Preliminary investigation of PWR feedwater line cracking

Circumferential cracks were found in large diameter carbon steel piping that carries feedwater on the secondary side in PWR nuclear power plants. The cracks were confined to a narrow band in the vicinity of the counterbore region of the piping near the weld joining the piping to the feedwater nozzle on the steam generator. In most cases, the cracks were numerous, but small; a typical penetration was less than 20% through the wall. However, in some cases, the cracks were much greater in depth and, in one instance, propagated through the pipe wall, approximately 1·6 cm (0·65 in.). The environment was high temperature water containing less than 1 ppm oxygen.

A metallurgical analysis, in which both light and electron microscopy were utilized, led to the conclusion that the cracks, particularly the large ones, were caused by corrosion fatigue. This conclusion was based upon crack morphology, crack location, the presence of bench marks and striations, fracture appearance, and the presence of corrosion products. The next step in the failure analysis was to determine the source(s) of the cyclic stresses that resulted in the corrosion fatigue.

Stress and fatigue analyses were made in an effort to determine the combinations of loads and cycles necessary to produce the observed phenomena. Environmental effects were taken into account in carrying out the analyses. The results showed that plant specific mechanisms must have been responsible for the rapid fatigue growth. An analytical and experimental programme involving the investigation of thermal fluctuations was planned to determine the precise source of the required combinations of loads and cycles.