Primary Water Stress Corrosion Cracking Analysis in Alloy 600 Steam Generator Nozzle of a Pressurized Water Reactor

Steam generator (SG) drain nozzles on the cold leg side in a pressurized water reactor (PWR) have exhibited stress corrosion cracking (SCC). The nozzles were made of forged and annealed Alloy 600 (UNS N06600) base material containing welds with Alloys 82 (UNS N06082) and 182 (UNS W86182). Carbides were distributed at the grain boundaries, but no grain boundary Cr depletion was observed in the base metal. Circumferential cracks developed at the root of a J weld inside a thick wall pipe. Axial cracks were distributed with a length around 6 mm to 10 mm long. All cracks were initiated from the inner surface of the pipe. Some more shallow cracks were also observed, but they were not detected by the field non-destructive examination using eddy current test (ECT) and ultrasound test (UT). Two out of the 12 cracks penetrated through the pipe wall, and therefore the primary water was leaking during the operation. The high tensile stress region calculated by a finite element modeling (FEM) model coincided with the crack locations. The highest tensile stress region was at the root of the J weld, as judged from metallography and residual stress analyses. Residual tensile stresses from welding are considered to be the main contributor to the overall stress.