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.
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机译：New Brunswick Power Nuclear公司已经成功地管理了Point Lepreau发电站的碳钢馈线管道的晶间轴向开裂，并且该管道也正在变薄，这主要是通过对退役材料进行积极的检查，维修和测试来实现的。对于过去的三次维修中断，已使用概率安全评估（PSE）来证明，对最高风险位置的年度检查可以将核安全风险保持在可接受的低水平。 PSE利用失效评估图（FAD）模型来预测部分贯通壁裂缝的失效。对两个带有部分贯通壁裂缝的退役管段进行的爆破测试表明，该FAD模型在预测组件测试的破坏压力时效果显着。使用此FAD模型会将不希望的保守性引入PSE评估中，这些评估用于优化支线管道检查和维护计划。本文介绍了另一种有限元方法，可用于提供更具代表性的结构模型，用于PSE评估。这种替代方法采用了弹塑性大应变有限元公式。使用具有代表性的材料特性;考虑空间微观结构分布;解释工作硬化率的影响;对所有变形过程进行建模，即均匀变形，局部缩颈以及失效模拟和传播。在2006年的爆破压力测试中显示了出色的测试前预测。尽管冷加工的送料弯管与母材直管相比降低了断裂韧性，但测试后的冶金检查表明在测试温度（90°C）时失败是一个延性过程，与替代的有限元方法一致。这些有限元模拟还表明，预应变为35％的冷加工材料具有比母直管更大的承载能力（即更大的破坏压力）。
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