Alloy 800 steam generator tube stress corrosion cracks -detection and root cause

摘要

Intergranular stress corrosion cracking in Alloy 800 steam generator tubes has historically been an unknown field damage mechanism for this material. Recently, such cracks have been detected within the tube sheet between the upper and lower mechanical tube expansions (these indications were limited to the outer tube bundle periphery) and in the area of the tube support (grid assembly). Detection and especially sizing of these types of defect were not reliable enough with the common NDT methods. Steam generator tube integrity constitutes the main barrier against the release of radioactivity into the environment. Under the effect of the pressure difference between the primary and secondary circuits, primary water can leak into the secondary circuit via through-wall defects. Such defects may have a safety relevance because crack growth can be very fast and the critical crack size is very small. Therefore, optimised inspection methods as well as monitoring during operation are necessary for tube integrity assurance. The overall requirement in this particular case is to guarantee the tube integrity until the next inspection by placing special emphasis on NDT methods. In this paper, NDT methods for the inspection of defects such as intergranular stress corrosion cracks, as well as a possible monitoring procedure, are presented. Some statements about the root cause of this special damage mechanism will also be described.