High Temperature NDE Ultrasound Transducers for Condition ...

High Temperature NDE Ultrasound Transducers for Condition Monitoring of Superheated Steam Pipes in Nuclear Power Plants Marko Budimir Institute for Nuclear Technology - INETEC Dolenica 28, HR-10250 Zagreb, Croatia [email protected] Abbas Mohimi, Cem Selcuk, Tat-Hean Gan Brunel Innovation Centre, Brunel University Uxbridge, Middlesex, UB8 3PH United Kingdom [email protected], [email protected], [email protected] ABSTRACT High temperatures and pressures in superheated steam pipes in nuclear power plants (as well as in classic fuel power plants) can lead to formation of flaws and defects due to material degradation mechanisms such as corrosion, creep (in aging nuclear plants and in classical plants) and fatigue over time. If undetected in advance, these critical features can cause catastrophic damage and failures which in turn could result in environmental and financial consequences. Inspection using conventional non-destructive evaluation (NDE) techniques such as visual testing, eddy current testing and ultrasonic testing are employed to assess the integrity of the steam pipelines during planned outages for nuclear power plant maintenance. However, if the condition of a pipeline is not monitored in between outages, this may present a problem for an aging power plant. In situ condition monitoring techniques therefore need to be developed to retain reliability and extend the lifetime of nuclear power plants. Long Range Ultrasonic Testing (LRUT) approaches to inspection of pipeline have successfully been employed at low temperatures, but at high temperatures present practical challenges. The key challenge is the development of transducers that can operate at elevated temperatures. This paper presents a review of different types transducers that have been reported to work at elevated temperatures, and the challenges in developing transducers for a LRUT system. 1

INTRODUCTION

A quasi-periodic repetition of catastrophic accidents in nuclear power plants around the world (among others Three Mile Island loss of coolant and partial core meltdown in 1979, Chernobyl steam explosion and meltdown in 1986, Mihama Nuclear Power Plant steam explosion in 2004, Fukushima Nuclear power plant cooling failure in 4 reactors followed by multiple meltdowns in 2011) has already been causing human losses, appalling injuries, massive financial losses, power cuts and reductions and strong negative public opinion towards the exploitation of nuclear energy for decades. Considering the lack of efficient shortterm alternatives to nuclear power plants, the question of nuclear plants safety thus remains 502.1

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crucial. Research and development of technologies for improving structural health control of such complex systems or its parts will continue to be of vital importance for many years to come. Maintaining the integrity of the pipelines that transport superheated steam is one of the critical issues, since high temperatures and pressures in superheated steam pipes in nuclear power plants can lead to formation of flaws and defects due to material degradation mechanisms such as corrosion, creep and fatigue over time. If undetected in advance, these critical features can cause catastrophic damage and failures which in turn could result in environmental and financial consequences. For a structural health inspection, each nuclear power plant has to follow rigorous standards, regulations and codes approved by national regulatory institutions. Accordingly, all inspections of nuclear power plants structural health have to be performed during plant outages. Since the inspections are performed during outages, the inspection techniques and technologies are adapted to be efficient at room temperatures. Inspection using conventional NDE techniques such as visual testing, eddy current testing and ultrasonic testing are commonly employed to assess the integrity of the steam pipelines during planned outages for nuclear power plant maintenance. However, if the condition of a pipeline is not monitored in between outages, this may present a problem for an aging power plant. In situ condition monitoring techniques therefore need to be developed to retain reliability and extend the lifetime of nuclear power plants. When selecting a suitable NDE technique that could be used for in situ condition monitoring of pipes, Long Range Ultrasonic Testing (LRUT) is an option that has been successfully used for inspection of pipelines at ambient temperatures [1]. LRUT employs Ultrasonic Guided Waves (UGW) to detect defects, whilst requiring limited access, which means that large areas can be screened from a single location. Unlike conventional ultrasonic testing (UT), which tends to operate at MHz range frequencies, LRUT operates at lower frequencies (typically in the kHz range), therefore less scattering and attenuation is experienced resulting in screening long lengths, but with poorer spatial resolution. LRUT is hence often used as a screening tool before employing other NDE techniques to examine the defective regions more closely. LRUT is beneficial in a power plant where the pipes are insulated, it can screen long lengths of pipes from a single location without the need to remove insulation, claimed to be saving time and money. LRUT of pipes generally involves mounting an array of transducers around the pipe to excite an axially-symmetric wave mode and capturing interaction of the wave modes with defects. This paper is based on an ongoing research and development project aiming to develop a LRUT system for continuous in service inspection and structural health monitoring of high temperature superheated steam pipes in power generation plants. The system is thus planned to work at elevated temperatures, up to 580°C, while the power plant is in operational state. The key challenge is the development of transducers that can operate at elevated temperatures. This paper presents a review of different types of transducers that have been reported to work at elevated temperatures, and therefore being considered for possible application to LRUT. 2

NON-CONTACT TRANSDUCERS

There are advantages for using non-contact type transducers for ultrasonic inspection at elevated temperatures. Electromagnetic Acoustic Transducers (EMAT) and laser ultrasound are two of the common non-contact techniques used for NDE at elevated temperatures. An EMAT normally consists of a magnet, to generate a magnetic field, and a coil to generate eddy current by passing an alternating current through the coil. When the EMAT is in transmission mode, an alternating current (AC) is passed through the coil to generate Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011

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alternating eddy currents near the surface of the structure, which interact with the magnetic field and generates an alternating force in the material resulting in generation of ultrasound. In the reception mode, when the travelling ultrasound passes through the EMAT it will cause interference in the magnetic field, which will lead to an AC voltage being induced into the coil of the receiving EMAT and a signal may be detected [2]. It is important to emphasize here that for the conventional UT a liquid couplant is used to facilitate the transmission of ultrasonic energy from the transducer to the test piece due to the large acoustic impedance mismatch between air and solids, while there is no need for couplant when using EMAT’s as they work without making contact to the specimen. This has obvious advantages for operation at elevated temperatures. However, most EMAT designs employ permanent magnets, since they provide high magnetic fields and have compact size, but these magnets have low Curie points (e.g. 100-150°C in NdFeB) [3]. The low Curie point of the magnets means that EMAT’s using permanent magnets present a significant challenge for operation at elevated temperatures, unless the magnets are actively cooled [4,5,6], and, in general, they are reported to have relatively low efficiency in comparison to piezoelectric transducers. Laser Ultrasonic Testing (LUT) is another non-contact UT technique. LUT has a small footprint, can interrogate cracks even in the presence of local surface corrosion and has higher bandwidth than contact transducers [7]. There are commercial LUT systems available [8]. The optical ultrasound detection techniques are contact-free, have a high detection bandwidth in comparison with contact transducers and provide an absolute measurement of the ultrasonic signal. However, their detection sensitivity is an order of magnitude lower than contact transducers [9]. There is clearly a need to develop contact type piezoelectric transducers that can withstand high temperatures, because of their efficiency in comparison to the above mentioned alternative non-contact techniques such as EMATs and laser ultrasound. 3

HIGH TEMPERATURE ULTRASONIC TRANSDUCERS

To enable successful high temperature ultrasonic inspection of superheated steam pipes various engineering and materials parameters should be reconsidered. Some of the most important technological challenges for conventional piezoelectric type ultrasonic transducers to operate at high temperatures are the depolarization of the acoustically active piezoelectric element, difficulties in assembling the transducers using high temperature joining techniques, such as brazing in which the coefficient of thermal expansion mismatch of the materials within a transducer may cause them to fail, and problems with suitable acoustic coupling between the transducer and the specimen, which is generally hard to achieve. It can thus be concluded that the choice of acoustically active piezoelectric materials for ultrasound generation, and bonding techniques for constructing a high temperature NDE transducer should be considered differently than the room temperature conventional ultrasound transducers design. To generate ultrasound by using the piezoelectric effect at high temperatures, one has to choose an acoustically active piezoelectric material that can withstand high temperatures. Many piezoelectric materials that possess high Curie temperatures have been reported in the literature [10-13]. A common problem associated with materials with high Curie temperatures is the magnitude of their piezoelectric response, which is generally low in comparison with that of PZT ceramics at room temperature, the most widely used piezoelectric material for conventional ultrasound NDE transducers. Among high temperature piezoelectric materials, LiNbO3, has been a promising material to some extent. Lithium niobate has the Curie temperature of 1210°C and the Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011

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thickness coupling coefficient of kt=0.30 [14,15]. As a general rule of thumb the recommended safe operating temperature for piezoelectric materials is half their Curie temperature, which suggests in theory LiNbO3 can operate at temperatures up to 605°C, but, on the other hand, it has been reported by some authors that at 600°C it starts to lose oxygen to the environment [15]. LiNbO3 has thus been somewhat neglected for use in ultrasonic transducers. But, recent advances in signal processing have made its disadvantages less significant, for example, promising results within investigations of the potential of LiNbO3 composites for use in high temperature transducers for non-destructive testing have been reported [16,17]. Another material based on lithium niobate, a new ceramic, was fabricated using only the sol-gel based technology [15]. Since LiNbO3 starts to lose oxygen at 600°C, the idea here was to block oxygen loss by replacing some of the Lithium with Sodium which offers a better piezoelectric sensitivity than Li. The authors performed an exaustive examination of the Li1xNaxNbO3 family, with an atomic percentage variation of sodium between 0% and 25%. It was shown that the new piezoelectric material had no oxygen loss and unique performance was achieved. An interesting material to mention is also sodium substituted LiNbO3 ceramics, Li1-xNaxNbO3 (LNN) developed by Ferroperm. The working temperature of this material is a little higher than of LiNbO3 (To=650°C). Its sensitivity is higher as well, while the pulse shape is of good quality at high temperatures [18]. As for materials not related to lithium niobate the modified bismuth titanate, Kezite K15 (Piezo Technologies, Keramos), is a relatively new piezoelectric ceramic developed for high temperature applications. It has a high piezoelectric stability, its TC=600°C, and kt=0.15 [19,20]. The lead metaniobate, PbNb2O6, possesses a high Curie temperature of 540°C, a low mechanical quality factor and is suitable as a sensitive element for a sensor. However, it is difficult to fabricate dense PbNb2O6 ceramics that have good piezoelectric properties. Ceramics with a high density and a high piezoelectric effect were fabricated by adding various elements such as Mn and Ca to PbNb2O6 and by optimizing the sintering process [21]. Gallium orthophosphate, GaPO4, which belongs to the same group as quartz, is a new piezoelectric material for a wide range of high technology applications. The sensitivity and a high thermal stability makes it a very attractive choice for a wide range of un-cooled high temperature applications up to 900°C. Up to 700°C the piezoelectric constant d11 shows no measurable deviation from its room temperature value which is about twice that of quartz [22]. It is also useful to review papers on high temperature transducers and transducers that are as well commercially offered by some companies. This includes, among others, a high temperature transducer developed using LiNbO3 single crystal [23]. The LiNbO3 was bonded onto a stainless steel substrate, and the transducer was heated in an electric furnace while measuring the bottom echoes from the substrate. The authors confirmed that their high temperature transducer could work up to 1000°C. A different approach is reported for solving the problem at elevated temperatures, where the authors discuss the design of a hightemperature ultrasonic thickness gauge that bypasses some difficulties related to high temperature environments in which it should work [24]. The system uses a waveguide to isolate vulnerable transducer and piezoelectric elements from the high-temperature measurement zone. In addition, an integrated ultrasonic sensor with an acoustic waveguide has also been developed in a manner to overcome the thermal limitations of conventional ultrasonic transducers [25]. The sensor could be presently applied at maximum temperatures up to 600°C. One can find a comprehensive study of high temperature ultrasonic transducers and the development of a prototype ultrasonic probes [18]. It is emphasized there that at temperatures >200°C ultrasonic testing is preferred to be performed by electromagnetically generated Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011

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ultrasound using EMAT transducers. Another review of techniques for generation of ultrasound for extended periods at high temperatures has been done [26], on transducer materials and various types of adhesive bonds were considered. Measurements of insertion losses, the transducer coupling factor and the capacitance indicated that the most successful transducer-bond combinations were PZT-5A with a solderbond for use up to 250°C, and lithium niobate with either ceramic or gold-indium bond for use up to 400°C. In other works for manufacturing of miniature high temperature ultrasonic transducers the lead-free thick bismuth titanate films have been successfully deposited on steel substrates of different shapes by the sol-gel spray technique [27, 28]. Considering commercial transducers, several high-temperature ultrasonic transducers are supplied by companies, such as Etalon [29], Panametrics [30], Ultran [31], SIGMA transducers [32] amongst others. It is interesting to mention that for design reasons there are no high temperature contact or immersion transducers in the standard product line. If we take an example, SIGMA transducers, most of them operate at higher temperatures than it is generally available in the market place at large. The selection of the proper materials other than epoxies and solders allows operation of these units up to 230°C continuously, and their delay lines can prolong the operation additionally. A similar observations can be made for the commercial transducers of other companies [30,31,33-35]. A comparison between various commercial methods and materials for acoustic transduction, identifying their advantages and limitations is available from an earlier study [11]. Techniques and devices include simple piezoelectric sensors, accelerometers, strain gauges, proximity sensors, fiber optics and buffer rods. It is important to emphasize here that, in that paper, the piezoelectric ultrasound generation approach was pointed out as the one that offers advantages (including design cost and simplicity) over other approaches. An important parameter for use of high temperature transducers is related to high temperature acoustic coupling. There are three diferent concepts of acoustic coupling at high temperatures: dry coupling (with a high pressure), liquid (fluid) coupling, and solid coupling. In dry coupling a high quality surface finish is required, because very small airgaps (>0.01 μm) between a piezoelectric element and a front protective layer, which can substantially reduce the acoustic energy entering the material and pressures up to 300 MPa are required to expel the air even at an interface between highly polished surfaces [13]. Dry coupling is limited with the transducer λ/2 membrane, as it can not withstand the necessary coupling pressure. In a liquid coupling thin acoustical membranes can be used. However, in spite of numerous attempts, an optimal coupling fluid, concerning corrosive environment especially, is not yet available. Liquid couplants can be divided into two groups: liquid at the room temperature (silicone oil and specialized high temperature couplants), and glass solders, which are solid at the room temperature and melt prior to operation. Silicone oil as a couplant was used successfully up to 250°C [36,37], but evaporated gradually. During a lengthy operation of the transducer, the liquid couplant can flow out of the interfaces due to vibrations of a piezoelectric element. The chemical stability of this couplant is lost when the temperature increases for a longer period. An original high temperature ultrasonic couplant is offered in [38]. It is non-flammable and consists of grease-like silicone fluid heavily filled with zinc oxide. Couplant E (Ultratherm) is recommended for use between 260°C and 540°C [30]. In a solid coupling several approaches can be used; soldering, diffusion bonding, ultrasonic welding, cementing, epoxy bonding and sol-gel or CVD technology [25,39-41]. In a higher temperature range a solid couplant material Sono 1100 Film [42] is designed to melt at the temperature 370°C and is intended for use up to 593°C, but only for 15 seconds. Internal acoustic coupling in principle may be accomplished with glass solder which is fluid in the temperature range required [18,43]. Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011

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The difficulties in high temperature packaging lie not only in finding materials that can survive at temperatures exceeding 300°C, but in finding compatibility between materials and assembly techniques also [44]. At high temperatures incompatibilities between materials prevail or become apparent. Factors such as thermal conductivity, expansion coefficients, oxidation and diffusion become critical as operating temperatures increase and play influential roles in the selection of packaging materials. Conventional ultrasonic transducers can tolerate temperatures up to approximately 50°C. If the medium being tested is hotter than 50°C then high temperature transducers and special techniques should be employed [30]. To overcome the thermal restrictions of PZT piezoceramics a waveguide (buffer rod) approach has been widely used. A large variety of waveguide design solutions have been reported. An alternative approach is a direct contact transducer (with λ/2 or thicker protector, delay line and immersion) in which only a high temperature piezoceramic must be used, possessing a significantly lower coupling coefficient and requiring new solutions in the transducer design. A temperature compensated piezoelectric transducer assembly for operation up to 350°C was diffusion bonded [45], using a lithium niobate piezoelectric element which was mounted on a metal base using a structured copper interlayer. The structured copper is a bundle of parallel filamentary strands of copper closely packed together (density ~90 %). This bundle accommodated different thermal expansions of the assembly. Diffusion bonding was also used in making piezoelectric actuators [46]. Fabrication of the actuators involves the stacking and diffusion bonding of multiple thin piezoelectric layers coated with silver electrodes. Stacked piezoelectric layers were placed in a press for the diffusion bonding process. The stack is pressed and heated at “a specified curing temperature and pressure for a specified curing time” [46]. Wear plates are normally made of aluminum oxide which is high temperature resistance material. The wear plate is used to protect the piezoelectric element in conventional ultrasonic probes where they are manually moved along the surface of the specimens. However, for a permanently installed system at a single location it may not be necessary to have a wear plate, as it will present joining issues when it needs to be bonded to the piezoelectric element. The option of applying a ceramic coating (aluminum oxide) via for example sputtering may be more suitable. High temperature cables should connect the high temperature transducers with the pulser/receiver system. There are cable manufacturers that produce a range of cables for operation at high temperatures such as Nickel Plated Copper or Pure Nickel conductors. In high temperature ultrasonic transducers mineral insulated signal transmission cables may be suitable such as Thermocoax [47]. Due to their exceptional properties, these cables can be used in highly aggressive media such as corrosive liquids or gases, under high pressure, vacuum, vibrations, in nuclear industry. They resist to a temperature up to 1200°C and can be welded or brazed. They also can carry relatively high currents as well as very low signals at high frequencies. 4

CONCLUSION

This paper reviews and emphasizes some advantages and disadvantages of contact and non-contact techniques that could potentially be related to non-destructive evaluation of structural health of nuclear power plant superheated steam pipes. It points out that the piezoelectric transducers are more favorable than other types of NDE techniques (EMAT and laser ultrasound) for such applications. The design of a high-temperature ultrasonic piezoelectric transducer essentially depends on the piezoelectric element and the bonding method selected. The other problems are related

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to high temperature couplants and cables. All these issues have been discussed in this paper and a literature survey has been performed. The LRUT transducers for in situ monitoring of power plant steam pipes should be designed in consideration of the existing material limitations, new material systems being developed and specific environment requirements. High temperature piezoelectric ceramics such as Lithium Niobate should be further investigated as a potential candidate for high temperature ultrasonic transducers applications. ACKNOWLEDGMENTS This work is part funded by the European Commission through the Seventh Framework Programme (FP7-SME-2010-1) under grant agreement no. 262574, as part of a collaborative project 'HOTSCAN', and by Croatian Unity through knowledge Fund, under grant agreement no. 47 as a part of the project 'SONDE'. The partners involved in the HOTSCAN project are: Plant Integrity Ltd, INETEC, Vermon SA, Polkom Badania SP ZOO, Brunel University, CERETETH and Technitest Ingeniros SL. REFERENCES [1] Rose, J. L., Cho, Y. and Avioli, M. “Next generation guided wave health monitoring long range inspection of pipes”, Journal of Loss Prevention in the Process Industries, Vol. 22, 2009., pp. 10101015. [2] Wilcox, P. D., Lowe, M. J. S. and Cawley, P. “The excitation and detection of lamb waves with planar coil electromagnetic acoustic transducers”, IEEE Transactions on ultrasonics, ferroelectrics, and frequency control, Vol. 52. No. 12, 2005. [3] Hernandez-Valle, F. and Dixon, S., “Preliminary tests to design an EMAT with pulsed electromagnet for high temperature”, AIP Conference Proceedings, 2009., pp. 936-941. [4] Idris, A., Edwards, C. and Palmer, S. B. 1994, Nondestr. Test. Eval., Vol. 11, pp. 195-213. [5] Baillie, I., et al. 2007, Insight., Vol. 49, pp. 87-92. [6] Hernandez-Valle, F. and Dixon, S. “Pulsed electromagnet EMAT for high temperatures”, Rev. Prog. in QNDE 2009. 2010, Vol. 29A&29B, pp. 957-963. [7] Klein, M., et al. ”Applications of laser ultrasonics in the pipeline industry”, 1st International symposium on laser ultrasonics: Science, technology and applications, Montreal, Canada : s.n., 2008 [8] Intelligent optical systems, Inc. [Online] http://www.intopsys.com/laserultrasound00.htm. [9] Krishnaswamy, S. “Theory and applications of laser ultrasonic techniques”. s.l. : CRC Press LLC, 2003. [10] Kazys, R., Voleisis, A. and Voleisiene, B. “High temperature ultrasonic transducers: review”, Ultragarsas (Ultrasound), Vol. 63, No. 2, 2008, p. 7. [11] Turner, R. C., et al. ”Materials for High Temperature Acoustic and Vibration Sensors: A Review”, Applied Acoustics, Vol. 41, 1994, p. 299. [12] Damjanovic, D. “Materials for high temperature piezoelectric transducers”, Current Opinion in Solid State & Materials Science, Vol. 3, 1998, pp. 469-473. [13] McNab, A., Krirk, K. J. and Cochran, A. “Ultrasonic transducers for high temperature applications”, IEE Proceedings - Science, Measurement and Technology, Vol. 145, 1998, pp. 229-236. [14] Acoustic properties of longitudinal piezoelectrics. Onda Corporation, Sunnyvale, CA. [Online] http://www.ondacorp.com/images/LongPiezo.pdf. [15] Innovation in Europe: Research and Results: "Non-destructive testing at 800C". [Online] http://ec.europa.eu./research/success/en/mat/0041e.html. [16] Schmarje, N., Kirk, K. J. and Cochran, S. “1-3 Connectivity lithium niobate composites for high temperature operation”, Ultrasonics, Vol. 47, 2007, p. 15. [17] Shepherd, G., et al. “1-3 connectivity composite material made from lithium niobate and cement for ultrasonic condition monitoring at elevated temperatures”, Ultrasonics, Vol. 40, 2002, p. 223. Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011

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