Abstract
The analytical flaw evaluation procedure of Subsubarticle IWB-3660 and Nonmandatory Appendix O of the ASME Boiler and Pressure Vessel Code, Section XI is available to PWR licensees to disposition the detection of primary water stress corrosion cracking (PWSCC) in Alloy 600 reactor vessel head penetration nozzles. This procedure can be applied to accept a detected axial flaw for continued service if the flaw dimensions projected to the end of the evaluation period do not exceed the acceptance criteria of Table IWB-3663-1. The crack growth prediction for future operation must consider the PWSCC crack growth due to applicable loadings, including the tensile weld residual stresses that can dominate the stress state.
Following the detection of leakage in an Alloy 600 reactor vessel closure head control rod/element drive mechanism (CRDM) penetration in a U.S. PWR, a retrospective review of examination data provided several operating cycles of size information for the axial PWSCC flaw in the nozzle that was determined to be the cause of the leakage. This experience is a unique opportunity to compare the detailed data for flaw size evolution with the PWSCC growth predicted for the earliest observed flaw dimensions per the Section XI flaw evaluation procedure as a validation exercise.
