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Nuclear Engineering and Design 241 (2011) 1889–1898

Contents lists available at ScienceDirect

Nuclear Engineering and Design

journa l homepage: www.e lsev ier .com/ locate /nucengdes

esign of copper/carbon-coated fiber Bragg grating acoustic sensor net forntegrated health monitoring of nuclear power plant

ee Yenn Chonga, Jung-Ryul Leea,∗, Chang-Yong Yuna, Hoon Sohnb

Department of Aerospace Engineering, Chonbuk National University, 664-14 Duckjin-dong, Duckjin-gu, Jeonju, Chonbuk 561-756, Republic of KoreaDepartment of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 335, Gwahangno, Yuseong-gu, Daejon 305-701, Republic of Korea

r t i c l e i n f o

rticle history:eceived 11 May 2010eceived in revised form 10 January 2011ccepted 28 January 2011

a b s t r a c t

A nuclear power plant (NPP) is a harsh environment that gives rise to age-related degradation of theplant structures, and eventually leads to radiation leakage that threatens humans. Integrated structuralhealth monitoring (ISHM) technology is a strong candidate for the prevention of the NPP accidents duringoperation. Prior studies have shown that fiber Bragg gratings (FBGs) and metal-coated fibers have goodradiation and high temperature resistance. In this study, a FBG acoustic sensor using a metallic adhesivefor installation and a relatively economical copper/carbon (Cu/C)-coated fiber is developed for ISHM ofhigh temperature NPP structures. A chemical method is proposed to remove the Cu/C coating. A 5 mm FBGwas successfully inscribed in a Ge-doped silica core through a 7 mm-long silica section with the coatingremoved. The Cu/C-coated fiber with the same core/clad structure as the standard SMF allowed no-loss

fusion splicing, and showed good adaptability to the economical standard fiber, adaptor, connector, andinstruments. It showed also good thermal resistance (<345 ◦C) with no degradation in optical powerduring the optical transmission. The metallic adhesive used to install the FBG in a one-end-free configu-ration showed superior bonding reliability during temperature cycles ranging from 25 ◦C to 345 ◦C. TheFBG reflectivity was stabilized at a 58% drop from the initial reflectivity, and the Cu/C-coated FBG sensorusing the metallic adhesive could successfully detect the acousto-ultrasonic waves generated by pencil

eam e

lead breaking and laser b

. Introduction

The demand for electricity has been increasing rapidly, andill almost double from 2004 to 2030 (World Nuclear Association,

009). Consequently, nuclear energy is the prime candidate forarge scale electricity generation with long term operation (>30ears) in comparison to other energy sources. However, the envi-onment of nuclear power plants (NPPs) is categorized as a harshnvironment, and it possesses aging mechanics such as radiationmbrittlement, stress corrosion cracking, fatigue, corrosion, andhermal aging that lead to age-related degradation on mechani-al structures, systems, and components (IAEA, 2009; Hrazsky andikus, 2007). Thus, continuous plant condition monitoring has

een proposed for cost-effective maintenance while maintainingafety and reliability (Oedewald and Reiman, 2003). A leakage pre-

ention system has also been considered as a critical system to bentroduced for NPP structures (Ferdinand and Magne, 2002). Anntegrated structural health monitoring (ISHM) system based onassive acoustic emission (AE) sensing is a strong candidate. The

∗ Corresponding author.E-mail address: leejrr@jbnu.ac.kr (J.-R. Lee).

029-5493/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2011.01.042

xcitation.© 2011 Elsevier B.V. All rights reserved.

ISHM system can perform in situ monitoring to determine struc-tural health by detecting AE events due to material failure andleakage (Wild and Hinckley, 2008).

For ISHM of a NPP, a fiber optic sensor network (FOSN) is veryattractive compared to the piezoelectric sensor networks beingapplied to structures in general environments because a FOSN haslow mass, small size, flexibility, durability, electromagnetic immu-nity, and corrosion resistance, as well as exceptional embedding,communication, and multifunctional sensing capabilities.

In general, fiber optic acoustic sensors have been developedusing standard telecommunication fiber, and their sensing princi-ples can be categorized into single fiber intensiometric, fiber opticinterferometric, and fiber Bragg grating (FBG) techniques (Wild andHinckley, 2008). However, in a harsh environment, these sensorsmust be redesigned by customizing their properties and structuresto best suit the specific application. In addition to sensing reliabil-ity, standard telecommunication fibers must be replaced by specialoptical fibers for FOSN construction.

Table 1 summarizes the special optical fibers which can be usedin a wide range of high temperature applications from 300 ◦C to2050 ◦C. The polyimide-coated fiber is the cheapest, but showsthe lowest maximum sustainable temperature of 300 ◦C. Sincethe operating temperature of the containment structures of NPPs,

1890 S.Y. Chong et al. / Nuclear Engineering a

Table 1Commercial price comparison of specialty fiber optics.

Type of optical fiber Maximum sustainabletemperature (◦C)

Price ($USD)per unit meter

Polyimide-coated single fiber 300 $0.7

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Metal-coated single fiberAluminum (Al) 450 $15.0Copper/carbon (Cu/C) 600 $15.0425 �m sapphire multimode fiber 2050 $1520.0

uch as the pressurizer and reactor vessel and their piping sys-ems, can range from 275 to 345 ◦C, the polyimide-coated fiber isot appropriate for FOSN construction in most NPP containmenttructures. Moreover, prior studies have shown that the aluminumAl)-coated silica fibers and the sapphire fibers possessed radiationesistance at an acceptable level of radiation-induced attenuationhen the optical transmission was in the infrared (IR) wavelength

ange (>750 nm) (Toh et al., 2004) and above the visible lightegion (>700 nm) (Sporea and Sporea, 2007), respectively. In addi-ion, the radiation resistance of the metal-coated fibers can bemproved by heat treatment at approximately 400 ◦C before irradi-tion (Bogatyrev and Semjonov, 2007; Toh et al., 2004).

In relation to the sensing techniques, the fiber optic interfer-metric technique was applied to the sapphire fiber to develop aemperature sensor (Wang et al., 1992a,b), a strain sensor (Xiaot al., 2003), and an acoustic sensor (Claus et al., 1995) for anxtremely high temperature environment. The sapphire fiber-ased acoustic sensor was capable of detecting the out-of-planearticle displacement induced by the surface acoustic wave trans-itted on an aluminum bar by a conventional PZT transducer

ctuated at 2.25 MHz. Besides that, a fiber-optic Sagnac interfer-meter has been designed for the noncontact detection of acousticmission for structural integrity monitoring in high-temperatureower plant applications (Carolan et al., 1997). The noncontactagnac interferometer showed the detectability of AE during thehermal cycling of an exservice T-piece to 550 ◦C with a veloc-ty resolution of 50 nm s−1 Hz−1/2. However, it is very difficult to

ultiplex or multichannel the interferometric acoustic sensorsue to their complex structure and high insertion loss caused byhe Fabry–Perot cavity formed inside the fiber (Fomitchov andrishnaswamy, 2003).

The fiber Bragg grating (FBG), which offers benefits such aself-calibration, multiplexing or multipoint sensing, and multi-arameter sensing, has also been used to develop fiber opticcoustic sensors (Wild and Hinckley, 2008; Lissak et al., 1998).egarding in FBG acoustic sensing technology, narrowband demod-lation schemes converting FBG spectral shift into optical intensityariation have been proposed to perform high sensitivity interro-ation of acoustic waves (Lissak et al., 1998; Tosi et al., 2008; Tsudat al., 2009). A FBG acoustic sensor with a one-end-free FBG con-guration, called the fiber acoustic wave grating (FAWG) sensor,as been developed and simultaneous multipoint sensing technol-gy based on FAWG sensors was presented (Lee et al., 2008). Theensor configuration had a resonance feature induced by a stand-ng wave formation, and alternation of the resonance frequencyould be made by changing the one-end-free fiber length and fiberroperties. The authors also presented theoretical equations thatescribed how to control the resonance frequency (Lee and Jeong,010). However, new ideas to overcome the harsh NPP environ-ent must be included in the sensor and FOSN designs so that

BG acoustic sensors can be applied as built-in acoustic sensors

or ISHM.

In the past decade, potential applications of FBGs for harshnvironments have been studied extensively. Gratings inscribed iningle mode fibers (SMFs) have shown strong potential in providingood resistance to radiation and high temperature. The effects of

nd Design 241 (2011) 1889–1898

MGy dose level �-irradiation on the parameters of gratings writtenin H2-loaded Ge-doped fiber, N-doped fiber, and highly Ge-dopedfiber (10 mol.%, photosensitive) have been reported (Gusarov et al.,1999, 2000). These works has reported that the radiation sensitivityof FBGs exposed to high total doses (>1.5 MGy) strongly dependedon the chemical composition of the fibers. Gratings written inhighly Ge-doped photosensitive fiber exhibited the lowest sensi-tivity to �-radiation. The Bragg wavelength shift was saturated at20 pm with respect to an accumulated dose of 100 kGy, and nei-ther the amplitude nor full-width at half-maximum (FWHM) ofthe FBG spectrum were influenced by radiation. Then, the researchwork was continued by installing the gratings fabricated in thephotosensitive and the standard fibers in the BR1 low-flux nuclearreactor at SCK·CEN in Mol, Belgium from 2000 to 2008 to evalu-ate the suitability of FBG under long-term exposure environment(Gusarov, 2010). During this time the reactor was operational4690 h and the gratings received a total thermal/fast neutron flu-ence ∼16.9/1.47 × 1017 n/cm2 and a gamma-dose ∼10 MGy. Theresults demonstrated that the 5-mm Bragg gratings, which writ-ten in the photosensitive fiber (10 mol.% GeO2) without H2-loading,pre-irradiation and pre-aging, could withstand under the long termexposure and showed no significant degradation of the reflectiv-ity with only a moderate shift of the Bragg peak. In addition, thesurvivability of in-core FBGs in fission reactor, where the gratingswere written in 5% Ge-doped fiber, 20% Ge-doped fiber, and B-Ge co-doped fiber, has been reported (Fielder et al., 2004). Eventhough the work showed saturating effect of Bragg wavelength shiftas those reported in Gusarov et al. (1999, 2000), the FBG sensorsstill showed an 87% survival rate at the total dose of 2 GGy gammaand 5 × 1019 n/cm2 neutron fluence using a high-sensitivity OpticalFrequency Domain Reflectometry (OFDR) as a interrogator.

Thermal stabilization of the grating inscribed in the SMF hasbeen greatly improved based on various grating inscription tech-niques with different dopant types (Nikogosyan, 2007; Mihailovet al., 2008; Canning, 2008). The gratings were categorized intotypes I, I-IR, II, IIA, II-IR, and R (regenerated) which possesseddifferent levels of thermal resistance. For the UV-induced type Igrating, the grating written in the photosensitive Ge-doped fiber(Nufern GF1) showed good thermal resistance to temperatures ashigh as 600 ◦C (Pal et al., 2003). Recently, Aslund et al. (2010) pre-sented a pre-annealing sequence method to improve type I thermalstabilization of a grating inscribed in H2-loaded Ge-doped fiber.Aging tests showed a reflectivity reduction of less than 3 dB at400 ◦C. Then, the UV-induced type II grating (called the damagegrating) also showed remarkable high temperature stability up to1000 ◦C (Canning, 2008). In addition, Groothoff and Canning (2004)have presented an enhanced type IIA gratings, written in standardboron-codoped germanosilicate fiber, has gratings resistant in thetemperature range of 400–700 ◦C and as high as 800 ◦C for a mod-erate length of time (∼2.9 h). Formation of the other two gratingtypes was developed using an ultrafast IR laser (Mihailov et al.,2008; Li et al., 2008). These type I-IR and II-IR gratings were writ-ten in Ge-doped fiber (Corning SMF-28) and showed relatively goodthermal stability. The type I-IR grating showed the ability to sustainits initial reflectivity at a temperature of 500 ◦C, and the type II-IRgrating showed the ability to retain its initial reflectivity at a tem-perature of 1000 ◦C. On the other hand, Bandyopadhyay et al. (2008)and Canning et al. (2008) presented a regenerated fiber Bragg grat-ings in standard photosensitive fibers with transmission rejection>10% cm−1 and which can tolerate temperatures as high as 1295 ◦C.Regenerated Bragg gratings are gratins that have grown through

thermal processing at high temperature (∼900 ◦C) after the seedgrating written by UV light is erased. Furthermore, Canning et al.(2009) have also shown the realistic of the approach to producesuperposed gratings and Moire gratings that can operate at hightemperature.

ring and Design 241 (2011) 1889–1898 1891

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S.Y. Chong et al. / Nuclear Enginee

Recently, Grobnic et al. (2004) successfully inscribed a retro-eflective grating in multimode crystalline sapphire fiber using anR femtosecond laser and phase mask. The sensor showed no degra-ation of grating strength at a temperature up to 1500 ◦C. Laterork (Grobnic et al., 2006a,b) incorporated a tapered SMF andas connected to a multimode sapphire fiber Bragg grating (SFBG)

o enhance single and low-order mode reflection/transmissionesponses that could improve the spectral resolution of the SFBGrom 6 nm to 0.33 nm at FWHM. However, the taper coupling intohe sapphire fiber caused a coupling loss of ∼3 dB in the reflectedBG spectrum. In addition, taper coupling with index matchingil as the coupling medium cannot be placed in hot NPP struc-ures without contrivance of a special protective technique. Evenf sapphire fiber is used only for sensor fabrication (not a FOSNonstruction), a length of several tens of millimeters is needed forrating inscription and coupling. As shown in Table 1, therefore, therice of the sapphire fiber is unreasonable for FOSN applications.

ndeed, the unique temperature resistance capability of the sap-hire fiber was intended for much higher temperature applicationshan NPPs.

According to the preceding literature review, we consideredhe high temperature and irradiation resistance of FBGs for built-n acoustic sensor development at a reasonable cost, single modeperation providing low loss and a superior FBG spectral profile,nd excellent adaptability to conventional fiber optic communi-ation of the metal-coated fiber for built-in FOSN construction. Inhis study, we designed a FBG acoustic sensor using a metal-coatedber optimized for the NPP environment. A chemical method wasroposed to remove the coatings of the Al-coated fiber and the cop-er/carbon (Cu/C)-coated fiber. Regarding the grating inscriptionechnique, according to the conclusion of aforementioned litera-ure survey, the type I grating written in photosensitive Ge-dopedber that can endure 600 ◦C was optimal for NPP applicationsonsidering grating inscription cost, mechanical strength, and theigh temperature and irradiation environments. Unfortunately, aetal-coated photosensitive Ge-doped fiber is not yet commer-

ially available. Therefore, the type I grating was inscribed in the

ore of the available metal-coated 5 mol.% Ge-doped fiber withouturther verification on irradiation. Then, the FBG was installed withone-end-free FBG configuration using high temperature metallicdhesive on a stainless steel vessel. The bonding quality and reflec-ivity of the FBG were tested under high temperature cycles, and

Fig. 2. (a) Copper coating removal setup, (b) carbon coating remova

Fig. 1. Structures of (a) Cu/C-coated, and (b) Al-coated optical fibers (Cu/C: cop-per/carbon, Al: aluminum).

the AE wave and laser-induced ultrasound detection capabilities ofthe new sensor were demonstrated through pencil lead breakingtest and Q-switched laser excitation.

2. Fabrication of fiber Bragg grating in acopper/carbon-coated fiber

The specifications of the Cu/C-coated and Al-coated fibers areshown in Fig. 1. The Cu/C-coated fiber (IVG Fiber, Cu1300) has acarbon coating for the first layer and copper coating for the secondlayer. The carbon layer (hermetic coating) improves the reliabilityof the fibers, thereby preventing strength degradation caused bymoisture attack on the fiber surface and preventing the diffusion ofhydrogen into the core of the fiber (Lemaire and Lindholm, 2007).On the other hand, the commercially available Al-coated fiber (IVGFiber, Al1300) has only aluminum coating, and the core chemical

composition of both fibers is 5 mol.% Ge-doped silica.

To write a Bragg grating of 5 mm in the core of the Cu/C-coatedfiber, the 7 mm section of the copper/carbon coatings was removedusing the apparatus shown in Fig. 2. The two ends of the Cu/C-

l setup, and (c) its sectional-view (l: coating removal length).

1892 S.Y. Chong et al. / Nuclear Engineering and Design 241 (2011) 1889–1898

F the (cc

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ig. 3. Microscopic images (a) before, and (b) after the copper coating removal; atoating removal; (e) before, and (f) after the Al coating removal.

oated fiber were taped onto two sponges as shown in Fig. 2(a).hen, a volume of approximately 2 �l nitric acid (11 mol/L HNO3)t the tip of a 10 �l pipette was carefully moved to the Cu/C-coatedber where 1 mm of the copper coating section was soaked in theitric acid for chemical reaction. The colorless nitric acid changed topale blue color during the chemical reaction because the copperas dissolved in the nitric acid solution. This process was repeatedntil the 7 mm-long copper coating was completely removed. Thehemical reaction took about 5 min to remove the 7 mm copperoating. Fig. 3(a) and (b) shows microscopic images before and afteremoval of the copper coating. As shown in Fig. 3(b), the carbonoating as the first layer and the partial sections of the bare silicalad were revealed. It is thought that some carbon coating parts noterfectly bonded to the silica clad were peeled off together with thehin carbon coating during the nitric acid reaction. This is becausehe carbon coating did not show a chemical reaction, and couldot be removed for further application of the nitric acid. There-

ore, sodium hydroxide solution (10 mol/L NaOH) was applied toemove the carbon coating as the first layer using the setup shownn Fig. 2(b) and (c). Since the chemical reaction between the carbon

oating and the sodium hydroxide was very slow, the apparatusas modified so that the carbon coating could soak in the sodiumydroxide solution for a longer period. Both sides of the top sur-

ace of the thin glass slide were taped to form a groove using andhesive thickness of 50 �m. The width of the groove was set to

) edge, and (d) in the center of the Cu/C coatings-removed section after the carbon

the same length (l = 7 mm) as the removed copper coating length,as shown in Fig. 2(b) and (c). The carbon coating section was thenlaid on the groove. Then, the Cu/C-coated fiber was taped at bothedges as shown in Fig. 2(c). Warmed sodium hydroxide solution(about 65 ◦C) was used to accelerate the chemical reaction. Thewarm sodium hydroxide solution was filled inside the groove todissolve the carbon coating. The amount of the solution (<5 �l) wascarefully controlled to prevent overflow. During the chemical reac-tion, the colorless sodium hydroxide solution showed no changein color although the carbon was being dissolved. The total timeto remove the carbon coating was reduced from 12 h to 5 h dueto the use of the warmed solution. Fig. 3(c) and (d) shows micro-scopic images at the edge and center of the clean bare fiber section,respectively. As shown in Fig. 3(c), the diameters of the silica cladand core were measured as 126.39 �m and 8.94 �m, respectively,and were almost the same as the specification shown in Fig. 1(a).

The coating of the other metal-coated fiber, i.e. Al-coated fiber,was also removed using the setup shown in Fig. 2(a) and thesodium hydroxide (10 mol/L NaOH) that used to remove the carboncoating from the Cu/C-coated fiber. During the chemical reaction,

the colorless sodium hydroxide solution showed no change incolor although the aluminum coating was dissolved. The aluminumcoating was completely removed after approximately 5 min. Micro-scopic images before and after the aluminum coating removalare shown in Fig. 3(e) and (f), respectively. Fig. 3(f) shows that

S.Y. Chong et al. / Nuclear Engineering and Design 241 (2011) 1889–1898 1893

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Since the FBG acoustic sensor was designed for harsh environ-

ig. 4. (a) FBG spectra for the four FBGs inscribed in the Cu/C-coated fiber, (b) theu/C-coated fiber with a 5 mm FBG and spliced with the standard SMF with a FC/PConnector (FBG: fiber Bragg grating, SMF: single mode fiber).

he diameter of the bare silica fiber was approximately 155 �m,hich is slightly larger than the manufacturing specification

n Fig. 1(b).In the grating inscription process, even if both metal-coated

bers have thermal resistances appropriate for NPP application,he Cu/C-coated fiber was considered because copper has betteradiation resistance than the aluminum (Maher, 2006). We pre-ared four samples of the Cu/C-coated fiber with a 7 mm-longoating-removed section to inscribe a 5 mm-long grating. A KrFxcimer laser (248 nm) and a phase mask were used to induceragg gratings at a pulse energy of 16 mJ, and the refractive indexodulation was set to 10−3. As shown in Fig. 4(a), the gratingsere successfully inscribed in the silica cores of the Cu/C-coatedbers. As shown in Fig. 4(b), one Cu/C-coated fiber with a 5 mmrating could be then spliced into the standard SMF without lossecause the silica core/clad structure of the Cu/C-coated fiber was

dentical to the standard one. The initial reflectivity of the FBGsas greater than 71%, and all the FWHMs of the FBG spectra wereearly 400 pm as shown in Fig. 4(a). These results demonstratehat a FBG can be successfully fabricated in a Cu/C-coated fiberue to the proposed coating removal process, and the FBG canrovide the same optical quality as the FBG written in a standardelecommunication SMF.

. Design of Cu/C-coated fiber Bragg grating acoustic sensor

In Fig. 4(a), FBG 4 was used to fabricate a FAWG sensor usingone-end-free FBG configuration. Its resonance frequency can

Fig. 5. (a) Schematic, and (b) photo of configuration of the Cu/C-coated FAWG sen-sor using metallic adhesive for high temperature application (FAWG: fiber acousticwave grating sensor).

be predetermined using the following equation (Lee and Jeong,2010):

fn = 2n − 14l

cf (1)

where n is a positive integer on the order of a wave mode, andcf = 5.75 m/ms is the group velocity of the fiber-guided acousticwave for the bare fiber. To install the FBG in the one-end-freeconfiguration, an adhesive should be applied between the fibersection having the coating and the structure because the bare silicasection with the grating is fragile to adhesive deformation duringcuring and to thermal deformation during high temperaturecycles. In other words, the bare fiber section should be cleanedfrom the adhesive. As shown in Fig. 5(a), the metallic adhesivewas applied only until the coating end and the other end was cutfor the one-end-free configuration. Therefore, the one-end-freelength l, defined as the sensing fiber length, was 7 mm; thus,the theoretical resonance frequencies of the FBG acoustic sensorwere f1 = 205.4 kHz and f2 = 616.2 kHz. In fact, the sensing fiberlength was controlled from the coating removal step because theresonance frequency of the FBG acoustic sensor should be in auseful range for NPP application. The first resonance frequencyof the FBG acoustic sensor is suitable to detect leaks in the con-tainment building, which have a frequency range from 100 kHz to400 kHz (Chokshi et al., 2005). For a different resonance frequencydepending on the failure event, the sensor head can be redesignedbased on selection of the coating removal length.

Fig. 5(b) shows a photo of the Cu/C-coated FAWG sensor witha one-end-free FBG and metallic bonding configurations. A hightemperature metallic adhesive (Stainless Steel Durabond 954-1)with an operating temperature of about 1000 ◦C was applied toonly the Cu/C-coating section as shown in Fig. 5(b), thereby nottouching the bare fiber section as shown in Fig. 5(b), and curedat room temperature for 24 h. Two metal supporters were used toplace the FBG and the Cu/C-coated fiber in the respective groovesto make the fiber straight, and to prevent the FBG from beingbent and thereby contact the target surface during adhesive curing.

mental conditions, the FBG acoustic sensor must not be debondedfrom the target surface at operating temperatures of NPP. Other-wise, the acoustic wave guided in the target structure cannot betransferred to the FBG via the bonding area. Finally, the metal

1894 S.Y. Chong et al. / Nuclear Engineering and Design 241 (2011) 1889–1898

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Time (hours)

ig. 6. Tested temperature cycles for (a) Cu/C-coated fiber, and (b) Cu/C-coatedAWG sensor.

upporters were removed so that the Cu/C-coated FAWG sen-or installed with the metallic adhesive may be used for acousticensing. In practice, the bare silica section would be protectedith a half-circle cylindrical cap for the real field as illustrated in

ig. 5(a) because the workers can break the barely visible bare fiberection.

. Experiment setup

In this work, experiments were conducted to evaluate the trans-itted optical power in a Cu/C-coated fiber and to investigate

he reliability of the optical spectra, acoustic signals, and sensorntegration of the Cu/C-coated FAWG sensor using the metallicdhesive. NPP structures will be maintained at their operatingemperature and be cooled to room temperature for the overhaulrocess at a period of a few years depending on the age of thePP. In this regard, the temperature cycle used to test the tem-erature effect on a Cu/C-coated fiber and a Cu/C-coated FAWGensor installed in a stainless steel vessel was modeled by ramp-ng from 25 ◦C to 345 ◦C over 30 min, holding for 2 h, then coolingver 6 h from 345 ◦C to 25 ◦C inside a furnace. This was repeatedour (ten) times for Cu/C-coated fiber (Cu/C-coated FAWG sensor)s shown in Fig. 6(a) and (b). For Cu/C-coated fiber case, the trans-itted optical power was acquired at points A, B, and C of each

emperature cycle (Fig. 6(a)) by a NTL source and a power meter.hile Cu/C-coated FAWG sensor case, the reflectivity of the FBG

pectrum was acquired at each room temperature part of the cyclesing a broadband light source and an optical spectrum analyzerith a resolution of 20 pm.

Fig. 7(a) shows the experimental setup used to evaluate theransmitted optical power of 1m-length Cu/C-coated fiber. Theu/C-coated fiber was spliced with the two SMFs. Then, the SMFsere connected to a NTL and a power meter respectively as shown

n Fig. 7(a). The NTL was tuned at a fixed-wavelength of 1561.1 nm

ith optical power of 4 mW, and the transmitted optical power in

he Cu/C-coated fiber was monitored in a power meter. Fig. 7(b) andc) shows the experimental setups used to investigate the acous-ic waveform characteristics of the Cu/C-coated FAWG sensor. Atrst, the frequency response of the Cu/C-coated FAWG sensor was

narrowband FBG demodulation system and installed in the vessel using metallicadhesive for pencil lead break test, and (c) photo for laser ultrasonic excitation setup(NTL: narrowband tunable laser, DO: digital oscilloscope, PD: photodetector).

investigated through AE input excited by a pencil lead breaking(Robin et al., 2007) at the point indicated in Fig. 7(b). Then, the signalresponse was interrogated using an acousto-ultrasonic demodula-tion system based on a narrowband demodulation scheme (Lissaket al., 1998). As shown in Fig. 7(b), a tunable laser with a line widthof 100 kHz was connected to the Cu/C-coated FAWG sensor via anoptical circulator, and the lasing wavelength was matched to theleft or right mid-reflection point of the FBG spectrum. A photode-tector with a bandwidth of 150 MHz at the 0 dB gain option wasconnected to the third pigtail of the optical circulator to convertthe variation of light intensity reflected from the FBG into an alter-

nating current (ac) voltage signal. A high pass filter was used toreject optical noise with a 20 kHz cut-on frequency before the dig-ital oscilloscope (DO). Then, laser-induced ultrasound was used toinvestigate the response characteristics of the Cu/C-coated FAWG

ring and Design 241 (2011) 1889–1898 1895

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S.Y. Chong et al. / Nuclear Enginee

ensor. The laser ultrasonic excitation (Davies et al., 1993) was per-ormed with the setup shown in Fig. 7(c). A Q-switched laser withwavelength of 1064 nm and a pulse energy of 1 mJ was used to

rradiate pulsed laser beams with a pulse repetition rate of 20 Hzt the same point on the stainless steel vessel as the pencil-leadreak test, and the stand-off distance between the laser pinholend the impinging point on the vessel was 500 mm. The ultrasonicave generated by thermal expansion of the laser-irradiated shal-

ow area modulated the grating pitch of the Cu/C-coated FAWGensor and was interrogated by the acousto-ultrasonic demodu-ation system using a band-pass filter from 170 to 190 kHz. Eachltrasonic wave was digitized in the form of a running average over12 acquisitions.

. Results and discussion

Fig. 8(a) shows the history of transmitted optical power and theptical power at the initial room temperature could be maintaineduring the four time–temperature cycles. The result implies theuitability of Cu/C-coated fiber to be used as a transmission mediumor FOSN in NPP applications.

As shown in Fig. 8(b), the Bragg wavelength was slightly shiftedrom 1561.1 nm to 1560.8 nm after the first cycle of the tempera-ure cyclic process shown in Fig. 6(b). The phenomenon occurredecause the FBG was isothermally annealed for 2 h at a temperaturef 345 ◦C corresponding to the first holding step. Then, for consecu-ive cycles, the Bragg wavelength showed no shift from 1560.8 nmven after the ten cycles because the maximum temperature forach cycle was no more than 345 ◦C. On the other hand, the reflec-ivity of the FBG dropped by 47% after the first cycle because themplitude of the core refractive index modulation of the gratingas decreased due to the temperature. As shown in Fig. 8(c), the

BG reflectivity was dropped by 58% at the third cycle and was thentabilized at the rest of cycles, which is similar to the result reportedy (Aslund et al., 2010)

Fig. 9 shows the AE waves and their spectra detected withhe acousto-ultrasonic demodulation system. Fig. 9(a) and (b) arebtained by launching the lasing wavelength to the left and rightid-reflection points of the FBG spectrum, respectively. The FAWG

ensor is a resonant type based on the standing wave formationetween the fiber end and the bonding edge (Lee and Jeong, 2010).s shown in the spectral responses in Fig. 9, the resonance featureemained in the Cu/C-coated FAWG sensor. The detected resonancerequencies for both slope cases were the same: fl = 180 kHz and2 = 550 kHz. The differences between the experimental and theo-etical results in the first and second resonance frequencies were5.4 kHz for fl and 66.2 kHz for f2, respectively.

Fig. 10 shows the laser ultrasonic experimental result obtainedsing the setup from Fig. 7(b) and (c). The ultrasonic signals forhe left mid-reflection point exhibited a higher sensitivity thanhose for the right mid-reflection point. This is because the leftlopes of the FBG spectra in Fig. 8(b) were steeper than the rightlopes. A steeper slope provides higher sensitivity because theltrasound-induced small wavelength shift is converted into a light

ntensity variation along the slope in this narrowband demod-lation. Fig. 10 also shows that the Cu/C-coated FAWG sensoran clearly detect zeroth-order symmetrical (S0) and asymmet-ical (A0) modes (called the fundamental modes of Lamb wave)uided in the plate-like structure. The amplitude of the S0 modebtained before the temperature cyclic process was less sensitive

han that obtained after the first cycle, although the reflectivityefore the temperature elevation was higher than after the firstycle. Even if the reflectivity was dropped by 48%, the sensitivityf the Cu/C-coated FAWG sensor was enhanced because ultra-onic energy transfer was improved by metallic adhesive hardening

Fig. 8. High temperature effect: the histories of (a) transmitted optical fiber andFBG spectrum in (b) profile, and (c) reflectivity.

due to the temperature elevation. In addition, the Cu/C-coatedFAWG sensor, even after the tenth cycle, still showed similarsensitivity.

The laser ultrasonic measurement was not performed at hightemperature because the furnace did not have an optical windowfor laser beam transmission. However, the FAWG sensor using themetallic adhesive was not debonded and no damage was found inthe Cu/C-coated sensor line and the FBG spectrum during the tencycles from 25 ◦C to 345 ◦C including 2 h holding at 345 ◦C; there-fore, this sensor configuration will be able to be applied to hightemperature NPP structures because the temperature effects on

the ultrasonic wave are restricted in change of its ultrasonic ampli-tude and time of flight (Raghavan and Cesnik, 2008; Konstantinidiset al., 2006; Lu and Michaels, 2005; Lanza di Scalea and Salamone,2008).

1896 S.Y. Chong et al. / Nuclear Engineering and Design 241 (2011) 1889–1898

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

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0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

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Left mid-reflection point of FBG spectrum Right mid-reflection point of FBG spectrum

(b)(a)

Fig. 9. Time and frequency responses with respect to the AE wave generated by pencil lead breaking when the lasing wavelength of NTL was matched to the (a) left, and (b)right mid-reflection points of the FBG spectrum (AE: acoustic emission).

Right mid-reflection point of FBG spectrumLeft mid-reflection point of FBG spectrum

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(a) (b)

Fig. 10. Time and frequency responses with respect to the ultrasonic wave generated by pulsed laser beam impinging when the lasing wavelength of NTL was matched tothe (a) left, and (b) right mid-reflection points of the FBG spectrum.

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S.Y. Chong et al. / Nuclear Enginee

. Conclusion

The design constraints of a FBG acoustic sensor in relation toarsh environment applications were studied. Prior works con-luded that FBGs and metal-coated fibers have good radiation andigh temperature resistance. Metal-coated fibers were the mostconomical and reliable for both optical communication and sens-ng applications in a NPP environment. Thus, a Cu/C-coated FAWGensor using metallic adhesive was developed for ISHM of highemperature NPP structures. A chemical method and its appa-atus were proposed to remove Cu/C and Al coatings. A 5 mmBG was successfully written in the Ge-doped silica core through

7 mm-long coating-removed silica section of a Cu/C-coatedber. Another advantage was the Cu/C-coated fiber had the sameore/clad structure as the standard SMF, which allowed no-lossusion splicing and good adaptability to the economical standardber, adaptor, connector, and instruments. It showed also goodhermal resistance (<345 ◦C) without degradation in optical poweruring the optical transmission. The high temperature metallicdhesive showed superior bonding reliability during temperatureycles from 25 ◦C to 345 ◦C, including a 2 h holding at 345 ◦C. Theeflectivity of the type I grating written in the 5 mol.% Ge-dopedore was stabilized at a 58% drop from the initial reflectivity.he Cu/C-coated FAWG sensor with metallic adhesive successfullyetected acousto-ultrasonic waves generated by pencil lead break-

ng and laser beam excitation. The type I grating was inscribedn the 5 mol.% Ge-doped core because the highly Ge-doped pho-osensitive Cu/C-coated fiber was not yet commercially available.f a photosensitive Ge-doped Cu/C-coated fiber becomes avail-ble, we can expect a sensitivity improvement of at least 2.75imes because the grating inscribed in the photosensitive Ge-dopedore will maintain its reflectivity at high temperature, as indicatedn Fig. 8(b) and (c).

cknowledgements

This work was supported by the MEST/NRF (Nuclear R&Drogram, 2009-0091279), and partially supported by the Koreainistry of Land, Transport and Maritime Affairs as Haneul Project.

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