Failure Analysis of Steam Generator Tubes

Abstract

Steam generator fabricated of alloy 600 tubes has been replaced in the last decades by newer versions built with alloy 690 or alloy 800 tubes. At first, it was thought that both alloys were resistant to SCC mechanisms but after several years of operation,some NPPs developed denting processes and IGSCC, specially in alloy 800. Corrosion products accumulated in the crevice area between tube-sheet and secondary tube surface of SG increase the stresses and produce a chemical more aggressive to the environment. These two points are the main factor to occur “denting” and increase the possibility of ODSCC can be developed. This chapter describes the failure analysis of an alloy 800 SG tube with only nine years of operation. Microstructural characterization of sludge was carried out after the cleaning by mechanical tube sheet water lancing. SEM/EDX, surface analysis by Auger, and XPS spectroscopy were used to identify chemical species over the OD surface and the fracture surface of some open cracks. “Denting” mechanism is related to flow accelerated corrosion and oxidant particles introduced into secondary side of SG. Therefore, it is convenient to prevent the impurities intake into SG and perform a chemical cleaning process to avoid the hard sludge formation.