Through-wall failure of a main steam bypass line socket weld and the flaw evaluation methodology

A recent event of through-wall leakage occurred on a 1 inch Class 2 turbine driven Auxiliary Feedwater steam supply bypass line at Surry Nuclear Power Station. The cause of the leakage was identified as a very small defect in a socket weld near the fitting side weld toe. The as-found weld flaw in the affected socket weld was evaluated based on a degraded cross-section methodology to demonstrate the acceptability of this condition, which formed the basis to seek relief from the NRC and allowed the time needed to design and install a structural clamp per ASME Section XI to ensure long-term stability of the welded joint. Recognizing the limitation of the in-house methodology and the need for a common methodology for resolutions to similar events associated with socket weld failures from across the industry, a flaw evaluation methodology based on Linear Elastic Fracture Mechanics, similar to the approach used in Code Case N-513 of ASME Section XI was proposed, along with new stress terms defined for socket welds. Finite Element models were developed for the bypass line socket weld to evaluate stresses in detail. Stress intensity factors were calculated for postulated semi-elliptical flaws along the shortest distance from the socket weld root to its surface, as an attempt to investigate the effects of the socket weld geometry on the weld strength. It is demonstrated that such an approach is feasible through the use of geometrical factors to account for the effects of the geometry on the stresses in the vicinity of the socket weld. The scope and applicability of the proposed flaw evaluation methodology for socket welds were also discussed, along with future work required to provide the bases for establishing the acceptance criteria and the parameters used in calculating the stress intensity factors.