New Brunswick Power Nuclear has been successfully managing intergranular, axial cracking of carbon steel feeder piping, that is also experiencing thinning, in the Point Lepreau Generating Station, primarily by an aggressive program of inspection, repair and testing of ex-service material. For the past three maintenance outages, a probabilistic safety evaluation (PSE) has been used to demonstrate that annual inspection of the highest risk locations maintains the nuclear safety risk from cracking at an acceptably low level. The PSE makes use of the Failure Assessment Diagram (FAD) model to predict the failure of part-through-wall cracks. Burst testing of two ex-service pipe sections with part-through-wall cracks has shown that this FAD model significantly under predicts the failure pressure from the component tests. Use of this FAD model introduces undesirable conservatism into PSE assessments that are used to optimize feeder piping inspection and maintenance plans. This paper describes an alternative finite element approach, which could be used to provide more representative structural models for use in PSE assessments. This alternative approach employs the elasto-plastic large strain finite element formulation; uses representative material properties; considers the spatial microstructural distribution; accounts for the effect of work hardening rate; models all deformation processes, i.e., uniform deformation, localized necking, and failure imitation and propagation. Excellent pre-test prediction was shown for the burst-pressure test in 2006. Although cold-worked feeder bends have reduced fracture toughness compared to the parent straight pipe, post-test metallurgical examinations showed that failure at the test temperature (90°C) is a ductile process, which is consistent with the alternative finite element approach. These finite element simulations have also shown that the cold-worked material with 35% pre-strain has greater load-carrying capacity (i.e., larger failure pressure) than the parent straight pipe.

This paper presents burst pressure tests of two ex-service, 2 1/2-inch, SA-106 Grade B pipe bends with multiple part-through-wall axial cracks. The two pipes tested were sections of outlet feeders that were removed from the Point Lepreau Generating Station after 18.6 and 19.5 effective full-power years of service. The field of cracks was tightly distributed in the longitudinal and transverse directions of pipe bends on both inside surface and outside surfaces. The total length of the crack fields were 40 mm and 100 mm, respectively. The deepest crack was 73% through-wall. The burst pressure tests were performed at 90°C. The failure pressure was more than 4 times the design pressure. Post-test metallurgical examinations revealed mixed mode failures on the main fracture surface. The failure of the specimens was controlled by local plastic collapse of the ligaments of a small number of the deepest cracks in the field of cracks.