Development of probability of detection functions for evaluating integrity of axial outside diameter stress corrosion cracks in steam generator

The reliability of an eddy current testing (ECT) inspection system depends upon the inspection technique and quality of analyst. In evaluating the integrity of a steam generator (SG) tube, degradation detection and sizing accuracy are considered performance measures of the nondestructive evaluation (NDE) system. A probability of detection (POD) model serves as a functional measure of the ability of an NDE system to detect degradation. It is one of the inputs in the operational assessment, and it is used to estimate the degradation during service via ECT of the SG tube. In this study, the POD functions of the inspection technique and analyst were obtained to quantitatively analyze the ECT bobbin probe for axial outside diameter stress corrosion cracks in SG tubes. This should serve to evaluate the integrity of the SG tubes. The depth and amplitude of defects were used as parameters of the POD model. Hit (detection) and miss (no detection) binary data obtained from destructive and nondestructive inspection of cracked tubes were also used.     

A case study on detection and sizing of defects in steam generator tubes using eddy current testing

Eddy current testing (ECT) method is widely used to detect various types of defects occurring in nuclear steam generator tubes. Therefore, the reliability of its detection and sizing accuracy for defects should be validated. For this purpose, two tubes with defect signals were pulled from an operating steam generator and destructively examined. The defect type was a circumferential crack for one tube and an intergranular attack (IGA) for the other tube. The plus point coil probe showed a better capability to detect and size both a circumferential crack and a volumetric IGA than pancake and bobbin coil probe. The destructive results are correlated with the ECT results obtained during the in-service inspection.     

蒸汽发生器传热管胀管过渡段水浸超声检查技术

近年来,核电厂提出了对蒸汽发生器传热管胀管过渡段进行检查的要求。由于采用常规涡流检查技术在传热管的胀管过渡段存在盲区,因此开展了传热管胀管过渡段水浸超声检查技术研究,并开发出一套完整的检查技术。通过试验结果分析,证明该技术完全满足蒸汽发生器传热管胀管过渡段检查要求,能够对传统传热管涡流检查形成补充,同时也能够应用于其他薄壁小径管道的检查。      

Prototype EMAT system for tube inspection with guided ultrasonic waves

A prototype system with full computer support for ultrasonic inspection of ferritic tubes using guided waves is described. The ultrasonic waves are launched and received with the aid of electromagnetic acoustic transducers which are layed out as linear phased arrays. The array structure provides a good axial directivity for the transducers so that the probe can be positioned anywhere along the tube length sequentially transmitting ultrasonic pulses in the foreward and backward directions. While the probe is fixed at one axial position during inspection the tube length is measured by the system and flaws are detected from returning ultrasonic echos. Results of the inspection of tubes with natural flaws are given and the wavelength-spectrum of the ultrasonic mode used for the inspection is discussed with respect to flaw depth sizing.     

Detection and Mode Identification of Axial Cracks in the Steam Generator Tube of the Nuclear Power Plant Using Ultrasonic Guided Wave

For those people who are involved in NDE, there is a growing concern regarding the significant traveling distance of a guided wave in a structure, which ensures the inspection of a large area of the structure from a single location. A significant number of studies on the guided wave have therefore been made to apply the foregoing to a nondestructive evaluation in many different industries and resulted in an increase in the efficiency of practical guided wave inspection. Unlike the previous studies based mainly on the detection of circumferential flaws, this study is focused on the axial flaw detection in the steam generator tubes of Korean standard nuclear power plants by generating the guided wave by changing frequency and selecting the applicable mode from the dispersion curve for the steam generator tube calculated in this study, where the dispersion-based short-time Fourier transform (D-STFT) algorithm is used to enhance mode identification. In conclusion, the L (0,1) mode at 2.25MHz is found to be most sensitive in detecting axial flaws in a steam generator tube.     

Enhancements of eddy current testing techniques for quantitative nondestructive testing of key structural components of nuclear power plants

In this paper, studies on upgrade of eddy current testing (ECT) techniques for inspection of stress corrosion cracks (SCC) in key structural components of a nuclear power plant are reported. Access and scanning vehicle (robot), advanced probes for steam generator (SG) tube inspection, developments and evaluations of new ECT probes for welding joint, and ECT-based crack sizing technique are described, respectively. Based on these techniques, it is demonstrated that ECT can play as a supplement of ultrasonic testing (UT) for the quantitative inspection of welding zone. It is also proved in this work that new ECT sensors are efficient even for inspection of a stainless steel plate as thick as 15 mm.     

含缺陷Inconel 690蒸汽发生器传热管的强度及堵管准则研究

针对缺陷对传热管强度的影响以及传热管判废准则问题展开研究,研制了适用于小管径蒸汽发生器传热管极限载荷及爆破压测试的实验装置,对含体积型缺陷及面型缺陷的Inconel 690蒸汽发生器传热管进行了实验研究,并采用有限元法对极限载荷及爆破压进行了估算.在此基础上,研究了传热管的堵管准则,提出了两级评定方法.该评定方法可根据缺陷的深度、轴向及环向长度来综合评价.     

Potential steam generator tube rupture in the presence of severe accident thermal challenge and tube flaws due to foreign object wear

This study develops a methodology to assess the probability for the degraded PWR steam generator to rupture first in the reactor coolant pressure boundary, under severe accident conditions with counter-current natural circulating high temperature gas in the hot leg and SG tubes. The considered SG tube flaws are caused by foreign object wear, which in recent years has emerged as a major inservice degradation mechanism for the new generation tubing materials. The first step develops the statistical distributions for the flaw frequency, size, and the flaw location with respect to the tube length and the tube's tubesheet position, based on data of hundreds of flaws reported in numerous SG inservice inspection reports. The next step performs thermal-hydraulic analysis using the MELCOR code and recent CFD findings to predict the thermal challenge to the degraded tubes and the tube-to-tube difference in thermal response at the SG entrance. The final step applies the creep rupture models in the Monte Carlo random walk to test the potential for the degraded SG to rupture before the surge line. The mean and range of the SG tube rupture probability can be applied to estimate large early release frequency in probabilistic safety assessment.     

控制棒检测中对显示信号判定技术的改进

针对控制棒涡流检测中出现的一个显示信号不能充分判定,制作了含有人造裂纹的试验样件,采用在涡流检测线圈中加入磁芯的磁饱和线圈,消除控制棒上可能存在的磁性影响。并将3点对中的涡流探头组件改进成6点对中,改善了检测条件,提高了检测的可靠性。试验研究和检测结果表明,在现有技术的前提下,穿过式线圈及点式涡流线圈均可检测出周向和轴向裂纹;穿过式线圈不能区分单个或多个裂纹,多个点式线圈存在实现这种区分的可能性;轴向裂纹的涡流信号明显,但结构信号可能会影响对周向裂纹的判伤。     

Performance demonstration for an automated ultrasonic testing system for piping welds

This paper addresses the implementation of an automated ultrasonic testing (AUT) system qualification by performance demonstration (PD) as imposed by the ASME Boiler and Pressure Vessel Code Section XI. To improve the reliability of the ultrasonic testing results for nuclear power plant (NPP) components, almost all engineering codes related to NPP inspection require the ultrasonic inspection systems to be qualified by passing a PD examination. In this study, an AUT system developed to inspect pipe welding parts in NPPs is introduced. To acquire a Korean Performance Demonstration (KPD) qualification, the developed system had a KPD. System obtained the qualification for flaw detection, length, and depth sizing from KPD.     

基于声发射技术的蒸汽发生器传热管点腐蚀研究

蒸汽发生器传热管的腐蚀是影响核动力装置安全运行的重要问题之一,传热管的腐蚀以点腐蚀的危害最为常见。利用声发射仪器,对蒸汽发生器传热管进行腐蚀实验时的信号进行采集和分析,并对腐蚀点进行了准确定位。实验结果表明,传热管的点腐蚀经历3个阶段:发展期、平稳期和迅速发展期。声发射技术能比其它任何无损检测方法更早地发现传热管腐蚀损伤,可对蒸汽发生器的安全和运行情况进行在线实时监测,具有重要的意义。     

Structural integrity significance of round robin testing trials application to PISC III Action 3

Non-destructive examination (NDE) plays an important role in the safety and structural integrity of plant components. Hence there is a need to assess if NDE will be able to furnish correct information on the status of structures concerning presence, location and sizing of flaws. It is to provide an experimental basis for such knowledge that NDE round robin testing trials like the programme for the inspection of steel components (PISC) carried out.An approach for evaluating NDE results in their structural integrity significance is presented in this paper and applied to PISC III Action 3 for assessing capabilities and limitations of NDE procedures and techniques. PISC results are processed so as to allow analyses and comparisons in terms of indices related to safety margins.The influence of inspection variables (equipment related parameters and flaw categorizations) and the performance of single teams are evaluated and presented in their structural integrity significance.     

Evaluation of the burst characteristics for axial notches on SG tubings

Some events of steam generator tubes have been reported in some nuclear power plants around the world. Main causes of the leakage are from various types of corrosion in the steam generator (SG) tubing. Primary water stress corrosion cracking (PWSCC) of steam generator tubing have occurred in many tubes in Korean plants, and they were repaired using sleeves or plugs. In order to develop proper repair criteria, it is necessary to ascertain the leak behavior of the tubings. A high-pressure leak and burst testing system was manufactured. Various types of electro-discharged-machined (EDM) notches having different lengths were machined on the o.d. of test tubes to study SG tube behavior. Leak rate and ligament rupture pressure as well as the burst pressure were measured for the tubes at room temperature. Rupture pressure of the part through-wall defect tubes depends on the defect depth and length. Water flow rates after the rupture were independent of the flaw types; tubes having 20–60 mm long EDM notches showed similar flow rates regardless of the initial defect depth. A fast pressurization rate generated a lower burst pressure than the case of a slow pressurization.     

卧式蒸汽发生器传热管涡流检查系统研制与应用

核电厂卧式蒸汽发生器由于集流管深度大，传热管弯道多、弯曲半径小、管数多以及材料均匀特性差等特点，给传热管涡流检查带来难度。研发了一套卧式蒸汽发生器传热管涡流检测系统（C-SMART），包括机械定位驱动装置、控制系统和控制软件、涡流数据采集和分析软件等。系统具备快速精准定位、单管标定、高效等特点，实现对传热管涡流的自动检查，具有显著的经济和社会效益。      

Quantification of the reliability required of non-destructive inspection of PWR pressure vessels

This paper presents a method of quantifying the reliability required of non-destructive inspections of PWR pressure vessels. It gives a strategy for improving the effectiveness of ultrasonic non-destructive testing in assuring the integrity of a PWR vessel and allows targets of inspection reliability to be set in order to achieve the requisite level of vessel integrity. To do this the failure rate of PWR pressure vessels is predicted on the basis of a probabilistic fracture mechanics model. We use various models of the reliability of non-destructive inspection to discover the minimum level of reliability which is consistent with the desired integrity of the structure and to demonstrate how improvements can be made most effective.The reliability of inspection is usually modelled by a function giving the probability of leaving an unacceptable defect in the vessel. This function B(a) is really the “unreliability” of inspection and so 1 - B(a) gives the usual reliability. A reliable inspection is one which detects and correctly classifies defects according to some criterion usually based on size. A reliable inspection must use a technique which is intrinsically capable of detecting and sizing defects in the required size range and it must be reliably applied in practice.We find that, based on certain stated assumptions, that an inspection reliability of 80% of detecting and correctly sizing a defect of 15 mm through-wall extent yields a predicted failure rate of 10⁻⁷ per vessel year. The failure rate includes a frequency of a major accident such as a large loss of coolant (LOCA) of frequency 10⁻⁴ per vessel year. The predicted failure rate can be reduced to 10⁻⁸ per vessel year if the sizing accuracy of the technique is improved so that the chance of undersizing a 15 mm defect falls from 0.19 to about 0.01. However, the failure rate of the vessel is not predicted to decrease further with any subsequent improvement in sizing accuracy unless there is also an improvement in the asymptote of the reliability of inspection. This asymptote is due to factors beyond the capability of the technique such as, for example, human error.     

大亚湾核电站蒸汽发生器传热管的涡流检查

介绍了大亚湾核电站蒸汽恨生器传热管的涡流检查设备与技术,１号机组蒸汽发生器首闪在役检查的关键路径,技术改进及其检查结果。首次在役检查结果表明;３台蒸汽发生器没有发现传热管破损,保证了核电站的安全运行。     

核电厂围板螺栓超声检测技术研究

围板螺栓是核电厂堆内构件的关键连接部件,长期服役下可能产生辐照应力腐蚀裂纹（IASCC）等缺陷,有必要对其结构完整性进行无损检测。分析围板螺栓的结构特点和在役检查工况,开发针对外六角头结构螺栓的组合晶片超声检测方案,介绍探头设计选型原则和缺陷评定技术,确保良好的声场有效覆盖以及检测出螺栓不同部位的裂纹缺陷。通过对含缺陷试块的试验验证了超声检查工艺的可行性,结果表明该技术能够有效检测30%螺杆横截面当量的裂纹缺陷,信噪比可达12 dB以上,满足在役检查要求。      

Steam generator mock-up for assessment of inservice inspection technology

A steam generator mock-up has been assembled for round-robin (RR) studies of the effectiveness of currently practiced inservice inspection technology for detection of current-day flaws. The mock-up will also be used to evaluate emerging inspection technologies. The 3.66 m (12 ft)-tall mock-up contains 400 tube openings, each consisting of nine test sections that can be used to simulate current-day field-induced flaws and artifacts. Included in the mock-up are simulations of tube support plate (TSP) intersections and the tube sheet (TS). Cracks are present at the TSP, TS, and in the free span sections of the mock-up. For initial evaluation of the RR results, various eddy current methods, as well as multivariate models for data analysis techniques are being used to estimate the depth and length of defects in the mock-up. To ensure that the RR is carried out with procedures as close as possible to those implemented in the field, input was obtained from industry experts on the protocol and procedures to be used for the exercise. One initial assembly of the mock-up with a limited number of flaws and artifact has been completed and tested. A second completed configuration with additional flaw and artifacts simulations will be used for the round robin.     

核电站蒸汽发生器降质预防和在役检查

介绍了法国核电站蒸汽发生器在运行初期所发生的传热管降质现象,重点论述了大亚湾核电站１ 号机组蒸汽发生器对降质预防所采取的措施和在役检查,包括二回路水化学监控、泄漏率监测、传热管涡流检验、二次侧的机械清洗、清洁度检查和外来物取出等。实践证明,采取了上述降质预防措施和在役检查,对核电站的安全运行起到了重要作用     

蒸汽发生器传热管结垢厚度的涡流检测方法与应用

蒸汽发生器二回路中有较多的沉积物存在并危害传热管安全,利用涡流检测方法可以对传热管二次侧泥渣进行有效检测。通过模拟传热管结垢的不同厚度并进行实验,可获得厚度与幅值的对应关系。本文描述了对蒸汽发生器传热管结垢的检测方法及幅值与厚度的对应关系,为统计蒸汽发生器传热管外壁结垢情况提供了较为有效的参考基准量。