LBB泄漏率计算与热力学非平衡效应影响评估

裂纹泄漏率计算是破前漏（LBB）在核电站管道和设备上应用的基础。在Fauske模型基础上,整个裂纹内流体流动假设为等焓过程且充分考虑摩擦效应对裂纹临界泄漏率的影响,利用Mathcad计算得到了管道裂纹两相泄漏率,与已有文献中实验数据进行对比,将其发展成为可准确计算裂纹泄漏率的计算机程序。同时根据两相流动不平衡理论,对模型进行热力学不平衡参数影响修正。结果表明：随裂纹长径比(L/D)增大,两相泄漏率减小;随裂纹入口滞止压力增大,两相泄漏率增大;裂纹入口流体过冷度增大,两相泄漏率增大,数学模型计算结果与实验结果趋势一致,但忽略热力学非平衡效应,数学模型计算得到的临界流量小于实验流量。对于热力学不平衡参数修正后模型,模型计算得到的结果均与实验数据符合很好,故由修正后模型编制的Mathcad程序可完成裂纹泄漏率的准确计算,为LBB在核电站管道上的应用提供基础。     

LBB管道裂纹泄漏率计算软件PICLES验证研究

管道裂纹泄漏率计算是破前漏（LBB）分析中的关键技术，采用与有效软件进行对比和与实验结果进行对比的方式，对国内自主研发的泄漏率计算软件PICLES进行验证研究。与已有成熟工程应用的国际同类软件（PICEP和SI-PICEP）对比，PICLES与其计算结果相差较小；与管道裂纹泄漏率实验结果对比，PICLES计算出的泄漏率与其相差?80.23%~?43.79%，PICLES计算的泄漏裂纹长度与实测裂纹长度相差21.84%~79.07%，说明将PICLES用于过冷水管道LBB分析具有较高的保守性。因此，PICLES可用于实际工程中的LBB分析。      

穿透裂纹临界泄漏率分析程序的研制与验证

建立考虑裂纹形态参数影响的周向穿透裂纹临界泄漏率的计算模型,以此为基础编制计算程序PC-Leakflow2。介绍程序的计算流程及求解方法,对影响裂纹临界泄漏率的各个输入参数进行敏感性分析,用文献中的临界泄漏率试验数据对PC-Leakflow2程序的计算结果进行验证。用PC-Leakflow2程序和经典的临界流模型对相同的例题进行计算,计算结果表明:临界泄漏率的大小受裂纹形态参数的影响较强;经典的临界流模型会显著地高估紧密裂纹的临界泄漏率。     

管道穿透裂纹泄漏率计算程序

穿透裂纹泄漏率计算程序是为了计算管道发生微小穿透裂纹时管内流体向外泄漏的临界流量而专门编制的程序采用的基本数学模型是由Abdollahian和Chexal修正过的Henry的两相临界流模型．该模型适合于计算流道长度与流道直径之比大于12的长管泄漏.     

含环向贯穿裂纹管道断裂力学工程方法影响函数的计算研究

为拓宽美国电力研究所(EPRI)工程方法的应用范围,本文通过一系列三维弹性、弹塑性断裂力学有限元分析,计算了含裂纹管道的裂纹张开位移(COD);基于有限元COD结果研究了EPRI方法中的关键影响甬数h2,并详细阐述了拉-弯组合载荷情况下h2的计算方法.为了验证该方法,将计算的h2值与EPRI已有的h2值进行比较;将基于计算的h2值所求得的COD结果与管道裂纹评定程序(PICEP)中工程实例的COD结果进行比较.结果表明,计算的h2值、COD结果均与参考值吻合良好,证明了本文h2值计算方法的正确性.     

LBB设计中管道贯穿裂纹张开位移及泄漏率计算研究

在核电站管道破前漏设计(LBB)过程中,需要对管道的关注部位假设一个贯穿裂纹,然后计算该裂纹在正常运行工况下的裂纹张开位移(COD)以及流体泄漏率,从而验证管道LBB设计准则的适应性。本文介绍了COD以及泄漏率计算的意义,然后探讨了各种计算方法的优缺点和发展方向,最后给出了自主开发程序的验证算例。文中讨论的问题可为LBB技术在我国核电厂中的应用提供参考。     

管道裂纹泄漏率计算软件开发

管道裂纹泄漏率是破前漏评价的关键参数,用于确定管道能否应用破前漏技术。开展管道裂纹泄漏率计算理论研究,基于研究成果,开发管道裂纹泄漏率计算软件PICLES,并结合工程应用实际,对软件功能进行扩展,以便直接计算泄漏监测能力下的裂纹长度。采用与目标软件对比计算的方式验证软件的有效性。结果表明:软件的计算结果与目标软件一致,可应用于管道破前漏评价;可扩展功能有效地提高了工作效率。     

管道裂纹泄漏率计算方法研究

介绍了常见的管道环向和轴向贯穿裂纹在拉伸和弯曲载荷下的张开位移计算方法,给出了基于Henry均匀非平衡流模型的管道裂纹泄漏率计算方法。将上述方法程序化,利用公开文献中的裂纹张开位移和泄漏率的试验结果与相应的计算结果进行对比研究。结果表明,利用上述计算方法能够有效预测管道裂纹张开位移和泄漏率,可用于LBB裂纹泄漏评估;但上述方法不是精确计算方法,在工程应用时需考虑一定的保守系数。     

基于有限元软件的核电厂管道破前漏裂纹稳定性分析

在核电厂管道的破前漏裂纹稳定性分析中,比较常用的是使用J积分-撕裂模量汇交方法 (简称J-T方法)来计算极限载荷和临界裂纹长度。本文利用有限元方法,建立含有裂纹的管道模型,指出裂纹尖端网格设置的关键点;然后用Abaqus软件对含有裂纹的管道模型进行J-积分分析;最后根据计算的结果,运用J积分-撕裂模量汇交方法计算出临界裂纹尺寸。将所得的计算结果与理论计算结果进行对比,可以看出二者在计算临界裂纹尺寸上具有高度的一致性。可见在小范围屈服(Small-Scale Yielding,简称SSY)情况下,使用有限元法进行破前漏裂纹稳定性分析是可行的。     

含环向穿透裂纹管道LBB裂纹扩展稳定性计算程序开发

在管道LBB设计和评定中,J积分和裂纹扩展稳定性的计算过程复杂,为提高计算精度和效率,需采用改进的计算方法,并实现程序软件的计算。本文基于增强参考应力法(ERS法)的J积分计算和J积分稳定性评定图法,开发LBB裂纹扩展稳定性计算程序,此程序可计算含环向穿透裂纹管道的LBB失稳载荷和临界裂纹长度。所开发的程序实现了单纯拉伸载荷、弯曲载荷及拉弯复合非比例加载下的裂纹扩展稳定性计算,并提供了两种材料真应力-真应变关系参数的输入计算方法,拓宽了现有计算方法的局限性。通过有限元计算结果和文献中管线试验结果与程序计算结果的对比分析,验证了计算程序的准确性。     

Leak rates through cracks and slits in PHT pipes for LBB

The Leak Before Break (LBB) concept is widely used in the nuclear industry to eliminate the consideration of postulated double ended guillotine ruptures. This is based on the premise that a detectable leak will develop before a catastrophic break occurs. A ‘throughwall flaw’ is postulated at critical sections so that any leakage from the postulated flaw is sure to be detected. The main purpose of the present study is to develop computer codes to estimate the leakage critical flow through a postulated crack. Flow through a crack may not always attain thermodynamic equilibrium; therefore Henry's Homogeneous Non-equilibrium Model (HHNM) and a Homogeneous Frozen Model (HFM) have been adopted for the present study. Here HFM has been modified taking into account the contribution of change in liquid phase kinetic energy. According to these models, critical mass flow is expressed in terms of inlet quality and exit pressure and quality. The exit pressure is related to the inlet pressure through the cumulative effect of various pressure losses, e.g. entrance loss, friction loss and loss due to flashing. A hybrid correlation for frictional pressure drop due to surface roughness has been incorporated in the present method. Computer codes have been developed based on these models. The results obtained have been verified against published data and are found to be in good agreement. Parametric studies have been carried out to examine the effects of different physical and thermal–hydraulic parameters and to assess the extent of leakage.     

Leak-Before-Break analysis for Pickering ‘A’ Unit 1 and Unit 4 large diameter main steam line pipes

This paper presents a Leak-Before-Break (LBB) analysis of large diameter main steam line pipes (i.e. NPS 28″ and 30″) running from reactor building to main steam balance header in Pickering nuclear plant Unit 1 and Unit 4. Recent development in LBB technology summarized in U.S. Nuclear Regular Commission report NUREG/CR-6765 was adopted. Based on the tiered approach of LBB philosophy, this LBB analysis belongs to level 2 or level 3 LBB analysis. Detailed fracture tolerance analyses and leakage rate calculations were performed. EPFM (elastic plastic fracture mechanics) theory of J-integral, resistance curve versus ductile crack extension was adopted in carrying out all fracture tolerance analyses. Through-wall cracks in axial and circumferential directions on both straight pipes and elbows were postulated and analyzed. The loads applied on the postulated cracked pipes were obtained from detailed piping stress analysis under deadweight load, design pressure, thermal expansion, seismic design based earthquake (DBE) and thrust load due to the opening of relief valves. J-resistance data were derived from the lowest fracture toughness testing data obtained from Ontario Power Generation's PHT (primary heat transport) LBB material testing programs. A margin of 2 on crack size was chosen in establishing maximum allowable crack sizes. Leakage rates were calculated using SQUIRT Windows Version 1.1 program. The fluid inside the main steam line pipes was assumed single phase steam at 100% quality. One tenth of the calculated leakage rates was proposed as the requirement for minimum leakage detection capability. The paper concludes that the absence of through-wall crack larger than 91.16 mm in length should be maintained in order to ensure the structural integrity of large diameter main steam line pipes. In lieu of this crack size requirement, a reliable leakage detection capability which could quantify mass steam leakage rate of 0.01678 kg per second, or volume leakage rate of 1.01 l/min, should be in place. If both of the above two requirements are met, the Leak-Before-Break of these large diameter main steam line pipes is warranted.     

Ontario hydro's leak before break approach: Application to the darlington (candu) nuclear generating station a

Ontario Hydro has developed a leak before break (LBB) approach for application to the large diameter heat transport piping for Darlington NGS A as an alternative to the provision of pipewhip restraints. This approach has been applied to pipe sizes which are equal to or greater than 530 mm (21 in. NPS). The proposed LBB approach incorporates assessments at several levels to provide assurance against catastrophic rupture. A comprehensive and systematic review of pipe failure mechanisms is considered the first important step in establishing role and applicability of the LBB concept. The elements integral to the approach are those related to demonstration of crack stability utilizing fracture mechanics methods and those related to leak rate predictions and leakage detection capability. For evaluation of crack stability the J-integral/tearing modulus (J/T) method has been selected. Results from an extensive material test program from actual heat transport piping, forgings, associated welds and heat affected zones as inpur to EPFM analyses provide the J-resistance and J—T curves. The details of EPFM analyses for a straight pipe with a circumferential crack and a piping elbow with a central longitudinal throughwall crack are presented here. Additionally, results of crack opening detail, the effects of crack face pressure, the predictions of LEAK RATE code and an assessment of the leakage detection capability are presented.     

压力管道破前漏分析的一种简化方法

介绍了一种用于核反应堆管道和压力容器破前漏（ＬＢＢ）分析的简化方法，它主要以线弹性断裂力学的基础，将ＬＢＢ分析中应力强度因子，裂纹张开面积和泄漏率等计算以解析公式表达出来。这种方法使用方便，而且满足有关的国家标准和国际规范的要求，适用于一些应力分布和几何形状比较简单的管道和压力容器，或用于ＬＢＢ性质的近似估计。     

A probabilistic method for leak-before-break analysis of CANDU reactor pressure tubes

In modern CANDU nuclear generating stations, pressure tubes of cold-worked Zr-2.5Nb material are used in the reactor core to contain the fuel bundles and the heavy water (D₂O) coolant. The pressure tubes operate at an internal pressure of 10 MPa and temperatures ranging from 250°C at the inlet to 310°C at the outlet. Over the expected 30 year lifetime of these tubes, they would be subjected to a total fluence of 3×10²⁶ n m⁻². In addition, these tubes gradually pick up deuterium as a result of a slow corrosion process. When the hydrogen plus deuterium concentration in the tubes exceeds the hydrogen/deuterium solvus, the tubes are susceptible to a crack initiation and propagation process called delayed hydride cracking (DHC). If undetected, such a cracking mechanism could lead to unstable rupture of the pressure tube. The service life of the pressure tubes is determined, in part, by changes in the probability for the rupture of a tube. This probability is made up of the probability for crack initiation by DHC multiplied by the sum of the probabilities of break-before-leak and leak-before-break (LBB). A probabilistic model, BLOOM, is described which makes it possible to estimate the cumulative probabilities of break-before-leak and LBB. The probability of break-before-leak depends on the crack length at first leak detection and the critical crack length. The probability of a LBB depends on the shut-down scenario used. The probabilistic approach is described in relation to an example of a possible shut-down scenario. Key physical input parameters into this analysis are pressure tube mechanical properties, such as the crack length at first coolant leakage, the DHC velocity and the critical crack length. Since none of these parameters are known precisely, either because they depend on material properties, which vary within and between pressure tubes, and/or because of measurement errors, they are given in terms of their means and standard deviations at the different temperatures and pressures defined by the shut-down scenario.     

Validation of simplified fracture mechanics methods by testing of real components

The evaluation of integrity of structural components is often based on the proof of leak-before-break (LBB). Leak-before-break behaviour in piping constitutes a fail-safe condition. Which means that, during multiplied loading conditions, a defect results at first in a leakage. The crack length which leads to the leakage is smaller than the critical through-wall crack length. Simplified fracture mechanics concepts are used for the demonstration of LBB. For this the conservative, safe calculation of the critical through-wall crack length for ductile failure is necessary. To validate simplified calculation methods for circumferential cracks (flow stress concept (FSC); plastic limit load (PLL)) and for axial cracks (Battelle approach (BMI); Ruiz approach (RUIZ)) all available experiments on real structural components, especially on pipes, were analysed and evaluated by the mentioned simplified methods (approximately 460 experiments). The methods were adapted by application of correction factors, mainly on the flow stress, to result in conservative (safe) and realistic (as near as possible to the experiments) predictions. Depending on method (FSC, PLL, BMI, RUIZ), crack orientation (circumferential and axial cracks) and type of material (ferritic and austenitic material) different definitions of flow stresses were established.     

Leak-before-break assessment of CANDU pressure tube considering leak detection capability

The leak before break (LBB) concept is employed as defense in depth in CANDU reactors in order to avoid an unstable failure in pressure tubes. The LBB in pressure tubes can be ensured when detection, confirmation and location of the leak are carried out and the reactor is placed in a depressurized condition before the crack exceeds critical crack length. Leak detection and location is provided by the annulus gas system (AGS). Therefore, the evaluation parameters for LBB assessment, such as leak detection and leak location capabilities, should be made available through AGS performance test. Recently, the AGS in-situ tests with a simulated moisture injection were carried out in one of the CANDU reactors in Korea. This paper presents the LBB assessment performed taking into account the leak detection capability and the modification of operating procedure therefrom.     

Development of modified piping evaluation diagram for leak-before-break application to Korean next generation reactor

Recently, the piping evaluation diagram (PED) is accepted in nuclear industry for an efficient application of leak-before-break (LBB) concept to piping system at an initial piping design stage. The objective of this paper is to develop the modified PED, which can account for the variation of the material properties of the PED development stage and those of the assembly stage. For this purpose, a parametric study was performed to investigate the effect of stress–strain curve on the detectable leakage crack length and the effect of fracture resistance curve on the LBB allowable load. Finite element analyses were also performed to investigate the effect of stress–strain curve on the LBB allowable load. Finally, a modified PED was proposed as a function of crack length and the allowable safe shutdown earthquake load. The LBB analyses based on the modified PED are in good agreement with those based on the traditional PED. By adopting the modified PED, the variation of material properties can be considered in the LBB analysis and the computing times required for the application of LBB during the design process can be considerably reduced.