Metallurgical Investigation of Cracking In the Reactor Vessel Alpha Loop Hot Leg Nozzle to Pipe Weld At the V. C. Summer Station

摘要

This paper presents the results of a metallurgical investigation conducted to establish the mechanism and cause of cracking and leakage of the Loop 'A' hot leg nozzle weld at the V. C. Summer station. A hot cell investigation was centered on a six-inch wide spool piece containing the affected weld. The reactor vessel nozzle was buttered with Alloy 182 and welded to Type 316 stainless steel piping using Alloy 82 filler metal. The investigation included surface examinations, metallographic and fractographic evaluations, chemistry and mechanical property tests, residual stress assessments, a review of the weld fabrication and repair history, and a review of a finite element stress analysis results of the weld procedures. The results of the investigation suggested that the cracking in the Alloy 182 cladding/butter and Alloy 82 weld occurred by Primary Water Inter Dendritic Stress Corrosion Cracking (PWIDSCC) mechanism. High residual stresses due to weld repair and heavy grinding at the ID surface are believed to have contributed to the crack initiation. Residual stresses and hot cracking, in the weld from multiple repairs, were identified as the most probable contributors to crack initiation.