New applications using phased array techniques

In general, the application of phased array techniques used to be limited to heavy components with large wall thicknesses, such as those in the nuclear power industry. With the improvement of the phased array equipment, including phased array search units, other application areas are now accessible for the phased array inspection technique, e.g. the inspection of turbine blade roots, weld inspection with a wall thickness ranging from 12 to 40 mm, inspection of aircraft components, inspection of spot welds and the inspection of concrete building components. The objective for the use of phased array techniques has not significantly changed since their first application, e.g. instant adjustment of the sound beam to the geometry of the test object by steering incidence angle, skew angle and/or sound field focusing. Because some new phased array technique applications are still in the experimental (laboratory) stage, this article will focus on some examples for practical, real-weld applications.     

Availability study of a phased array ultrasonic technique

A phased array ultrasonic testing method is expected to be applicable to nuclear power plants for electrical ultrasonic beam control and real time B-scope image display. The availability of the phased array ultrasonic technique has been investigated and it has been verified that the sound beam is electrically steered to the intended angle and is electrically focused at the intended position. From the results of experiments, the dynamic aperture sizing technique was established as a practical method and this technique was demonstrated for artificial flaws.     

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.     

Development of an ultra sonic phased array system for nondestructive tests of nuclear power plant components

A phased array ultrasonic inspection (PAULI) system is being developed to obtain electronically scanned ultrasonic images of the inside of nuclear power plant components for nondestructive evaluation. The development strategy of PAULI system was the modification of a medical ultrasound imaging system that had 64 individual transceiver channels. Optimization of array transducers has been also pursued based on the systematic investigation of the radiation beam field simulated by the use of the boundary diffraction wave models. 7.5 MHz phased array transducers was, then, fabricated and tested with the carbon steel specimen having side-drilled holes. For the nondestructive tests on power plant component, a sample mockup of turbine blade root with EDM notches was fabricated and the detection capability was demonstrated. The developed system can provide electronically scanned ultrasonic images in real time fashion and greatly enhance the efficiency and reliability in the flaw detection and location in comparison with the classical ultrasonic testing (UT) using A-scan signals. For the flaw classification, the analysis of the electronically scanned ultrasonic images was not sufficient at this moment but analysis of features obtained from A-scan signals of flaws at the various steering angles showed the potential capability.     

Application of advanced ultrasonic test techniques coupled with fatigue and leakage monitoring to assure integrity of BWR feedwater nozzles

As a result of feedwater nozzle cracking observed in Boiling Water Reactor (BWR) plants, several design modifications were implemented to eliminate the thermal cycling that led to crack initiation. BWR plants with these design changes have successfully operated for over ten years without any recurrence of cracking. To provide further assurance of this, the U.S. Nuclear Regulatory Commission (NRC) issued NUREG-0619, which established periodic ultrasonic testing (UT) and liquid penetration testing (PT) requirements. While these inspections are useful in confirming structural integrity, they are time consuming and can lead to significant radiation exposure to plant personnel. In particular, the PT requirement poses problems since it is difficult to perform the inspections with the feedwater sparger in place and also leads to additional personnel exposure. Clearly, an inspection and monitoring program that eliminates the PT examination and still verifies the absence of surface cracking would be extremely valuable in limiting costs as well as radiation exposure. This paper describes a program involving the application of advanced UT techniques coupled with fatigue and leakage monitoring to assure integrity of BWR feedwater nozzles. The inspection methods include: (1) scanning with optimized transducers and techniques from the outside vessel wall surface to inspect the nozzle inner radius region, and (2) scanning from the nozzle forging outside-diameter to inspect the nozzle bore region. Methods of analyzing the data using 3-D graphics displays have been developed that show crack location, size, and maximum depth of penetration into the nozzle inner surface. These techniques have been developed to the point where they are now considered a reliable alternative to the liquid penetrant requirements of NUREG-0619. An important supplement to the UT program is the use of automated fatigue, leakage and crack growth monitoring to verify the absence of cracking. This approach provides for a continuous assessment of the integrity of the nozzle structure by tracking the actual fatigue duty, measuring thermal sleeve bypass leakage and performing crack growth predictions based on actual thermal duty. Collectively, the monitoring and inspection program provides technically sound assurance of nozzle integrity and a firm basis for plant operational planning.     

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.     

A model for predicting vessel failure probabilities including the effects of service inspection and flaw sizing errors

A numerical approach has been developed to predict the probability that a fabrication flaw in a reactor pressure vessel will extend by fatigue crack growth mechanisms and become a through-wall flaw. The fracture mechanics model treats the size of the flaw, the location of the flaw, and the parameters governing the fatigue crack growth rates as stochastic variables that are described by histograms that represent their statistical distributions. A latin hypercube approach forms the basis for efficient numerical calculations of vessel failure probabilities, in particular for those cases having very low probabilities that are not readily calculated by use of more conventional Monte Carlo simulations. A second aspect of the vessel failure model evaluates the benefits of in-service inspections at prescribed inspection time intervals and with prescribed nondestructive examination capabilities (probability of detection as a function of flaw size). A third aspect of the paper evaluates flaw sizing accuracy, and the impacts of flaw acceptance criteria. For representative values of flaw detection probability, flaw sizing errors, and flaw acceptance criteria, detection capability is the most limiting factor with regard to the ability of the in-service inspections to reduce leak probabilities. However, gross sizing errors or significant relaxations of current flaw acceptance standards could negate the benefits of outstanding probability of detection capabilities.     

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.     

Integration of nondestructive examination reliability and fracture mechanics

A multi-year program at the Pacific Northwest Laboratory is in progress to determine the reliability of ultrasonic ISI that is performed on light-water reactor primary systems, using probabilistic fracture mechanics (FM) analysis to determine the impact of NDE unreliability on system safety, and to evaluate advanced ultrasonic techniques. This paper is a review of the last year's highlights. Emphasis is placed upon the results of a pipe inspection round robin, advanced technique evaluation, joint study with Westinghouse, qualification document, underclad crack detection sizing studies, and a FM analysis using the PRAISE code for studying inspection parameters.     

Improved ultrasonic testing by phased array technique and its application

The phased array ultrasonic testing method is available for beam steering and focusing, and then the focused beam divergence angle can be narrowed and the sound level can be intensified at the focused region. In the present study, two examples of phased array application which are the linear scanning mode operation and the sector scanning mode operation, were conducted in order to establish the availability of this technique as an actual NDT technique. As the result of the experiments, it was recognized that both types of scanning mode had the capability for defect detection and successful estimation of the defect size. Furthermore, it was well realized that the real time B-scope display without mechanical scanning was effective to reduce the operating time of inspection.     

反应堆压力容器在役检查规范特性值与超声波检测数据分析方法之间的相互关系

对法国RSEM规范中的记录阈值、规范化阈值和验收标准之间的相互关系以及超声波检测中使用的波幅法和尖端衍射法显示定量技术之间的相互关系进行了详细的分析。明确界定了核电站反应堆压力容器役前和在役超声波检查中不同的规范特性值使用合适的数据分析和缺陷验收处理办法。该方法中采用的原则亦可应用于一回路其它部件役前和在役检查的数据分析和缺陷验收处理。     

Online monitoring method using Equipotential Switching Direct Current potential drop for piping wall loss by flow accelerated corrosion

The flow accelerated corrosion (FAC) phenomenon persistently impacts plant reliability and personnel safety. We have shown that Equipotential Switching Direct Current Potential Drop (ES-DCPD) can be employed to detect piping wall loss induced by FAC. It has been demonstrated to have sufficient sensitivity to cover both long and short lengths of piping. Based on this, new FAC screening and inspection approaches have been developed. For example, resolution of ES-DCPD can be adjusted according to its monitoring purpose. The developed method shows good integrity during long test periods. It also shows good reproducibility. The Seoul National University FAC Accelerated Simulation Loop (SFASL) has been constructed for ES-DCPD demonstration purposes. During one demonstration, the piping wall was thinned by 23.7% through FAC for a 13,000 min test period. In addition to the ES-DCPD method, ultrasonic technique (UT) has been applied to SFASL for verification while water chemistry was continually monitored and controlled using electrochemical sensors. Developed electrochemical sensors showed accurate and stable water conditions in the SFASL during the test period. The ES-DCPD results were also theoretically predicted by the Sanchez-Caldera's model. The UT, however, failed to detect thinning because of its localized characteristics. Online UT that covers only local areas cannot assure the detection of wall loss.     

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.     

Steam generator small bore piping socket weld inspection using the phased array ultrasonic technique

As nuclear power plants age, the likelihood of failures in the small bore piping used in those plants caused by exposure to mechanical vibrations during plant operations increases. While small bore piping failures rarely cause plant shutdown, the management of small piping has been a keen area of interest since their repair or maintenance requires a reactor trip. Steam generator (SG) drain pipe socket welds are small diameter piping connections (nominal pipe schedule 3–4 inches) susceptible to mechanical vibration. SG drain pipe socket weld failures have caused coolant leakage. Therefore, more reliable inspection technologies for small bore piping need to be developed to detect problems at an early stage and prevent pipe failures. This research aims to improve the reliability and accuracy of small bore piping inspections through the design, manufacture and application of a new phased array ultrasonic testing technique and inspection system for SG drain line socket welds.     

核电厂一回路管座焊缝超声波检测仿真研究

小尺寸支管接头(BOSS)焊缝作为核电厂一回路压力边界的薄弱环节,对其有效监控是核电厂日常和在役大修的重点和难点。采用仿真技术、工艺试验和现场应用验证等方法,设计并验证了BOSS焊缝的超声波相控阵检测工艺,解决了核电厂日常和在役大修中BOSS焊缝的监督难点。并得到类似超声波相控阵检测工艺的设计和验证方法。     

Computerised defect sizing using ALOK analysis in shrunk-on wheel of turbine rotor

A computerised defect evaluation system using an advanced ultrasonic technique for shrunk-on wheels of a turbine rotor has been developed. The flaw image was reconstructed by a modified ALOK method with attention to the tip diffraction echoes of stress corrosion cracking (SCC). The results of the application of this system to the mock-up wheel with artificial slits and real SCC cracks show that this system is very useful for crack sizing and locating.     

Ultrasonic signal characteristics by pulse-echo technique and mechanical strength of graphite materials with porous structure

Ultrasonic testing (UT) is an important non-destructive method to detect internal flaws and is widely applied to product control in industrial fields. In an investigation on ultrasonic signal characteristics in porous ceramics, the present authors developed an ultrasonic wave propagation model for the pulse-echo technique by improving an existing one for the transmission technique. A wave-pore reflection process was taken into account in the improvement. In the developed model, both diffusion and scattering losses can be treated as important factors of ultrasonic wave attenuation. The model was demonstrated by experimental data on ultrasonic signal characteristics of nuclear grade graphite. As an application of the model, the authors proposed a new approach combined UT signal with fracture mechanics to evaluate the mechanical strength of porous ceramics from UT signal. The combined approach was tried to apply to the acceptance test and the in-service inspection conditions of graphite components in the High Temperature Engineering Test Reactor (HTTR) as an example. This paper presents the developed propagation model for the pulse-echo technique as well as the combined approach. Moreover, both acceptance test and in-service inspection techniques of graphite components in high temperature gas-cooled reactors (HTGRs) using the combined approach was also proposed in this paper.     

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.     

Ultrasonic test of carbon composite/copper joints in the ITER divertor

The vertical targets of the ITER divertor consist of high flux units (HFU) actively cooled: CuCrZr tubes armoured by tungsten and carbon/carbon fibre composite (CFC). The armour is obtained with holed parallelepiped blocks, called monoblocks, previously prepared and welded onto the tubes by means diffusion bonding. The monoblock preparation consists in the casting of a layer of copper oxygen free (Cu OFHC) inside the monoblock hole.Each HFU is covered with more than 100 monoblocks that have to be joined simultaneously to the tube. Therefore, it is very important to individuate any defects present in the casting of Cu OFHC or at the interface with the CFC before the monoblocks are installed on the units.This paper discusses the application of non-destructive testing by ultrasound (US) method for the control of the joining interfaces between CFC monoblocks and Cu OFHC, before the brazing on the CrCrZr tube.In ENEA laboratory an ultrasonic technique (UT) suitable for the control of these joints with size and geometry according to the ITER specifications has been developed and widely tested. Real defects in this type of joints are, however, still hardly detected by UT. The CFC surface has to be machined to improve the mechanical strength of the joint. This results in a surface not perpendicular to the ultrasonic wave. Moreover, CFC is characterized by high acoustic attenuation of the ultrasonic wave and then it is not easy to get information regarding the Cu/CFC bonding. Nevertheless, the UT sharpness and simplicity pushes to perform some further study. With this purpose, a sample with artificial defects induced on the CFC/Cu interface during the Cu casting, has been manufactured and investigated, both by pulse-echo technique and by different techniques. The results obtained by the UT on this sample are reported and compared with X-ray method.     

Apparative developments for inservice inspections of reactor pressure vessels

Inservice inspections of primary circuit components are important preventive measures to guarantee nuclear power plant integrity, satisfying simultaneously reactor safety and economy in plant operation. Emphasizing pressurized water reactor pressure vessel (RPV) inspections, recent developments of new generations of automated and mechanized ultrasonic inspection equipment are presented. Starting from general equipment design and inservice implementation criteria, specific examples are given. Main attention is directed to equipment realization of phased array and ALOK inspection techniques, especially in their combination. Refined aspects of subsequent computer processing and evaluation of defect detection data are described. Analytical features and potential for further developments become evident. Remote controlled RPV inspections are stressed by describing a new generation of central mast manipulators, forming an integral part of total inservice inspection system.