Root Cause analysis of SG tube ODSCC indications within the tube sheets of NPP biblis unit A

¹ AREVA NP GmbH
² RWE Power AG Kraftwerk Biblis
³ RWE Power AG (Germany)

Abstract

In the nuclear power plant Biblis A during the regular SG tubing inspections using eddy current testing in 2005 and 2006, a few 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 tubes 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. Penetration of secondary side water is only possible by a penetration path along the upper expansion.

A comprehensive root cause analysis was initiated to clarify in which way such a penetration path can be developed and to identify the source of circumferential tensile stresses at the tube OD. This 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. The results are summarized in this paper.