Biblis Unit A is a Siemens/KWU type four loop PWR equipped with SG tubing made from Stainless Steel Alloy 800, operating from 1975 until the shutdown in 2011 due to the German political decision of a quick nuclear phase out. During the regular SG tubing inspections using eddy current testing in 2005 and 2006, a few SG tube indications were detected within the tube sheet between upper and lower mechanical tube expansion. These indications were limited to the outer tube bundle periphery. As a consequence two rubes with EC indications were pulled in 2006 for further investigations. The destructive examinations revealed axially oriented cracks starting from the outer tube surface (ODSCC). The analysis of the debris from the upper expansion area clearly indicated the presence of secondary side water in the volume between tube sheet and SG tube. Penetration of secondary side water is only possible by a penetration path along the upper expansion. The ODSCC found in this area within the tube sheet is of no safety relevance because it could at maximum lead to a limited leakage from primary to secondary side. Nevertheless, a comprehensive root cause analysis was initiated by the NPP operator to clarify in which way such a penetration path can develop. To identify the source of circumferential tensile stresses as precursor of SCC at the tube OD, investigations on the stress level of the heating tubes were carried out. The study includes an assessment of the manufacturing and operation of the SGs and the analysis of the deformation behavior in the upper expansion areas well as a consideration of the secondary side water chemistry data. One target of the root cause analysis was to focus future SG tube NDE. The results are summarized in this paper.
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