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。     

Application of eddy current inversion technique to the sizing of defects in Inconel welds with rough surfaces

This paper evaluates the applicability of eddy current inversion techniques to the sizing of defects in Inconel welds with rough surfaces. For this purpose, a plate Inconel weld specimen, which models the welding of a stub tube in a boiling water nuclear reactor is fabricated, and artificial notches machined into the specimen. Eddy current inspections using six different eddy current probes are conducted and efficiencies were evaluated for the six probes for weld inspection. It is revealed that if suitable probes are applied, an Inconel weld does not cause large noise levels during eddy current inspections even though the surface of the weld is rough. Finally, reconstruction of the notches is performed using eddy current signals measured using the uniform eddy current probe that showed the best results among the six probes in this study. A simplified configuration is proposed in order to consider the complicated configuration of the welded specimen in numerical simulations. While reconstructed profiles of the notches are slightly larger than the true profiles, quite good agreements are obtained in spite of the simple approximation of the configuration, which reveals that eddy current testing would be an efficient non-destructive testing method for the sizing of defects in Inconel welds.     

An experimental result on flaw sizing of steam generator tube at nuclear power plants using ultrasonic testing

A steam generator at a nuclear power plant consists of thousands of thin tubes, and is a highly important component in operation. Also, steam generator tubes play a critical role in maintaining pressure boundaries of the primary and secondary sides, and can be easily damaged due to operation conditions caused by high temperature and pressure. Therefore, considerable amount of efforts are being committed to evaluating structural integrity of steam generators during in-service inspection. Eddy current testing is the commonly used inspection technique to evaluate a steam generator tube's integrity, but it has limitations in accurately sizing flaws due to the nature of the technique which determines size based on the entire volume of a flaw. In this study, experiments were performed by using ultrasonic testing instead of eddy current testing for the inspection of steam generator tubes to detect various kinds of flaws and to see if the detected flaws can be sized accurately. Consequently, the ultrasonic testing technique could detect various types of flaw, and the flaw sizing results were reliable in length and depth.     

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.     

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.     

Qualification of the LF-eddy current technique for the inspection of stainless steel cladding and applications on the reactor pressure vessel

As part of the re-inspection of the reactor pressure vessel of the nuclear power plant, the low-frequency-eddy current technique was implemented during the 1995 outage. Since then, this inspection technique and the testing equipment have seen steady further development. Therefore, optimization of the entire testing system, including qualification based on the 1995 results, was conducted. The eddy current testing system was designed as a ten-channel test system with sensors having separate transmitter and receiver coils. The first qualification of the testing technique and sensors was performed using a single-channel system; a second qualification was then carried out using the new testing electronics. The sensor design allows for a simultaneous detection of surface and subsurface flaws. This assumes that testing is performed simultaneously using four frequencies. Data analysis and evaluation are performed using a digital multi-frequency regression analysis technique The detection limits determined using this technique led to the definition of the following recording limits for testing in which the required signal-to-noise ratio of 6 dB was reliably observed.

• Detection of surface connected longitudinal and transverse flaws:

• notch, 3 mm deep and 10 mm long, for weave bead cladding;

• notch, 2 mm deep and 20 mm long, for strip weld cladding.

• Detection of embedded planar longitudinal and transverse flaws:

• ligament of 7 mm for 8 mm clad thickness and 3 mm;

• ligament for 4 mm clad thickness, notch starting at the carbon steel base material with a length of 20 mm.

• Detection of embedded volumetric longitudinal and transverse flaws:

• 3 mm diameter side-drilled hole (SDH) for 8 mm clad thickness; ligament, 4 mm. For 4 mm clad thickness: diameter, 2 mm SDH; ligament, 2 mm. All SDHs are 55 mm deep.

Evaluation of stress corrosion cracking as a function of its resistance to eddy currents

This study discusses the equivalent conductivity, the equivalent width, and the equivalent resistance of stress corrosion cracks from the viewpoint of eddy current testing. Four artificial stress corrosion cracks were prepared for this study, and their eddy current signals were gathered using two absolute pancake probes and two differential type plus point probes. Then their numerical models were evaluated using finite element simulations on the basis of the measured eddy current signals and their profiles revealed by destructive tests. The results of this study revealed that whereas the equivalent conductivity and the equivalent width depend on the exciting frequency utilized, the equivalent resistance of a crack has much less dependency, which agrees well with an earlier report. This study also revealed that the resistance of a crack depends on probe utilized. Larger probes tend to lead to smaller crack resistance. Pancake type probes tend to lead to larger crack resistance than plus point probes. Analyzing the results together with earlier reports indicates that cracks with a large equivalent conductivity tend to have large equivalent width, and supports the validity of assuming the minimum resistance of a stress corrosion crack whereas considering the conductivity and the width individually would not be viable.     

Evaluation of the electromagnetic characteristics of type 316L stainless steel welds from the viewpoint of eddy current inspections

This study evaluated the electromagnetic characteristics of austenitic stainless steel welds from the viewpoint of eddy current testing. Seven welded plate specimens, which were welded using JIS Z3221:2010 YS316L welding metals, were prepared. Two welding metals and several welding conditions were adopted to discuss the generality. The results of eddy current examination of the specimens using three different types of eddy current probes, that is, an absolute pancake probe, a differential plus-point probe, and a uniform eddy current probe, confirmed that the welds cause a large noise especially when the absolute pancake probe was used. The analysis of the signals through finite element simulations revealed that the magnetic property of the welds is not negligible from the viewpoint of eddy current testing. Complementary experiments were conducted using a vibrating sample magnetometer to validate the results.     

Caution when applying eddy current inversion to stress corrosion cracking

This study evaluates the applicability of computer-aided eddy current inversion techniques to the profile evaluation of stress corrosion cracking in Inconel welds. Welded plate specimens, which model head penetration welds of pressurized water reactors, are fabricated; notches and stress corrosion cracks are artificially introduced into the weld metal of the specimens. Eddy current inspections are performed using a uniform eddy current probe driven at frequencies of 10 and 40 kHz. Since weld noise is observed uniformly along the weld line, a simple signal processing is applied to eliminate it. First, the artificial notches are reconstructed and good agreements between reconstructed and true profiles are provided, which demonstrates that the computer-aided eddy current inversion technique can deal with defects in welds. Then, numerical simulations are performed to evaluate the profiles of the stress corrosion cracks. In the numerical simulations, the stress corrosion cracks are modeled as a conductive region with a fixed width of 0.3 mm. The cross-sectional profiles of the cracks are reconstructed from measured eddy current signals directly above and along a crack. Although eddy current signals calculated from the reconstructed profiles agree well with measured ones, the true profiles revealed by destructive testing are found to be very different from the reconstructed ones. Whereas the most plausible reason for the difference is the unexpectedly volumetric profile of the stress corrosion cracks, this study has revealed that computer-aided eddy current inversion techniques that have been used to consider cracks in thin structures would not at this point be directly applicable to those in thick structures. It is also important to know in advance those crack features that can adversely impact accurate crack sizing including whether a detected crack is volumetric or not, namely there are many parallel cracks in a cluster or not.     

Development of a diagnostic expert system for eddy current data analysis using applied artificial intelligence methods

A diagnostic expert system that integrates database management methods, artificial neural networks, and decision-making using fuzzy logic has been developed for the automation of steam generator eddy current test (ECT) data analysis. The new system, known as EDDYAI, considers the following key issues: (1) digital eddy current test data calibration, compression, and representation; (2) development of robust neural networks with low probability of misclassification for flaw depth estimation; (3) flaw detection using fuzzy logic; (4) development of an expert system for database management, compilation of a trained neural network library, and a decision module; and (5) evaluation of the integrated approach using eddy current data. The implementation to field test data includes the selection of proper feature vectors for ECT data analysis, development of a methodology for large eddy current database management, artificial neural networks for flaw depth estimation, and a fuzzy logic decision algorithm for flaw detection. A large eddy current inspection database from the Electric Power Research Institute NDE Center is being utilized in this research towards the development of an expert system for steam generator tube diagnosis. The integration of ECT data pre-processing as part of the data management, fuzzy logic flaw detection technique, and tube defect parameter estimation using artificial neural networks are the fundamental contributions of this research.     

应力作用对电涡流法测试碳纤维复合材料位移的影响机理

电涡流法是测试碳纤维复合材料（CFRP, carbon fiber reinforced composite）位移的常用技术。CFRP通常工作在较高应力水平,本研究针对应力作用下电涡流法测试CFRP位移开展了理论与实验研究。建立了应力作用下电涡流传感器测试CFRP位移的理论模型,分析了应力对该测试方法的影响规律。建立了原理性实验测试装置,测试了不同应力水平下电涡流法测试CFRP位移的传感器“输入-输出”曲线,获得了应力水平对传感器灵敏度的影响规律。通过理论与实验结果综合比较分析,证实了理论模型的有效性,并分析了应力对该测试方法的影响机理。     

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。      

An efficient modeling of fine air-gaps in tokamak in-vessel components for electromagnetic analyses

A simple and efficient modeling technique is presented for a proper analysis of complicated eddy current flows in conducting structures with fine air gaps. It is based on the idea of replacing a slit with the decoupled boundary of finite elements. The viability and efficacy of the technique is demonstrated in a simple problem. Application of the method to electromagnetic load analyses during plasma disruptions in ITER has been successfully carried out without sacrificing computational resources and speed. This shows the proposed method is applicable to a practical system with complicated geometrical structures.     

Detection of embedded fatigue cracks in Inconel weld overlay and the evaluation of the minimum thickness of the weld overlay using eddy current testing

This study evaluates the applicability of eddy current testing to the detection and sizing of fatigue cracks embedded in Inconel weld overlays. Welded plate specimens, which model head penetration welds and their weld overlays, are fabricated, and fatigue cracks are artificially introduced into the specimens. Eddy current inspections are performed using a uniform eddy current probe driven with 10 kHz, and all of the fatigue cracks are detected with clear signals. Subsequent numerical inversions estimate that the minimum thicknesses of the weld overlays are 1.47, 2.17, and 2.23 mm, whereas true thicknesses revealed through destructive testing are 1.51, 3.25, and 2.10 mm, respectively. Thicknesses are also evaluated using potential drop and ultrasonic testing methods; the results demonstrate that eddy current testing is the most efficient of the three methods.     

Design and Development of Rogowski Coil Sensors for Eddy Currents Measurement on Toroidal Vessel

In tokamak, eddy currents are produced due to change in plasma positions during plasma instabilities that result into generation of electromagnetic forces on interaction with the induced currents. Measurement of this current is essential to design a mechanical structure that can withstand this force. Principle objective of this paper is the development of Rogowski coil sensor to measure eddy currents on a toroidal vessel. The paper presents an elaborative and practical construction technique of a Rogowski coil. The calibration method for the Rogowski coil is also presented. Rogowski coils as an eddy currents diagnostics are tested and experiments to measure induced currents on the toroidal vessel are performed using the coils. Experimental values of eddy currents are compared with the ANSYS simulation results.     

In-service inspection of PWR steam generators by eddy currents; French achievements in defect detection and evaluation

In a nuclear plant the steam generator tubes must be efficiently inspected. The highest possible detection sensitivity is necessary to get a clear decision for plugging the rejected tubes. For this reason multi-frequency eddy current examinations have been developed at the Commissariat à l'Energie Atomique and industrialized by Intercontrôle. Defects are characterised case by case as the needs arose on the site: sludge height determination; measurement of tube deformation under pressure of oxides; determination of tube degradation due to the pressure of foreign bodies; detection of cracks under coatings; detections of cracks at the end of flanging. Problems unsolved by standard probes were dealt with by rotating-head machines. This report sums up part of the research undertaken in the field of eddy current testing.     

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.     

Numerical Simulation Approaches to Evaluating the Electromagnetic Loads on the EAST Vacuum Vessel

Numerical simulation approaches are developed to compute the electromagnetic forces on the EAST vacuum vessel during major disruptions and vertical displacement events, with the halo current also considered. The finite element model built with ANSYS includes the vacuum vessel, the plasma facing components and their support structure, and the toroidal and poloidal field coils. The numerical methods are explained to convince of its validity. The eddy current induced by the magnetic flux variation and the conducting current caused by the halo current are also presented for discussion. The electromagnetic forces resulting from the numerical simulation are proven to be useful for structure design optimization. Similar methods can be applied in the upgrades of the EAST device.     

An elastoplastic phase-field model for the evolution of hydride precipitation in zirconium. Part II: Specimen with flaws

An elastoplastic phase-field model, described in Part I, was applied to bulk materials containing flaws such as sharp cracks and blunt notches. An additional set of long range order parameters, namely, stress-free strains for flaws, was introduced. The nucleation and growth of hydrides near a void or a crack were simulated by the proposed elastoplastic phase-field model. The effects of notch root radius, hydrogen concentration in solid solution, yield stress of the matrix and the level of externally applied stress on hydride morphology around flaws were studied. It is demonstrated that parameters such as the distribution of the tensile stress component perpendicular to the hydride platelet normal may be closely monitored during hydride growth near a flaw with or without externally applied stresses. Combined with a fracture criterion and real experimental data, the model is capable of predicting the rate and morphology of hydride precipitation, and crack initiation near flaws.     

Secondary side corrosion in steam generator tubes: lessons learned in France from the in-service inspection results

Non-destructive testing (NDT) has proved to be very important in the maintenance of steam generator tubing. This is particularly true in the case of secondary side corrosion, because this type of degradation leads to various morphologies which are often complex (intergrranular attack) (IGA), intergranular stress corrosion cracking (IGSCC), or a mixture of both. Their detection and characterization by the usual NDT techniques have been achieved through numerous laboratory studies, which were conducted in order to determine the performance and limitations of NDT. Pulled tube examination in a hot laboratory was very valuable, for both NDT and fracture mechanics aspects. The eddy current bobbin coil probe, used for multipurpose inspection of tubes, allows the detection of IGA-SCC at the tube support plate elevation. In France, the use of rotating probes is not required for that type of degradation, since the repair criterion is based on bobbin coil results only. The bobbin coil is also used for detection of IGSCC occurring in free spans, within sludge deposits. The eddy current rotating probe allows, in that case, characterization of main cracks. Concerning the outer diameter initiated circumferential cracks which occur at the top of the tube sheet, only the rotating probe is used. An ultrasonic (UT) inspection was performed several times, in order to obtain information on UT capabilities. The goal of tube inspection is obviously knowledge of the status of steam generators, but also to follow up degradations and to estimate their revolution, and to verify the beneficial effect of some corrective measures, e.g. boric acid injection.