Acoustic leak detection for reactor coolant systems

An acoustic leak detection (ALD) facility has been constructed at Argonne National Laboratory (ANL), and tests have begun on five laboratory-grown cracks (three fatigue cracks and two thermal-fatigue cracks) and two field-induced intergranular stress-corrosion cracks (IGSCC). The acoustic characteristics of the IGSCC were established and the minimum leak rate that is detectable at a distance of 50 cm under laboratory conditions was identified to be 0.001 gal/min for a frequency window of 300–400 kHz. Cross-correlation techniques were also demonstrated for locating leaks and distinguishing among leak types. A comparison of acoustic spectra of the two leaking IGSCC at flow rates of 0.005 gal/min revealed nearly identical signals in the frequency range from 200 to 400 kHz. Below 200 kHz, the differences in geometry between the two cracks can be reflected in a signal difference. These data suggest that geometrical effects may be less significant at frequencies above 200 kHz. It appears that leak rates may be more reliably related to acoustic signals at higher frequencies.     

Development of Acoustic Leak Detection and Localization Methods for Inlet Piping of Fugen Nuclear Power Plant

The development work carried out on Fugen NPP is focused on detection of a small leakage on the reactor's inlet feeder pipes at an early stage by an acoustic leak detection method with usage of high-temperature resistant microphones. Specifically, the leak rate of 0.046 m³/h has been chosen as a target detection capability for this system. A cross-correlation technique has been studied for leak detection under low signal-noise ratios. The study shows that the sound diffusion on piping causes distortion of leak signals that results in their low correlation. A leak-location estimator and multi-channel correlation value, associated with estimated leak position, have been employed to detect such low-correlated leak signals. A method based on cross-correlation of signal spectral components has been proposed to deal with non-stationary leak signals. Joint-Time-Frequency-Analysis has been applied to analyze such signals, whilst a Wavelet decomposition technique has been used to extract their short-term spectral fluctuations. Since the spectral components are less affected by signal distortion, they provide higher correlation value and can be applied for leak detection under lower signal-noise ratios. The possibility of detecting and locating a small leakage by the methods proposed has been demonstrated by a number of simulation tests conducted on the Fugen NPP site.     

核电厂声学泄漏监测系统的设计和验证

介绍VVER-1000型核电厂声学泄漏监测系统的设计基准和功能,给出判断泄漏过程、确定泄漏量和泄漏位置的系统算法。建立主回路声模型结构图,计算得到环路背景噪声分布,并和实际机组上的试验结果进行对比。建立管道模型的试验台架,并对管道模型进行了试验验证。根据试验数据得到用于计算泄漏量大小和泄漏位置的相关系数。通过核电厂声学泄漏监测系统的设计和验证,为开发田湾核电厂1、2号机组在线的核电厂声学泄漏监测系统奠定了基础。      

The Influence of Dissolved Hydrogen on the Corrosion of Type 304 Stainless Steels Treated with Inhibitive Chemicals in High Temperature Pure Water

In order to assess the influence of dissolved hydrogen on the intergranular stress corrosion cracking (IGSCC) characteristics of Type 304 stainless steels treated with inhibitive chemicals, electrochemical corrosion potential (ECP) measurements and slow strain rate tensile (SSRT) tests were conducted in high temperature pure water. A number of thermally sensitized specimens were prepared and then pre-oxidized in a 288°C pure water environment with the presence of 300ppb dissolved oxygen for 360h. Most of the specimens were then separately treated with various inhibitive chemicals including powdered zirconium oxide (ZrO₂), powdered titanium oxide (TiO₂), and zirconyl nitrate [ZrO(NO₃)₂] via hydrothermal deposition at 150°C. Test environments with a dissolved oxygen concentration of 300ppb and various dissolved hydrogen concentrations at 288°C were created. Test results showed that the ECPs of the treated specimens were lower than that of the untreated one no matter what the dissolved hydrogen concentration was. In addition, IGSCC was observed on all specimens (treated or untreated) in all tested environments. However, the untreated specimen exhibited lower elongation, shorter failure time, and more secondary cracks on the lateral surfaces. It was therefore suggested that inhibitive chemicals such as ZrO₂, TiO₂, and ZrO(NO₂)₂ did provide a certain degree of enhancement in improving the mechanical behavior of the treated specimens and in prolonging IGSCC initiation times.     

An assessment of acoustic emission for nuclear pressure vessel monitoring

Recent research has greatly improved our understanding of the basic mechanisms of deformation and fracture that generate detectable acoustic emission signals in structural steels. A critical review of the application of acoustic emission (AE) to the fabrication, proof testing and in-service monitoring of nuclear pressure vessels is presented in the light of this improved understanding. The detectability of deformation and fracture processes in pressure vessel steels is discussed, and recommendations made for improving source location accuracy and the development of quantitative source assessment techniques.

Published data suggest that AE can make an important contribution to weld fabrication monitoring, and to the detection of defects in lower toughness materials during vessel proof testing. In high toughness materials, however, the signals generated during ductile crack growth may frequently be too weak for reliable detection. The feasibility of AE for continuous monitoring has not yet been adequately demonstrated because of high background noise levels and uncertainty about AE signal strengths from the defect growth processes that occur in service. In-service leak detection by AE shows considerable promise.

It is recommended that further tests are carried out with realistic defects, and under realistic conditions of loading (including thermal shock and fatigue) and of environment.     

Leak behavior of SCC degraded steam generator tubings of nuclear power plant

A forced outage due to a steam generator tube leak in a Korean nuclear power plant has been reported [Kim, J.S., Hwang, S.S., et al., 1999. KAERI Internal Report (Korean). Destructive analysis on pulled tubes from Ulchin unit 1. Korea Atomic Energy Research Institute]. Primary water stress corrosion cracking has occurred in many tubes in the plant, and they were repaired using sleeves or plugs. In order to develop proper repair criteria, it is necessary to understand the leak behavior of the tubes containing stress corrosion cracks. Cracked specimens were prepared using a room temperature cracking technique, and the leak rates and burst pressures of the degraded tubes were determined both at room temperature and at a high temperature. Some tubes with 100% through wall cracks did not show a leakage at 10.8 MPa, which is the typical pressure difference of the pressurized water reactors (PWRs) during a normal operation. In some tests, the leak rates of the tubes increased with time at a constant pressure. In a high temperature pressure test at 282 °C one specimen showed a very small leakage at 18.6 MPa, which stopped after a small increase in the test pressure. Because stress corrosion cracks can develop at relatively low stresses, even 100% through wall cracks can be so tight that they will not leak at a normal operating pressure.     

基于泄漏监测数据综合的核电厂反应堆一回路压力边界泄漏诊断技术研究

泄漏监测系统用于监测反应堆冷却剂系统压力边界（RCPB）完整性,也是破前漏（LBB）技术应用的先决条件。泄漏综合诊断是泄漏监测系统的核心功能。本文从系统可用性、数据可靠性、单仪表泄漏报警、泄漏综合诊断、报警响应策略、泄漏率定期试验自动计算6个方面构建泄漏综合诊断技术方案。泄漏监测系统的灵敏度和准确性是泄漏综合诊断技术的重要性能指标,也是监测RCPB完整性和LBB技术应用的关键要求。先确定触发单仪表报警的保守阈值范围,以保证检测的灵敏度,再经泄漏综合诊断技术方案复核并调整有效单仪表报警阈值,以保证报警的准确性。通过理论计算、数据分析、多信号的一致性判断,泄漏监测系统能及时准确地诊断出泄漏情况,充分应用智能化泄漏诊断技术,减少运行人员复核报警工作量。      

Development of microphone leak detection technology on Fugen NPP

A method of leak detection, based on high-temperature resistant microphones, was originally developed in JNC to detect leakages with flow rates from 1 m³/h to 500m³/h. The development performed on Fugen is focused on detection of a small leakage at an early stage. Specifically, for the inlet feeder pipes the leak rate of 0.2gpm (0.046m³/h) has been chosen as a target detection capability. Evaluation of detection sensitivity was carried out in order to check the capability of the method to satisfy this requirement. The possibility of detecting and locating a small leakage has been demonstrated through the research.     

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.     

Low-cycle fatigue properties of Zircaloy-2 cladding

The low-cycle fatigue properties of specimens of recrystallized Zircaloy-2 cladding have been investigated using a bend-testing rig at 20, 250, 300, 350 and 400°C. The results followed the Coffin-Manson law for low-cycle fatigue at all temperatures. Irradiated specimens were tested at 20 and 300°C. At 20°C the life of irradiated specimens was shorter than that of unirradiated specimens, whilst at 300°C the irradiated specimens fall on the same Coffin-Manson plot as the unirradiated specimens. Generally the number of cycles to failure in the present investigation was a factor of three smaller than in other investigations using push-pull testing machines. Fracture surfaces were investigated in a scanning electron microscope in order to determine the characteristic features of low-cycle fatigue failures. Twins found along the path of the fatigue cracks were explained as being a consequence of the cracks rather than as promoters of crack propagation.     

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.     

The development of techniques for the surveillance of LMFBRs

The development of surveillance techniques of LMFBRs is determined by the interaction of three factors: the specification of requirements, improvements in technique and the physical analysis of the processes involved. The specification of requirements, which sets the structure for the discussion, is mainly concerned with public safety. Two main divisions are identified: those concerned with thermal events in the nuclear core and those concerned directly or indirectly with the mechanical integrity of components. The necessary developments are then discussed in terms of the signal analysis techniques to anticipate various modes of failures. The importance of an adequate understanding of the failure mode is emphasised in optimising the surveillance technique.

Core surveillance may be achieved by monitoring individual sub-assemblies or by monitoring bulk conditions. The important features of sub-assembly monitoring are discussed and the advantages of temperature analysis explained. The specification of the temperature-monitoring systems is identified and the conflicting requirements for the reactor sensor discussed, viz adequate band width as against a robust and reliable construction. A theoretical treatment using Monte Carlo techniques allows a full examination of the choice of method of temperature analysis. This shows that, although a filtered rms value has been the preferred choice for detecting either local blockage or sodium boiling, it may be possible to distinguish the temperature signals of blockages from those of power gradients by an amplitude probability density plot. The advantages of acoustic monitoring using the noise of boiling sodium to detect overheating, leading to core damage, are examined. An important consideration is the thermal-acoustic process of sodium boiling, and evidence is submitted from a range of out-of-pile experiments involving local sub-cooled boiling and bulk boiling in discussing the merits of pulse analysis and power spectral density techniques. An important factor in discriminating background from signal is the extent of cavitation in reactor components. Experiments are mentioned in which pulse techniques have been used to locate boiling sources by spatial correlation. The interpretation of reactor signals requires a detailed knowledge of the transmission of acoustic waves in reactor pools and structures and the effect of gas bubbles. Measurements in PFR and sodium loops have helped to lead to a more quantitative assessment of the sensitivity of the acoustic techniques.

Structural integrity depends on detecting failure modes, particularly those arising from crack propagation. Manufacturing defects or pre-existing cracks may be identified by ultrasonic inspection or by stress-wave emission. On-line monitoring for stress-loaded cracks by a stress-wave emission is seen as intrinsically difficult because of low signal strength and high attenuation but initial experiments have indicated possibilities for detecting stress-corrosion cracking. Mechanical failure from fatigue may be anticipated from a understanding of the vibrational modes of the sodium and its coupling with the structure. A one-eighth scale model of a LMFBR design has recently demonstrated the likely vibrational modes. A major handicap in supervising mechanical operation in sodium systems is the opacity of the sodium. Visualisation techniques of the major parts of the core structure are being developed. An important aspect is the study of the information processing required to present an image easy for the reactor operator to understand. Advances may be made using transform methods to improve object boundaries by modifying the spatial frequencies of the display or record.     

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.     

Influence of δ phase precipitation on the stress corrosion cracking resistance of alloy 718 in PWR primary water

The negative influence of δ phase on the intergranular stress corrosion cracking (IGSCC) resistance of alloy 718 is commonly taken for granted. In addition, δ phase formed at low temperature (about 1023 K) do not present the same characteristics than the one formed at higher temperatures (from 1173 to 1273 K). The aim of the present study is then to understand how δ phase precipitation could enhance crack initiation in alloy 718, whatever the form of δ phase is. For that purpose, several heat treatments leading to δ phase precipitation were realized on two alloy 718 heats, one sensitive to IGSCC and the second not. Specific slow strain rate tensile tests carried out on thin tensile specimens in simulated PWR primary medium at 633 K conclusively prove that δ phase has no effect on the intrinsic sensitivity to intergranular crack initiation of tested heats.     

Methods for leak detection for KWU pressurized and boiling water reactors

Leakage monitoring is an essential criterion to rule out the possibility of double ended pipe rupture in the primary coolant system. Subcritical cracks can be detecred with a considerable margin before they extend to critical crack lengths resulting in spontaneous failure. In those KWU PWRs which went into operation recently, a Leakage Monitoring System was installed that is based on thermodynamic analysis. It utilizes the following measured parameters: dew point temperature, accumulated condensate inside aircoolers, air temperature, sump water level, gully monitoring. In KWU's BWRs, although the measurement concept has to be slightly changed because of a different design of buildings and components, the same instrumentation will be used. Besides this installed monitoring system, different approaches like acoustic leak detection systems or the application of moisture sensitive instrumentation have been considered. Both systems have been successfully tested.     

Essential results of analyses accompanying the leak rate experiments E22 at HDR

During phase III of the HDR Safety Programme (HDR: decommissioned overheated steam reactor in Karlstein, Germany), experiments were performed in test group E22 on small-bore austenitic straight piping and on pipe elbows and branches containing through-wall cracks. The main aim was the determination of crack opening and leak rate behaviour for the cracked components under almost operational pressure and temperature loading conditions, especially including transient bending moments. In addition to machined slits, naturally grown fatigue cracks were also considered to cover the leakage behaviour. The experiments were accompanied by calculations, mainly performed by GRS. The paper describes the most important aspects and the essential results of the calculations and analysis. The main outcome was that the crack opening and initiation of crack growth can be described with the finite element techniques applied with sufficient accuracy. However, the qualification of the leak rate models could not be completed succssfully, and therefore more sophisticated experiments of this kind are needed.     

Evaluation of stainless steel pipe cracking: Causes and fixes

This paper discusses (1) studies of impurity effects on susceptibility to intergranular stress corrosion cracking (IGSCC), (2) intergranular crack growth rate measurements, (3) finite-element studies of the residual stresses produced by induction heating stress improvement (IHSI) and the addition of weld overlays to flawed piping, (4) leak-before-break analyses of piping with 360° part-through cracks, and (5) parametric studies on the effect of through-wall residual stresses on intergranular crack growth behavior in large diameter piping weldments. The studies on the effect of impurities on IGSCC of Type 304 stainless steel show a strong synergistic interaction between dissolved oxygen and impurity concentration of the water. Low carbon stainless steel (Type 316NG) appear resistant to IGSCC even in impurity environments. However, they can become susceptible to transgranular SCC with low levels of sulfate or chloride present in the environment. The finite-element calculations show that IHSI and the weld overlay produce compressive residual stresses on the inner surface, and that the stresses at the crack tip remain compressive under design loads at least for shallow cracks.     

快堆蒸汽发生器声学泄漏探测技术的研究──Ⅰ . 系统信号特性

根据在实验Ｎａ回路上的实验结果，分析了快堆电站蒸汽发生器中Ｎａ噪声、Ｈ２Ｏ噪声、泵和风机噪声等泄漏信号和背景噪声的时域、频域特性，对泄漏噪声的提取方法提出了设想，为提高泄漏探测系统的灵敏度提供了实验依据。     

Acoustic emission measurements during welding of a 250 mm testplate

The capability of the acoustic emission monitoring system of Prine (Gard Corporation, Illinois) should be demonstrated for a multi-layer submerged arc weld of a 250 mm test plate. The question had to be answered, if the measuring system developed by Prine and the evaluation models are suitable for production control of multi-layer submerged-arc welding, respectively if defect detection with interpretation of defect type, classification of size and triangulation is possible.The Prine system works with regard to electronic equipment as well as the interpretation quite satisfactory. The agreement of the defect indication with the defects visible during welding is very well. Hot and cold cracks as well as slag inclusions were indicated and normally evaluated correctly. Pores were not detected during welding.Partly a lot of low signals were realized, especially if residual stresses (closure seam) were expected. These small indications at the sensitivity level of the system could not be precisely explained. A clear evaluation of the results will be possible after completing nondestructive and destructive testing of the weld.     

Current in-service inspections of austenitic stainless steel and dissimilar metal welds in light water nuclear power plants

A tendency towards growing requirements for the inspection of austenitic piping can be observed in several countries. In Germany the revised KTA rule demands the UT inspection of austenitic and dissimilar metal welds in piping with diameters of 200 mm or more.On the basis of experience gained from austenitic piping with integranular stress corrosion cracking (IGSCC), longitudinal waves and mode conversion techniques are used. Depending on the geometry, material and grain orientation, spurious signals can be observed which require additional evaluation or analysis measurements.A promising new technique is based on horizontally polarized shear (SH) waves generated by electromagnetic acoustic transducers (EMATs). Investigations in the laboratory and field inspections showed that SH waves are well suited for the detection of longitudinal flaws, especially where the weld can be examined from one side only.For the complete solution of a given inspection problem SH waves can be combined with well-known standard techniques in order to provide redundant information for the characterization and sizing of indications.The investigation of possibilities of SH waves showed that the problem of cast austenitic steel inspection might not be solved using this technique. However, measurements using low frequency UT transducers showed promising results.