ASCO steam generators operating experience. Safety criteria for defect management and effectiveness of preventive measures

ASCO NPP is a two W-PWR 930 Mwe Units. Each Unit is provided with three Westinghouse Model D3 steam generators which are of preheater type and Inconel 600 MA as tube material. The Secondary side was designed and erected with copper alloys. Unit I: 81.072 EFPH, and Unit II: 69.720 EFPH. The results of the Eddy Currents Inspections performed during the first refueling outage showed Denting at tube support plates and PWSCC at roll transition zone in Unit I and Denting in Unit II. Later inspections showed other types of damages, such as: (1) ODSCC at tube support plates intersections. (2) Circumferential cracks OD and ID at roll transition zone. (3) Wear at antivibration bars and preheater baffles level. Consequently, in order to limit the plugging rate, A.N. ASCO decided to license new plugging criteria in addition to the 40% depth criterion included in Technical Specification. The new licensing criteria and surveillance requirements, varying with tube zone, are explained in the paper