Alternative methodology for assessing part-through-wall cracks in carbon steel bends removed from Point Lepreau Generating Station

Autor: John Slade, Xinjian Duan, Michael J. Kozluk, Tracy Gendron
Rok vydání: 2011
Předmět:
Zdroj: Nuclear Engineering and Design. 241:630-637
ISSN: 0029-5493
DOI: 10.1016/j.nucengdes.2010.07.035
Popis: In 2008 April Point Lepreau Generating Station entered an extended refurbishment outage that will involve the replacement of key reactor components (fuel channels and connecting feeder pipes). Prior to the refurbishment outage, New Brunswick Power Nuclear had been successfully managing intergranular, axial cracking of carbon steel feeder piping, that were 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 previous three maintenance outages, a probabilistic safety evaluation (PSE) had 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-pressure testing of two ex-service feeder pipe sections with part-through-wall cracks showed that this FAD model significantly under predicts the failure pressure measured in 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 performed 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.
Databáze: OpenAIRE
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