Integration of nondestructive examination reliability and fracture mechanics

A multi-year program on the Integration of Nondestructive Examination and Fracture Mechanics (NDE/FM) has been funded by the U.S. Nuclear Regulatory Commission at the Pacific Northwest Laboratory. Many activities are being pursued under this program. This paper highlights some of the activities: input to the NRC Pipe Crack Task Group, an evaluation of manual ultrasonic testing of centrifugally cast stainless steel, interaction matrix, advanced UT technique evaluation, qualification document, evaluation of crack characterization techniques, international NDE reliability work, siamese imaging technique for imaging planar-type radial defects in reactor piping, fracture mechanics analysis for PTS-type flaws and piping reliability, and a position paper on piping ISI.     

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.     

Flaw behavior in mechanically loaded clad plates

A small crack near the inner surface of clad nuclear reactor pressure vessels is an important consideration in the safety assessment of the structural integrity of the vessel. Four-point bend tests on large plate specimens, six clad and two unclad, were performed to determine the effect of stainless steel cladding upon the propagation of small surface cracks subjected to stress states similar to those produced by pressurized thermal shock conditions. Test results have shown that the tough surface layer composed of cladding and/or heat-affected zone has enhanced the load-bearing capacity of plates under conditions where unclad plates have ruptured. The results are interpreted in terms of fracture mechanics. The behavior of flaws in clad reactor pressure vessels is examined in the light of the test results.     

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.     

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.     

The newest two-phase flow control devices in LWR equipment based on ultrasonic and WAT-technology

This paper deals with new acoustic methods of two-phase flow diagnostics used to carry out research in the fields of nuclear power thermophysics and nuclear power plant (NPP) technologies equipment control. All the designs are to be used under extreme conditions, characteristic for water coolant, with temperature up to 350°C and pressure 20 MPa. All the safety and reliability requirements are met. The methods use waveguide ultrasonic transducers for longitudinal and bending waves, made according to specially designed technology (waveguide acoustic transducers — WAT technology). This paper deals with the operating principles of transducers and processing device physical models as well as some results on the practical use of this equipment. The method of acoustic impedance is based on measuring attenuation of a longitudinal or bending ultrasonic wave in a thin-walled tube diameter vapour fraction or the level of the coolant in the tank. The waveguide transducers, designed by the centre, use bending waves of a surface type. They enable us to carry out diagnostics of the liquid film on the inner surface of the tube or discover gas inclusions in the liquid flow. The paper touches upon the method of acoustic emission for measuring moisture content in a steam flow.     

Requirements related to in-service inspection of reactor vessels

The in-service inspection of nuclear facilities should make it possible to detect all flaws capable of deterioration. The size of a flaw is not normally characteristic of its seriousness: a large, stable defect which is not developing is of no importance for the safety of the facility whereas a small but developing defect may be dangerous and hence of importance for safety. It is essential to detect all defects, so that any development they may be undergoing can be assessed, and to use, for that purpose, a technique with a maximum detection probability; but the main thing is to characterize all developing defects. For the in-service inspection of pressurized-water reactor vessels the “Commissariat à l'Energie Atomique” has conducted studies on inspection methods with a view to ensuring first the best possible detection and secondly the best possible characterization of flaws. Since 1973 many publications have appeared on the subject and this paper will only recall the essentials giving particular attention however to the characterization of defects, which is the principal purpose of the studies currently being conducted.     

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.

Flaw density examinations of a clad boiling water reactor pressure vessel segment

As part of the Oak Ridge National Laboratory's Heavy-Section Steel Technology Program, studies have been conducted to determine flaw density in a section of reactor pressure vessel cut from the Hope Creek Unit 2 vessel. This boiling water reactor vessel was never in service. One objective was to evaluate the approximate 0.7- by 3-m (2- by 10-ft) segment of the vessel provided using ultrasonic flaw detection methods performed with both ASME Code techniques and supplemental ultrasonic methods. A second objective was to evaluate the inner surface stainless steel cladding for cracks with a high sensitivity penetrant examination. Both objectives were successfully completed. Five Code-recordable indications were detected ultrasonically; however, all were found to be anomalies associated with the cladding. One flaw was detected by the supplemental ultrasonic tests, and it was analyzed destructively. This flaw was pipelike indication, about 20 mm (0.8 in.) long extending along the length of the longitudinal weld in which it was located and was about 20 mm below the cladding surface. The flaw had a through-wall dimension (or length) of about 6 mm (0.24 in.) for an approximate 3-mm (0.1-in.) distance along the 20-mm major length. No flaws were detected by the penetrant examination of the cladding surface.     

Waveguide ultrasonic liquid level transducer for nuclear power plant steam generator

This paper deals with the development and utilization of a new acoustic method to control the level of the coolant in industrial power-generating equipment. The development research was carried out on the basis of the method of acoustic impedance, based on measuring attenuation of a longitudinal ultrasonic wave in a waveguide sensing element. Ultrasonic attenuation is determined by the degree of submergence of the sensing element in liquid. For this purpose waveguide ultrasonic transducers are used, made with specially designed technology (waveguide acoustic transducers—WAT-technology). The transducers are adapted for operation under extreme conditions in the water coolant with temperatures up to 350°C and pressure up to 20 MPa. The paper tackles: principle of operation of transducers, methods of testing under laboratory conditions, design and results of calibration in a thermophysical test facility, results of the operational trial of the level transducer at the nuclear power plant in Zaporozhye.     

Inspection procedure assessment using modelling capabilities

Before carrying out inspections on technical components, criteria have to be met to qualify the inspection procedure. The inspection qualification can be performed by technical justification or performance demonstration. The qualification of non-destructive testing methods can be checked on full scale mock-ups where real or realistic defects are implemented. To minimize costs it will become mandatory to model the inspection procedure. In the case of ultrasonic testing, the modelling includes the ultrasonic probe, the pulse-defect interaction and the probe geometry. The applied modelling code is the elastodynamic finite integration technique (EFIT) which includes mode conversion effects. The results of the code are either wave fields, A-scans at different probe positions or complete r.f. data fields. Examples demonstrate the efficiency in modelling angle beam probes which transmit both pressure, shear and subsurface longitudinal waves. These waves are scattered by horizontal or surface breaking cracks. The modelled r.f. data fields are used within the synthetic aperture focusing approach to predict the ultrasonic image which would be obtained in performing the experiment. In addition it will be shown that the EFIT wave fronts for anisotropic homogeneous media are explicitly related to the group velocity in these media.     

Monitoring the filling level of the rpv using stationary emus-probes

The determination of the filling level and the initiation of growth of bubbles in vessels and pipes containing fluids is an essential component of monitoring during operation.The ultrasonic pulse-echo-method is a measuring procedure suited for this purpose and applicable from the outside. Piezoelectric ultrasonic transducers can be used in principle at a temperature of 300°C, but in practice these transducers are not preferred because of the expense and inconvenience in coupling them to the vessel wall.These problems are solved using lectro agnetic- ltrasonic-(EMUS-)transducers. Due to physical reasons a longitudinal wave is generated in the fluid by the refraction of a shear wave in the vessel-wall. The filling level is measured in a pitch and catch-technique by a mirror reflection of the longitudinal wave at a construction element inside the vessel. This paper reports on laboratory investigations concerning the applicability of the technique and first experiences with an EMUS-prototype system installed in a nuclear power plant.     

Inspection qualification programme for VVER reactors and review of round robin test results

Experience obtained, especially from in-service inspections of VVER 440-type reactor pressure vessels and from the Czech round test trials with international participation of ultrasonic teams, has highlighted the need for an in-service inspection qualification programme in the Czech Republic focused on NDT procedures, equipment and personnel. Recently, several national and international regional projects included in the PHARE programme (projects 4.1.2/93 and 1.02/94), briefly described, have been initiated. These projects are to cover step by step the programme of the in-service inspection qualification in view of technical justification as well as of practical assessment—performance demonstration—for all the main VVER-type primary circuit components.     

Models for the computation of ultrasonic fields and their interaction with defects in realistic NDT configurations

Developments in quantitative ultrasonic non-destructive testing (subsequently denoted by NDT) require simulation tools for cost-effective research and engineering works. The present paper reviews the two models developed for several years at the French Atomic Energy Commission (CEA) for this purpose. The first one is dedicated to the computation of the ultrasonic field radiated by arbitrary transducers into pieces under examination. The second simulates actual testing configurations, that is, includes beam/defect interaction as well as the transducer scan over the piece so as to synthesise images typical of those actually measured in NDT experiments. Examples of their application in the context of nuclear engineering are given. They demonstrate the usefulness of such modelling tools in ultrasonic NDT either to study and optimise testing configurations, including transducer design or to help NDT-experts to interpret actual data, possibly by means of model-based automatic data inversion.     

Current status of the engineering design of the test modules for the IFMIF

Under Broader Approach (BA) Agreement between EURATOM and Japan, IFMIF/EVEDA (International Fusion Materials Irradiation Facility/Engineering Validation and Engineering Design Activities) has been performed since the middle of 2007. IFMIF presents three main facilities (the Accelerator Facility, Li Target Facility and Test Facilities). A previous design of IFMIF was summarized in the comprehensive design report [1]. The present EVEDA phase aims at producing a detailed, complete and fully integrated engineering design of IFMIF. The delivery of the “Intermediate IFMIF Engineering Design Report” is foreseen mid-2013. The goal of IFMIF is to obtain the indispensable design database to allow the design and licensing of DEMO and ensuring commercial reactors thanks to the materials data set obtained from planned evaluation tests such irradiations in high flux test modules (HFTM-vertical rig, HFTM-horizontal rig), medium flux test modules (creep fatigue test module, tritium release test module, liquid breeder validation module) and low flux test modules of IFMIF. In addition, the Startup Monitoring Module will be used for IFMIF commissioning. This paper is summarizing the overall current progress status of the engineering and conceptual design of the test modules in IFMIF/EVEDA.     

Development of an ultrasonic inspection system using array transducers

A new ultrasonic inspection system was developed to obtain ultrasonic images of defects. This system adopts an electronic beam control method using array transducers. The beam control modes are a compound scanning mode and a linear scanning mode. Both modes are performed by timing control of ultrasonic wave transmission and reception at each transducer element.In the focal beam, the refraction of the ultrasonic wave at the boundary between water and metal in immersion testing is utilized to improve the lateral resolution of the ultrasonic beam. In steel, the improvement is a maximum of 25% for focal lengths from 5 to 35 mm as compared to lateral resolution not utilizing refraction.This system successfully imaged two side drilled holes in a steel block.     

Fatigue and fracture behavior of straight pipe with flaws in inner surface

Fatigue and fracture tests of piping models with flaws in the inner surface were carried out to investigate the fatigue crack growth, coalescence of multiple cracks and fracture behavior.Two straight test pipes with and without weldment in the test section of AISI type 304L stainless steel were tested under almost the same test conditions by imposing moment loads. Three artificial defects were machined in the inner surface of the test section of the test pipes and the fatigue test was performed until the cracks coalesced and grew through the thickness. Subsequently, a static load was imposed on the test pipe which contained a large crack in the test section.The fatigue test results are compared with an analytical crack growth behavior predicted by the method described in the Section XI of ASME Code, and show slower crack growth than that of the prediction. From the fracture test results, it is found that the test pipes can endure considerably high load.     

Validation of room-temperature primary creep crack-growth analysis for surface-cracked pipes

This paper briefly reviews the theoretical considerations that underlie the development of an engineering analysis of ductile, time dependent flaw growth as adapted to axial surface flaws in cylindrical containers such as pipes and tanks. Thereafter the validation of this analysis by comparison of the predicted and observed behavior of an extensive database for part-through-wall defects in pipes is presented. The results of this comparison indicate that the analysis provides an accurate means to characterize the flaw growth and related failure pressure for this class of flawed structures. The results indicate a close correspondence of predicted and observed behavior that is free of bias with respect to flaw length and depth and the grade of steel used to fabricate these full scale test specimens. Readers interested in more complete documentation of the analysis and the validation studies are directed to the report covering model formulation and validation (Leis et al, June 1991).     

Quantitative Characterization of Tissue Using Ultrasound

Present medical ultrasound systems are based on energy detection methods and therefore only utilize echo intensity information. However, phase, as well as spectral, information is recorded by the transducer, which is a pressure sensitive device, but is not utilized in present display or measurement schemes. This additional information may be of diagnostic significance since the interaction between sound and tissue is exceedingly complex, since many types of tissue can be categorized in terms of their acoustical properties, and since changes in tissue acoustical properties can be correlated with specific pathological states. Thus, in principle, in vivo techniques could be devised which would extract and separate the medically significant features of the ultrasound interactions with tissue and would display ultrasonic tissue signatures appropriate for a differential diagnosis. The development of such quantitative techniques for the measurement of ultrasonic tissue parameters and/or the display of ultrasonic tissue signatures is known as ultrasonic tissue characterization. In this paper we review the physical and clinical basis for ultrasonic tissue characterization, describe several characterization schemes currently being explored, and attempt to assess the importance such techniques may play in the future of diagnostic medicine.