operating experience with Incoloy 800 tubed steam generators shows that there is only one area of concern with respect to corrosion attack: The tube wastage, a phosphate induced wall-thinning which occurs in a specific zone within the hot and cold side sludge pile above the tube sheet. From the inspection findings and metallographic examinations of removed tubes it was concluded that three remedial actions have to be implemented: (i) minimization of low-flow zones in the steam generator; (ii) prevention of condenser leakages; (iii) reduction of corrosion product transport into the steam generators. To meet these objectives, several improvements and modifications were introduced into the steam generators and the steam-water-cycle of plants under construction, i.e. high integrity condensers using Cu-free materials, filtration of the feedwater, All Volatile Treatment (AVT) with pH-values above 9.8 in the feedwater etc. First experience from a unit operating according to this philosophy is excellent. To all units operating on phosphate chemistry it was recommended to implement the above-mentioned features emphasizing the high integrity condensers. (author)