해석적 방법에 의한 감육곡관 손상압력에 미치는 굽힘하중 영향 평가

Abstract

원전 2차측 배관은 유체유동에 의한 가속부식(FAC, Flow Accelerated Corrosion) 현상이 급속히 증가되어 벽두께 일부가 허용두께 이하로 얇아지는 3차원 형태의 결함으로 나타난다. 원전 2차계통 주증기 또는 주급수 파단시 그에 따른 피해 발생 정도가 크기 때문에 감육 결함 발생은 원전의 안전성과 운전성 측면에서 중요 대상으로 관리하고 있다. 원전 감육 배관의 건전성 평가와 관련한 연구는 1990년대 이후부터 활발히 진행되고 있지만 지금까지 수행되어온 대부분의 연구들은 단순내압 조건이나 Displacement Constant와 단순내압 조건에서만 수행되어 왔다. 본 연구에서는 굽힘 하중이 걸리는 상태에서 내압이 존재할 때 감육 곡관의 손상에 미치는 감육 결함의 영향을 파악하고, 단순 내압 조건이나 Displacement Constant와 단순내압 조건에서의 손상 영향을 해석적 방법으로 비교해 보았다.|Under normal operating conditions, piping systems in nuclear power plants(NPPs) are subject not only to internal pressure but also to bending loads induced by deadweight and thermal expansion. Bending is thus considered to be an important factor in evaluating the integrity of defective piping components. This study investigated the effects of bending load on the failure of wall-thinned pipe bends under internal pressure. A previous study experimentally evaluated the bending load effects on the failure pressure of wall-thinned elbows under displacement controlled in-plane bending load, but the numbers of experimental data were insufficient to determine the effects of bending load on the failure pressure of wall-thinned pipe bends. Therefore, the present study systematically evaluates the effects of bending load on the failure pressure of wall-thinned pipe bends using parametric finite element analyses. To evaluate the effect of bending load on the failure pressure of wall-thinned pipe bends, the failure pressures were evaluated under combined bending and internal pressure and they were normalized to those evaluated under simple internal pressure. For most wall-thinning cases the bending load decreased the failure pressure of wall-thinned pipe bends, although it slightly increased the failure pressure in some cases. The result showed that bending load effect was more significant when the constant moment was applied than when the constant displacement corresponding to the moment was applied. This is associated with the relaxation of bending load during the pressurization. The previous experimental study showed that the bending moment by constant displacement was relaxed as the plastic deformation occurred at wall-thinned area by internal pressure. Thus, the bending effect is relatively minor when applying the displacement controlled bending compared to the load controlled bending. The reduction of failure pressure by bending load was less than 10% for shallow wall-thinning cases, (tnom-tp)/tnom≤0.5, regardless of the thinning length and circumferential angle, location, and bending mode. For deeper wall-thinning cases ((tnom-tp)/tnom=0.75), however, the effect of bending load on the failure pressure was considerable and depended on the bending mode, thinning location, thinning length, and circumferential angle. For intrados wall-thinning of θ/π=0.5 subjected to opening mode bending and extrados wall-thinning of θ/π=0.5 subjected to closing mode bending, the reduction of failure pressure by the bending load became significant as the thinning length decreased. As thinning length increased, however, the bending effect became significant for narrow intrados wall-thinning (θ/π=0.0625) under opening mode bending. This is because the dominant stress to initiate the failure at wall-thinned area depends on thinning geometry. For shorter wall thinning with large circumferential angle, the axial stress is dominant compared to circumferential (hoop) stress under internal pressure. Thus, the failure occurs at lower pressure when the bending provides tensile axial stress at wall-thinned area, whereas the failure occurs at higher pressure when the bending provides compressive axial stress. For wall thinning with small angle or long length, the failure is governed by circumferential stress. In this case, the bending load enhances the circumferential stress for both intrados and extrados wall-thinning cases. In particular, the enhancement of circumferential stress is considerable for longer intrados wall-thinning with small angle under opening-mode bending. Thus, the failure pressure decreases with thinning length for narrow intrados wall-thinning cases.