不同材料的核电厂主管道LBB评估对比研究

我国核电厂主管道的材料主要有铸造奥氏体不锈钢(CASS)和锻造不锈钢(WSS)。针对CASS和WSS两种材料的主管道,依据美国核管会的SRP3.6.3进行主管道硬前漏(LBB)评估的对比研究。考虑热老化效应获取可信的材料性能数据,根据材料性能差异采用极限载荷法或J积分撕裂模量汇交法计算临界裂纹尺寸。根据Henry均匀非平衡双相流模型计算泄漏裂纹尺寸,并通过环向表面裂纹和贯穿裂纹的扩展分析论证了裂纹疲劳扩展不会导致管道的突然断裂。研究结果表明,WSS材料和CASS材料相比具有更好的LBB性能。     

LBB裂纹稳定性分析方法研究

破前漏（LBB）分析是一种先进的管道分析评价技术,其关键技术是裂纹的稳定性分析,决定着LBB技术的可用性。本文从核级管道的失效模式出发研究了LBB分析的裂纹稳定性分析问题,并探讨了修正J积分撕裂模量汇交方法(J-T方法)、修正极限载荷分析法以及两倍弹性斜率法的优缺点和适用范围,最后通过工程算例对不同分析方法进行检验,其结论可为我国核电站管道的LBB设计提供参考。     

残余应力作用下的不锈钢管道环向穿壁裂纹闭合效应研究

管道环向穿壁裂纹在不同载荷水平作用下的张开位移预测值是破前漏技术应用的关键核心参数。针对具有代表性几何尺寸的奥氏体不锈钢管道，采用数值分析和对比验证相结合的方法，基于工程中实际测得的材料性能曲线研究了典型焊接残余应力作用下穿壁裂纹临界闭合应力的变化规律。分析结果表明，目前的通用电气有限公司/美国电力研究院（GE/RPRI）方法和美国核管会技术报告NUREG/CR-6837修正方法均低估了由美国机械工程师协会（ASME）规范工作小组推荐的简化残余应力场所导致的管道环向穿壁裂纹闭合效应。此外，分析了环向穿壁裂纹闭合状态下管道的失效模式，在此基础上进一步讨论了裂纹闭合效应对破前漏技术应用的影响，为后续工程实践提供了可借鉴的技术观点。     

破前漏(LBB)分析方法在钠冷快堆管道分析中的应用探讨

采用破前漏(LBB)分析方法,对中国实验快堆余热排放系统中间回路进行断裂力学分析.假设在其受力最严重的管段,反应堆运行前已存在一个贯穿裂纹,根据系统寿期内的运行工况,运用J积分撕裂模量汇交方法,求出在综合最恶劣工况(即合成出包络工况)下裂纹的临界长度;通过Paris公式求出该裂纹在交变载荷作用下的稳定扩展量;利用CASTEM2000程序建立力学模型,分别求出包络工况下寿期末裂纹和临界裂纹的张开而积.结果证明,该管道在其寿期内可能出现的最大裂纹张开面积小于(DT)/4(D为管道直径,T为管壁厚).     

非中心穿透裂纹对LBB技术应用的影响分析

随着LBB技术的发展和应用推广,可运用详细的裂纹扩展分析技术,通过周密的分析论证,以证明带缺陷管系在使用寿期内同样能够满足SRP3.6.3中关于LBB技术应用的、与泄漏探测能力、裂纹稳定性和载荷相关的裕量要求。在这种前提下,一些额外的分析就必不可少。其中之一即为需要考虑载荷对称中心与裂纹对称中心不重合情况,即所谓的非中心裂纹,对LBB技术应用的影响。本文以压水堆核电厂中DN150、DN350和DN550管径的核1级高能管道中非中心裂纹为研究对象,先从偏心角度对裂纹张开面积的分析着手,进而研究其对泄漏率分析与裂纹稳定性分析的影响,并对非中心裂纹对LBB技术应用的影响做了综合性的分析总结,为今后含缺陷管道应用LBB技术的分析提供参考与借鉴。     

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。     

LBB分析中J积分撕裂模量汇交方法研究

介绍了管道破前漏(LBB)分析中极限载荷和临界裂纹长度求解的J积分-撕裂模量汇交方法(J-T法).利用有限元分析软件(ABAQUS)计算3种裂纹长度J积分,并将J积分与裂纹长度采用多项式函数进行拟合,最后在多项式函数的基础上求解极限载荷和临界裂纹长度.采用FLET软件提供的算例对J-T方法的正确性进行验证.结果表明,运用基于ABAQUS的J-T法能提高失稳J积分的计算精度,能适用更广泛范围、更复杂结构的LBB分析.     

破前漏中的断裂力学分析研究

在核级管道的设计中,若能证明其满足破前漏(LBB)准则,在设计时可不予考虑假想的双端剪切断裂事故的发生,从而减少防甩防冲击等附加设备及在役检查工作量。因此对反应堆管道系统运用破前漏设计可降低复杂性,提高安全性。管道破前漏中的断裂力学特性分析内容主要包含给定裂纹下的裂纹张开量(COD)、张开面积(COA)计算及裂纹稳定性判定。本文总结了当前工程中最常用的两种破前漏近似准则并程序化其分析过程,然后利用有限元软件的二次开发,建立了含贯穿裂纹的压力管道数值分析程序,完成了校验计算。通过对工程准则与有限元计算结果的比对,讨论了工程准则的保守性,比对结果可给出不同工程近似方法的适用区间。     

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。      

主管道材料高温力学性能研究

提供核电厂破前漏（LBB）设计所需材料性能参数需要测量主管道母材和焊接材料在高温下的力学性能（包括材料在地震环境下的高温动态力学性能）。基于万能伺服材料试验机和高速材料试验机测量了核电厂主管道母材控氮00Cr17Ni12Mo2不锈钢及焊接材料OK Tigrod 316L在高温（350℃）下的静动态拉伸力学性能、裂纹扩展率和静动态断裂韧性。与主管道母材和焊接材料的常温力学性能相比,2种材料在350℃下的静动态拉伸力学性能以及OK Tigrod 316L在350℃下的静动态断裂韧性都较常温有较大幅度的降低,2种材料在350℃下的抗裂纹扩展性能较常温略有下降。研究成果可为核电厂管道的LBB设计提供试验技术和材料参数支持。     

Low alloy steel piping test for fracture criteria of leak before break

A total of seven pipe fracture tests were performed to provide the data for establishing fracture criteria of leak before break for the low alloy steel pipe, which is expected to be applied to reactor coolant piping and feedwater piping in advanced PWRs in Japan. Test pipes were 6-inch and 8-inch diameter pipes made of SFVQ1A or STPA24 low alloy steel. A circumferential through-wall crack was introduced at the center of a pipe, and four-point bending load was applied without internal pressure. Stable crack extensions were observed in all of the experiments. The net-section criteria (NSC), R6 method option 2 and option 3 were used to estimate the maximum applied load. The predicted values by three kinds of evaluation methods were compared with the experimental loads. Most of the predicted maximum loads agreed well with the experimental maximum loads within 20% difference. The NSC gave the most accurate prediction but also gave unconservative results in some test cases. The predicted maximum loads by R6 option 2 were conservative in all of the test cases. From the viewpoint of conservativeness R6 method can be used for evaluation of the low alloy steel pipe fracture. Therefore, the leak before break (LBB) concept could be applied to the protective design standard against pipe break for the material.     

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.     

Ductile fracture behavior of circumferentially cracked type 304 stainless steel piping under bending load

A ductile pipe fracture test program has been conducted in Japan Atomic Energy Research Institute (JAERI) to investigate the ductile fracture behavior of circumferentially cracked pipes and to demonstrate the validity of the leak before break concept in LWR pipings.In the paper are described the scope of the pipe test program and current test results for 6-inch diameter type 304 stainless steel pipes. Test pipes with a through-wall or a part-through crack in the circumferential direction were bent under low or high compliance condition, and stable or unstable pipe fracture behavior was investigated. J based tearing instability criterion and the net section collapse criterion are compared with the pipe test results, and the validity of these fracture criteria is discussed. Furthermore, geometries of acceptable flaws in pipes are evaluated considering the leak before break condition.     

Crack stability evaluation of nuclear main steam pipe considering load reduction effect

The objective of this paper is to evaluate the crack stability of a nuclear main steam pipe, considering load reduction effect due to the presence of circumferential through-wall crack. Also, optimization techniques are adopted to simulate the crack effect on the elbow component of piping systems. By using a general beam element that contains a discontinuous cross-section, piping analysis is performed to obtain the reduced load. Considering this reduced load, LBB design concept is applicable to the nuclear main steam pipe system. Also, by combining an optimization program and a general finite element analysis software, the appropriate dimensions are simplified and an equivalent beam element representing the effect of crack in the elbow could be successfully obtained.     

Elastic-plastic finite element analyses of stable crack growth and fracture instability in a girth welded pipe

Detailed elastic-plastic finite element fracture mechanics analyses were conducted on a 16 inch diameter Type 304 stainless steel pipe containing a circumferential through-wall crack located in a girth weld. Calculations were performed to analyze the welded pipe treated as (1) a monolithic pipe entirely composed of the base metal, and (2) a composite of base metal and weldment. In the latter, each constituent was assigned distinct mechanical and fracture properties. In both solutions applied J values were calculated for a fixed axial load combined with a monotonically increasing applied bending moment. The material J-resistance curves appropriate for the two problems were each used to initiate and grow the initial crack in a stable manner until fracture instability occurred under load control. It was found that the extent of stable crack growth and the applied loads at fracture instability are distinctly different in the two analyses. It is concluded that more precise fracture mechanics approaches than those now in current use are required for accurate assessments of weld cracking problems.     

Tests on large scale LMFR piping Part I: crack resistance properties of through-cracked straight 700 mm nominal diameter pipes tested under bending at ambient temperature and 550°C

Knowing the crack resistance properties of a structure is essential for fracture mechanics safety analyses. Considerable attention has to be given in many cases to the through-wall case, since this is generally believed to be the controlling case with regard to complete pipe failure. Within a cooperative fracture mechanics programme of Electricite de France (EdF), Novatome and Siemens/KWU, bending tests with monotonously increasing load on circumferentially cracked straight pipes of typical liquid metal fast reactor (LMFR) main piping dimensions were performed. In this paper a summary report is given on crack resistance curves based on the crack tip parameters S, J and JM. The data are compared with those of C(T) specimens. The experiments have demonstrated an enormous potential for stable crack extension under global bending which is a typical loading for LMFR piping structures. The results of checking the transferability of laboratory specimen crack growth characteristics to the cracked pipes on the austenitic stainless steel 316 L demonstrate that the fracture mechanics concept for a reliable transfer of crack resistance data from small specimen geometries to large structures needs further qualification for high toughness materials.     

Leak-before-break behavior of statically indeterminate piping system

Structural integrity considering the fatigue have become to be important, as energy-related structures such as nuclear piping systems especially have been usually operating under cyclic loads. This study is concerned about the leak-before-break (LBB) behavior when pipe including a circumferential crack is subjected to a cyclic load. Attentions were paid to the proposal of assessment models and conditions on the LBB behavior considering a crack shape at wall penetration under a cyclic load, the comparison of LBB condition between a static load and a cyclic load, and the characteristic of the fatigue LBB behavior under statically indeterminate system. It is concluded that LBB behavior is more likely to be occurred in statically indeterminate piping system, as the reduction of ultimate strength due to a sufficient redistribution of bending moment in this system was relatively small. This benefit on structural integrity of statically indeterminate system was applied to the fatigue LBB behavior envisioned the validity and the superiority of the fatigue LBB behavior in statically indeterminate piping system.     

Low cycle fatigue and ductile fracture for Japanese carbon steel piping under dynamic loadings

Dynamic fracture behavior of circumferentially cracked pipe is important to evaluate the structural integrity of nuclear piping from the viewpoint of the LBB concept under seismic conditions. Fracture tests have been conducted for Japanese carbon steel (STS410) circumferentially through-wall cracked pipes that are subjected to monotonic or cyclic bending loads at room temperature. In the monotonic-loading tests, the maximum load to failure increases slightly with increasing loading rate. The failure cycles can be expressed simply by ratio of the load amplitude to the plastic collapse load. Fracture analysis has been also conducted to model the pipe tests. A new equation for calculating ΔJ for a circumferentially through-wall cracked pipe subjected to bending has been proposed. The failure cycles under cyclic loads are satisfactorily evaluated using an elastic-plastic fracture mechanics parameter ΔJ.     

Prediction of leak areas and experimental verification on carbon and stainless steel pipes

This paper presents a study on the pipe crack opening area which is a task in the frame of the six-years Pipe Test Program that ENEA (Comitato Nazionale per la Ricerca e lo Sviluppo dell' nergia ucleare e delle nergie lternative) started aiming to address some of the uncertainties related to Leak-Before-Break (LBB) concept.The study focuses on the opening generated in a pipe containing a through wall circumferential crack loaded in pure bending, trying to assess the relative leak area of paramount importance in any evaluation of dynamic effects, i.e. thrust forces and jet impingement, and leak detection systems capability as well.The purpose of this report is to check some available analytical tools against the experimental results obtained on stainless and carbon steel pipes.In particular two different methods are considered, that is:

1. (a) Tada/Paris formula,

2. (b) General Electric Engineering Approach.

Calculaion are compared with experimental results from pure bending pipe tests, run at room temperature.