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A preliminary study on noncontact imaging inspection for internal defects of plate-type nuclear fuel by using an active laser interferometer
Nuclear Engineering and Design 256 (2013) 153–159
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
A preliminary study on noncontact imaging inspection for internal defects of plate-type nuclear fuel by using an active laser interferometer Nak-Gyu Park a , Seung-Kyu Park a,∗ , Sung-Hoon Baik a , Young-June Kang b a b
Korea Atomic Energy Research Institute, 1045 Daedeokdaero, Yuseong-gu, Daejeon 305-353, Republic of Korea School of Mechanical Design Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea
h i g h l i g h t s
An active laser interferometer was preliminarily studied to visually detect internal defects of plate-type nuclear fuel. The developed active optical interferometer system showed vivid defect images. The adopted flexible holder improved the detection efficiency for the internal defects by concentrating the strains. The filtering technique, which removes the whole-body variation, was effective in improving the defect detection capability. The developed noncontact imaging technique can be a useful nondestructive inspection tool for plate-type nuclear fuel.
a r t i c l e
i n f o
Article history: Received 26 July 2012 Received in revised form 22 November 2012 Accepted 24 November 2012 PACS: 07.60.Ly 07.20.-n
a b s t r a c t A non-contact optical imaging technique was preliminarily studied to detect internal defects of a platetype nuclear fuel specimen by using an active laser interferometer. A sinusoidal and periodic thermal wave induced periodic micro deformation including internal defect information of the specimen. The laser interferometer acquired a micro deformation image for the specimen surface, the shape of which varied according to the energy of the thermal wave. An adopted flexible holder of the specimen increased the capability of defect detection by concentrating the strains at the defects. A real-time inspection image including internal defect information was produced after the signal processing to remove noises. As a nondestructive inspection system, the developed visual inspection technique using an active laser interferometer can be a valuable tool to detect the internal defects of plate-type nuclear fuel. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Most research reactors have adopted the use of plate-type nuclear fuel, which consists of a fuel core in aluminum alloy. The production quality of nuclear fuel is an important part for an efficient and stable generation of thermal energy in research reactors. Thus, a nondestructive quality inspection for the internal defects of plate-type nuclear fuel is a key process during the production of nuclear fuel for safety assurance. Nondestructive quality inspections based on X-rays and ultrasounds have been widely used for the defect detection of plate-type nuclear fuel (Barrna, 1979; Ghosh et al., 1992; Knight and Morin, 1999; Katchadjian et al., 2002; Borring et al., 2002; Mucio et al., 2009). X-ray testing is a simple and fast inspection method, and provides an image as the inspection results (Barrna, 1979; Ghosh et al., 1992). Thus, testing can be carried out by a non-expert field worker.
∗ Corresponding author. Fax: +82 42 861 8292. E-mail address: [email protected] (S.-K. Park). 0029-5493/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2012.11.015
However, it is hard to detect closed-type defects because the Xray image is made from the density differences of the materials. In particular, it is hard to detect delamination defects that should be detected during the production of plate-type nuclear fuel. Thus, an X-ray inspection is generally used for an inspection of the filling status of nuclear fuel in aluminum alloy. Ultrasonic testing is a powerful tool for detecting internal defects including open-type and closed-type defects in plate-type nuclear fuel (Knight and Morin, 1999; Katchadjian et al., 2002; Borring et al., 2002; Mucio et al., 2009). The testing can also provide thickness information for each internal fuel plate. A high frequency ultrasonic signal of over 20 MHz is usually adopted because the thickness of each plate in plate-type nuclear fuel is about 1 mm to sub-millimeters thin. The inspection process is complicated because an immersion test should be carried out in a water tank. It is also a time-consuming inspection method because area testing is based on a scanning of the point-by-point inspections. In addition, this testing method needs to be carried out by an expert since the detection capability is greatly dependent on the inspector’s knowledge.
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Fig. 1. Block diagram of the developed noncontact optical imaging inspection system using an active laser interferometer.
In this paper, an optical imaging technique using an active laser speckle interferometer with periodic thermal energy was preliminarily studied to visually detect internal defects. The laser speckle interferometer is an optical technique used to measure micro changes in the surface topography (Malacar, 1992). This method is sensitive to micro-displacement at a nanometer resolution by superposing the speckle patterns of two different object states. Amplitude differences in deformation among intact and defective areas have been widely used for the detection of internal defects in plate specimens (Habib, 2005; Taillade et al., 2011; Huang et al., 2009; Liu et al., 2011; Hung et al., 2009). Phase information has been also used when the phases of the modulation frequencies in displacement among intact and defective areas are different during the periodic deformation of a plate specimen (Menner et al., 2011; Dolinko and Kaufmann, 2007; Gerhard and Busse, 2006; Menner et al., 2008; Zhu et al., 2011). Amplitude information on the modulation frequencies, and nonlinear information on the harmonic frequencies have been also used to detect internal defects including small closed-type defects (Menner et al., 2011; Dolinko and Kaufmann, 2007; Gerhard and Busse, 2006; Menner et al., 2008; Zhu et al., 2011; Zhang, 2009). The developed active laser interferometer measures an interference image, which is a micro-deformation image of a specimen surface including the internal defect information. Thus, the inspection can be carried out by a non-expert field worker. Here, the surface deformation is induced by a heater supplying a thermal wave with periodic sinusoidal intensities. The inspection images can be displayed on a monitor in real time, and visual images including internal defect shapes after signal processing to reduce noises can also be displayed. Computer simulations of thermal deformation and experiments using the developed active laser interferometer to detect the internal defects of a plate-type nuclear fuel specimen are described in this paper. 2. Experimental setup for noncontact imaging inspection A block diagram of the developed noncontact imaging inspection system using an active laser interferometer with a thermal wave is shown in Fig. 1. The noncontact imaging inspection system was configured using two halogen lamps with a driver (DDS 5600, LEVITON MFG. co) controlled by a signal generator, a test specimen held by a flexible holder, a digital speckle pattern laser interferometer (CW laser: 532 nm, 100 mW, Cobolt SambaTM 150) with a phase control driver (E-665.XR, LVPZT-AMP, PI), and a main control computer.
Fig. 2. Dimensions of the test specimen for plate-type nuclear fuel.
As shown in Fig. 1, the deformation shape of the specimen surface will vary according to the applied thermal energy at the back side of the specimen. The thermal wave is generated by two halogen lamps driven by an amplifier. The intensity of the thermal wave is a periodic sinusoidal pattern controlled by a signal generator. The digital speckle pattern interferometer acquires a deformation interference image for the front surface of the specimen. The deformation interference image is an interference image between a reference laser beam and a signal laser beam. As shown in Fig. 1, the reference laser beam reflected at the beam splitter (BS) goes into the CCD camera after passing through a mirror (M), a spatial filter (S. F), a mirror (M), and a focusing lens. The signal laser beam reflected from the front surface of a specimen goes into the CCD camera after passing through the beam splitter (BS), mirrors (M), and a spatial filter (SF). Here, the phase of the signal beam can be shifted using a piezoelectric transducer (PZT) controlled by a computer. Internal defect information is contained in the acquired interference image because the deformation difference is mainly caused by the reflection at the internal defects of the thermal wave. An extracted deformation image including internal defect information will be produced after signal processing of the laser fringe images by a computer. As shown in Fig. 1, a specimen was held by a flexible holder supported by spring force in the length direction of the specimen. Here, the specimen is freely movable in the thickness direction, but the movement is not free in the length direction when thermal energy is applied. This flexible holder will increase the detection capability of the internal defects by increasing the strain deformation to the defect area. Plate-type nuclear fuels consist of several thin plates containing a plate core of uranium compounds in a metallic matrix, such as dispersions of U3 O8 –Al and U3 Si2 –Al, with aluminum cladding. The dispersions, produced by powder metallurgy processes in the form of briquettes, are used to obtain the plate-type nuclear fuels using the picture frame technique. The briquette is mounted in a frame, and the surfaces of this set are covered by aluminum foils. Platetype nuclear fuel is then obtained by submitting the structure to a hot rolling process under controlled conditions. As shown in Fig. 2, a similar test specimen was prepared for the experiments in this paper owing to a lack of proper production facilities for plate-type nuclear fuel in our laboratory. A tungsten plate (B) with a similar density was used instead of uranium compounds. The thickness of the tungsten plate (B) was 1.0 mm, and
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Table 1 The mechanical properties of the materials used for the simulation specimens. Material
Tungsten
U3 Si2 –Al
Aluminum
Density Young’s modulus Poisson’s ratio Coefficient of thermal expansion Thermal conductivity
19.3 g/cm3 400 GPa 0.28 4.4 e−5 /C
6.11 g/cm3 58 GPa 0.34 1.48 e−5 /C
2.69 g/cm3 71 GPa 0.33 2.3 e−5 /C
0.163 W/mm C
0.181 W/mm C
0.237 W/mm C
Fig. 3. Simulated deformation image based on the thermal energy for plate-type nuclear fuel having a core plate of tungsten.
Fig. 4. Simulated deformation image based on the thermal energy for practical plate-type nuclear fuel having a core plate of U3 Si2 –Al.
the thicknesses of the aluminum alloy at the front and backside were 0.5 mm, respectively. Here, the typical thickness of each component widely used in the field was selected. Three plates were bonded using metal paste except the two areas of delaminationtype defects, D and E, as shown in Fig. 2. The sizes of the two internal defects were 3.0 mm × 3.0 mm and 5.0 mm × 5.0 mm, respectively. The dotted rectangle in A of Fig. 2 is the viewing area of the CCD camera in Fig. 1. 3. Simulation of thermal deformation and experiments on detecting internal defects by using an active optical noncontact imaging inspection A computer simulation was conducted by using ANSYS software to see the pattern of thermal deformation and the deformation difference between the prepared test specimen and practical platetype nuclear fuel. Two test specimens having a plate core of tungsten and U3 Si2 –Al, respectively, were designed using software. The structures and dimensions were the same as the test specimen of Fig. 2. The material properties used for the simulation are shown in Table 1. Here, the material properties of U3 Si2 –Al were selected from the published test data (Wang and Xu, 2003; Williams et al., 1985). To induce the thermal deformation of the specimen, a heat flow of 80 W was applied to the front aluminum plate for 0.5 s from an initial temperature of 40 ◦ C. The simulated deformation images for the plate-type nuclear fuel having a tungsten and U3 Si2 –Al core are shown in Fig. 3 and Fig. 4, respectively. The relative deformations from the minimum surface value for the delamination defects, 5 mm × 5 mm in size as shown in Figs. 3 and 4, are 1.41 m and 6.52 m, respectively. The relative deformations for the defects 3 mm × 3 mm in size are 1.04 m and 4.80 m, respectively. Here, the deformation amplitude from the U3 Si2 –Al
Fig. 5. Simulated deformation image for fixed plate-type nuclear fuel having a core plate of tungsten.
Fig. 6. Simulated deformation image for fixed plate-type nuclear fuel having a core plate of U3 Si2 –Al.
specimen is larger than the tungsten specimen, and the deformation difference between the two specimens is about several micrometers on the internal defects. From the simulation results, we can see that an optical interferometer can measure the thermal deformation image of plate-type nuclear fuel since the practical measurement resolution of the laser interferometer is usually about 0.01–0.05 m. Deformation images acquired by a computer simulation for fixed plate-type nuclear fuels having a tungsten core and U3 Si2 –Al core are shown in Figs. 5 and 6, respectively. Here, the length direction, left and right sides, of the Z-axis for each simulation sample was fixed as shown in Figs. 5 and 6. The relative deformations from the minimum surface value for the delamination defects 5 mm × 5 mm in size are 7.24 m and 10.24 m, respectively. The relative deformations for the defects 3 mm × 3 mm in size are 3.81 m and 7.01 m, respectively. The image distinctness of the internal defects from the acquired image was improved by fixing the length direction of the Z-axis. A periodic thermal wave with sine pattern intensities was applied to the specimen for the experiments. The shape of the specimen was periodically varied according to the energy of thermal wave. The applied pattern of the thermal wave and the signal processing flowchart of the developed noncontact imaging inspection system are shown in Figs. 7 and 8, respectively. To obtain a relatively deformed image, a reference phase image was extracted using a digital laser speckle interferometer at the user-set voltage time. In this experiment, the user-set time was selected as the lowest thermal energy time of (0), as shown in Fig. 7. Here, the reference phase image was extracted from four speckle images with phases of 0◦ , 90◦ , 180◦ , and 270◦ at time (0) using a 4-bucket algorithm (Cheng and Wyant, 1985). After extracting a reference phase image, the control computer extracted the current deformation image by subtracting a reference phase image from a current phase image, which was also calculated from the four phase-shifted speckle images at the current time. In this experiment, the control computer acquired about 30 deformation images in one period of 10 s. The information regarding the internal defects can be extracted after the signal processing to remove the noise components, such as the shape variation of the whole specimen surface according to the thermal energy, as shown in Fig. 8. This function was carried out by using a band-pass filter operated in the frequency domain since the frequency band of the whole shape variation is usually
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Fig. 7. Thermal wave pattern of a developed noncontact imaging inspection system.
Fig. 9. Wrapped phase maps during the 4th period of thermal energy from a flexible holder.
Fig. 8. Signal processing flowchart of the developed noncontact imaging inspection system.
low. Next, a low-pass filter in the time domain was carried out to remove impulse-type noise, which rarely exists in real environments. The computer can only display the inspected images when the input voltage of the heat is within the user set level band. This viewing technique can supply a more distinct defect image since defect information is well detected in a particular time band, such as the lowest energy time of each period. Wrapped phase maps during the 4th period of the thermal energy shown in Fig. 7 from a flexible holder supported by spring force and a one-sided free holder are shown in Figs. 9 and 10, respectively. Here, the one-sided free holder had a soft sponge holder, which was replaced on one side (B2) of the flexible holder shown in Fig. 1. The unwrapped phase images corresponding to those in Figs. 9 and 10 are shown in Figs. 11 and 12, respectively. As shown in Figs. 11 and 12, the flexible holder provided more distinct defect images than the one-sided free holder. The deformation images after signal processing for the images in Figs. 11 and 12 are shown in Figs. 13 and 14, respectively. The distinctness in the defect images was improved after removing the low-frequency components, which mainly contain the whole shape variation of the specimen surface according to the thermal energy. Herein, the applied band-passing range was 5–200 pixels
Fig. 10. Wrapped phase maps during the 4th period of thermal energy from a onesided free holder.
for the band-pass filter in the frequency domain. The image size was 512 × 512 pixels. As shown in Figs. 13 and 14, the defects were more distinct at around the lowest energy time of the thermal wave by virtue of the remaining strains in the defect areas. The normalized images for the internal defect area of Figs. 13 and 14 are shown in Figs. 15 and 16, respectively. By virtue of the strain concentration at the interior of the specimen, the flexible holder provided more distinct defect images than the one-sided free holder.
N.-G. Park et al. / Nuclear Engineering and Design 256 (2013) 153–159
Fig. 11. Unwrapped images for the wrapped phase maps during the 4th period of thermal energy from a flexible holder.
Fig. 12. Unwrapped images for the wrapped phase maps during the 4th period of thermal energy from a one-sided free holder.
The unwrapped deformation images at the lowest energy time of the thermal wave from the flexible holder and the one-sided free holder are shown in Figs. 17 and 18, respectively. Here, the images are from the 1st through 14th periods, and the 17th through 30th periods. As shown in Figs. 17 and 18, more distinct defect images were provided by the flexible holder. As the experimental results, internal defects were visually shown after the 2nd period of applied periodic thermal wave using
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Fig. 13. Deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a flexible holder.
Fig. 14. Deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a one-sided free holder.
a digital speckle pattern laser interferometer. A band-pass filter operated in the frequency domain improved the detection capability of defects by removing the noise of the whole body variation caused by a sinusoidal thermal wave, except for the defect information. In addition, the adopted flexible holder supported by spring force improved the detection efficiency of defects by concentrating the strains at the interior.
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Fig. 15. Normalized deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a flexible holder.
Fig. 16. Normalized deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a one-sided free holder.
Fig. 17. Unwrapped deformation images at the lowest energy time of the thermal wave from a flexible holder.
Fig. 18. Unwrapped deformation images at the lowest energy time of the thermal wave from a one-side free holder.
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4. Conclusion In this paper, a nondestructive testing technique using an active digital speckle pattern interferometer was preliminary investigated to visually detect internal defects for plate-type nuclear fuel. The developed nondestructive noncontact imaging inspection system was used to detect delamination defects of a similar test specimen, where such defects should be detected during the production process for safety assurance. The developed speckle interferometer system showed vivid defect images after the second period of the applied periodic thermal wave. The adopted flexible holder in the system improved the detection efficiency of the internal defects by concentrating the strains. In addition, the developed filtering technique, which removes the whole-body variation of the specimen surface according to the thermal energy except the defect components, was effective in improving the capability of defect detection. As the preliminary results of the computer simulation and experiments, the developed noncontact imaging technique using an active laser interferometer can be a useful nondestructive inspection tool for plate-type nuclear fuel. Acknowledgements This work was supported by the Ministry of Education, Science and Technology of Korea (grant No. WCI 2011-001). References Barna, B.A., 1979. The suitability of X-ray paper as an inspection tool for flat plate nuclear fuel. In: ANS Winter ‘79 Meeting, November 11-16, San Francisco, CA. Borring, J., Gundtoft, H.E., Borum, K.K., Toft, P., 2002. Non-destructive evaluation of the cladding thickness in leu fuel plates by accurate ultrasonic scanning technique. In: 1994 International Meeting on Reduced Enrichment for Research and Test Reactors, Williamsburg, VA (US), September 18–23, pp.178–190. Cheng, Y.Y., Wyant, J.C., 1985. Phase shifter calibration in phase-shifting interferometry. Applied Optics 24 (18), 3049–3052. Dolinko, A.E., Kaufmann, G.H., 2007. Enhancement in flaw detectability by means of lockin temporal speckle pattern interferometry and thermal waves. Optics and Lasers in Engineering 45, 690–694.
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Gerhard, H., Busse, G., 2006. Lockin-ESPI interferometric imaging for remote nondestructive testing. NDT&E International 39, 627–635. Ghosh, J.K., Muralidhar, S., Chandrasekharan, K.N., 1992. Nondestructive evaluation of plate type nuclear fuel elements for PURNIMA-III and KAMINI research reactors. Advanced Materials Manufacturing and Testing Information Analysis Center. Habib, K., 2005. Thermally induced deformations measured by shearography. Optics and Laser Technology 37, 509–512. Huang, Y.H., Ng, S.P., Liu, L., Li, C.L., Chen, Y.S., Hung, Y.Y., 2009. NDT&E using shearography with impulsive thermal stressing and clustering phase extraction. Optics and Laser in Engineering 47, 774–781. Hung, Y.Y., Chen, Y.S., Ng, S.P., Liu, L., Huang, Y.H., Luk, B.L., Chung, P.S., 2009. Review and comparison of shearography and active thermography for nondestructive evaluation. Materials Science and Engineering R 64, 73–112. Katchadjian, P., Desimone, C., Ziobrowski, C., Garcia, A., 2002. Detection of delamination defects in plate type fuel elements applying an automated c-scan ultrasonic system. In: International Meeting on Reduced Enrichment for Research and Test Reactors, Barioche, Argentina, November 3–8. Knight, R.W., Morin, R.A., 1999. Fabrication procedures for manufacturing high flux isotope reactor fuel. In: E ORNL/6852, December. Liu, Z., Gao, J., Xie, H., Wallace, P., 2011. NDT capability of digital shearography for different materials. Optics and Lasers in Engineering 49, 1462–1469. Malacar, D., 1992. Optical Shop Testing, 2nd ed. A Wiley-Interscience Publication/John Wiley & Sons, Inc., New York. Menner, P., Schmitz, P., Gerhard, H., Busse, G., 2008. Lockin-speckle-interferometry with modulated optical and inductive excitation. SPIE 7063. Menner, P., Gerhard, H., Busse, G., 2011. Lockin-interferometry: principle and applications in NDE. Journal of Mechanical Engineering 57, 183–191. Mucio Jose Drumond de Brito, Wilmar Barbosa Ferraz, Donizete Anderson de Alencar, Silverio Ferreira da Silva Jr., 2009. Nondestructive evaluation of plate type nuclear fuel elements during manufacturing stage using ultrasonic test method. In: 2009 INAC, Roi de Janeiro, RJ, Brazil, September 27. Taillade, F., Quiertant, M., Benzarti, K., Aubagnac, C., 2011. Shearography and pulsed stimulated infrared thermography applied to a nondestructive evaluation of FRP strengthening systems bonded on concrete structures. Construction and Building Materials 25, 568–574. Wang, X.S., Xu, Y., 2003. Mechanical characterizations of the dispersion U3 Si2 –Al fuel plate with sandwich structure. Applied Composite Materials Vol. 10, 159–167. Williams, R. K., Graves, R.S., Domagala, R.F., Wiencek, T. C., 1985. Thermal conductivities of U3 Si and U3 Si2 –Al dispersion fuels, Technical Report #-CONF851021-1, International Thermal Conductivity Conference, Cookeville, TN, USA. Zhang, K.Y., 2009. Nonlinear ultrasonic techniques for nondestructive assessment of micro damage in material: a review. International Journal of Precision Engineering and Manufacturing 10 (1), 123–135. Zhu, Y.K., Tian, G.Y., Lu, R.S., Zhang, H., 2011. A review of optical NDT technologies. Sensors 11, 7773–7798.
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
A preliminary study on noncontact imaging inspection for internal defects of plate-type nuclear fuel by using an active laser interferometer Nak-Gyu Park a , Seung-Kyu Park a,∗ , Sung-Hoon Baik a , Young-June Kang b a b
Korea Atomic Energy Research Institute, 1045 Daedeokdaero, Yuseong-gu, Daejeon 305-353, Republic of Korea School of Mechanical Design Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea
h i g h l i g h t s
An active laser interferometer was preliminarily studied to visually detect internal defects of plate-type nuclear fuel. The developed active optical interferometer system showed vivid defect images. The adopted flexible holder improved the detection efficiency for the internal defects by concentrating the strains. The filtering technique, which removes the whole-body variation, was effective in improving the defect detection capability. The developed noncontact imaging technique can be a useful nondestructive inspection tool for plate-type nuclear fuel.
a r t i c l e
i n f o
Article history: Received 26 July 2012 Received in revised form 22 November 2012 Accepted 24 November 2012 PACS: 07.60.Ly 07.20.-n
a b s t r a c t A non-contact optical imaging technique was preliminarily studied to detect internal defects of a platetype nuclear fuel specimen by using an active laser interferometer. A sinusoidal and periodic thermal wave induced periodic micro deformation including internal defect information of the specimen. The laser interferometer acquired a micro deformation image for the specimen surface, the shape of which varied according to the energy of the thermal wave. An adopted flexible holder of the specimen increased the capability of defect detection by concentrating the strains at the defects. A real-time inspection image including internal defect information was produced after the signal processing to remove noises. As a nondestructive inspection system, the developed visual inspection technique using an active laser interferometer can be a valuable tool to detect the internal defects of plate-type nuclear fuel. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Most research reactors have adopted the use of plate-type nuclear fuel, which consists of a fuel core in aluminum alloy. The production quality of nuclear fuel is an important part for an efficient and stable generation of thermal energy in research reactors. Thus, a nondestructive quality inspection for the internal defects of plate-type nuclear fuel is a key process during the production of nuclear fuel for safety assurance. Nondestructive quality inspections based on X-rays and ultrasounds have been widely used for the defect detection of plate-type nuclear fuel (Barrna, 1979; Ghosh et al., 1992; Knight and Morin, 1999; Katchadjian et al., 2002; Borring et al., 2002; Mucio et al., 2009). X-ray testing is a simple and fast inspection method, and provides an image as the inspection results (Barrna, 1979; Ghosh et al., 1992). Thus, testing can be carried out by a non-expert field worker.
∗ Corresponding author. Fax: +82 42 861 8292. E-mail address: [email protected] (S.-K. Park). 0029-5493/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2012.11.015
However, it is hard to detect closed-type defects because the Xray image is made from the density differences of the materials. In particular, it is hard to detect delamination defects that should be detected during the production of plate-type nuclear fuel. Thus, an X-ray inspection is generally used for an inspection of the filling status of nuclear fuel in aluminum alloy. Ultrasonic testing is a powerful tool for detecting internal defects including open-type and closed-type defects in plate-type nuclear fuel (Knight and Morin, 1999; Katchadjian et al., 2002; Borring et al., 2002; Mucio et al., 2009). The testing can also provide thickness information for each internal fuel plate. A high frequency ultrasonic signal of over 20 MHz is usually adopted because the thickness of each plate in plate-type nuclear fuel is about 1 mm to sub-millimeters thin. The inspection process is complicated because an immersion test should be carried out in a water tank. It is also a time-consuming inspection method because area testing is based on a scanning of the point-by-point inspections. In addition, this testing method needs to be carried out by an expert since the detection capability is greatly dependent on the inspector’s knowledge.
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Fig. 1. Block diagram of the developed noncontact optical imaging inspection system using an active laser interferometer.
In this paper, an optical imaging technique using an active laser speckle interferometer with periodic thermal energy was preliminarily studied to visually detect internal defects. The laser speckle interferometer is an optical technique used to measure micro changes in the surface topography (Malacar, 1992). This method is sensitive to micro-displacement at a nanometer resolution by superposing the speckle patterns of two different object states. Amplitude differences in deformation among intact and defective areas have been widely used for the detection of internal defects in plate specimens (Habib, 2005; Taillade et al., 2011; Huang et al., 2009; Liu et al., 2011; Hung et al., 2009). Phase information has been also used when the phases of the modulation frequencies in displacement among intact and defective areas are different during the periodic deformation of a plate specimen (Menner et al., 2011; Dolinko and Kaufmann, 2007; Gerhard and Busse, 2006; Menner et al., 2008; Zhu et al., 2011). Amplitude information on the modulation frequencies, and nonlinear information on the harmonic frequencies have been also used to detect internal defects including small closed-type defects (Menner et al., 2011; Dolinko and Kaufmann, 2007; Gerhard and Busse, 2006; Menner et al., 2008; Zhu et al., 2011; Zhang, 2009). The developed active laser interferometer measures an interference image, which is a micro-deformation image of a specimen surface including the internal defect information. Thus, the inspection can be carried out by a non-expert field worker. Here, the surface deformation is induced by a heater supplying a thermal wave with periodic sinusoidal intensities. The inspection images can be displayed on a monitor in real time, and visual images including internal defect shapes after signal processing to reduce noises can also be displayed. Computer simulations of thermal deformation and experiments using the developed active laser interferometer to detect the internal defects of a plate-type nuclear fuel specimen are described in this paper. 2. Experimental setup for noncontact imaging inspection A block diagram of the developed noncontact imaging inspection system using an active laser interferometer with a thermal wave is shown in Fig. 1. The noncontact imaging inspection system was configured using two halogen lamps with a driver (DDS 5600, LEVITON MFG. co) controlled by a signal generator, a test specimen held by a flexible holder, a digital speckle pattern laser interferometer (CW laser: 532 nm, 100 mW, Cobolt SambaTM 150) with a phase control driver (E-665.XR, LVPZT-AMP, PI), and a main control computer.
Fig. 2. Dimensions of the test specimen for plate-type nuclear fuel.
As shown in Fig. 1, the deformation shape of the specimen surface will vary according to the applied thermal energy at the back side of the specimen. The thermal wave is generated by two halogen lamps driven by an amplifier. The intensity of the thermal wave is a periodic sinusoidal pattern controlled by a signal generator. The digital speckle pattern interferometer acquires a deformation interference image for the front surface of the specimen. The deformation interference image is an interference image between a reference laser beam and a signal laser beam. As shown in Fig. 1, the reference laser beam reflected at the beam splitter (BS) goes into the CCD camera after passing through a mirror (M), a spatial filter (S. F), a mirror (M), and a focusing lens. The signal laser beam reflected from the front surface of a specimen goes into the CCD camera after passing through the beam splitter (BS), mirrors (M), and a spatial filter (SF). Here, the phase of the signal beam can be shifted using a piezoelectric transducer (PZT) controlled by a computer. Internal defect information is contained in the acquired interference image because the deformation difference is mainly caused by the reflection at the internal defects of the thermal wave. An extracted deformation image including internal defect information will be produced after signal processing of the laser fringe images by a computer. As shown in Fig. 1, a specimen was held by a flexible holder supported by spring force in the length direction of the specimen. Here, the specimen is freely movable in the thickness direction, but the movement is not free in the length direction when thermal energy is applied. This flexible holder will increase the detection capability of the internal defects by increasing the strain deformation to the defect area. Plate-type nuclear fuels consist of several thin plates containing a plate core of uranium compounds in a metallic matrix, such as dispersions of U3 O8 –Al and U3 Si2 –Al, with aluminum cladding. The dispersions, produced by powder metallurgy processes in the form of briquettes, are used to obtain the plate-type nuclear fuels using the picture frame technique. The briquette is mounted in a frame, and the surfaces of this set are covered by aluminum foils. Platetype nuclear fuel is then obtained by submitting the structure to a hot rolling process under controlled conditions. As shown in Fig. 2, a similar test specimen was prepared for the experiments in this paper owing to a lack of proper production facilities for plate-type nuclear fuel in our laboratory. A tungsten plate (B) with a similar density was used instead of uranium compounds. The thickness of the tungsten plate (B) was 1.0 mm, and
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Table 1 The mechanical properties of the materials used for the simulation specimens. Material
Tungsten
U3 Si2 –Al
Aluminum
Density Young’s modulus Poisson’s ratio Coefficient of thermal expansion Thermal conductivity
19.3 g/cm3 400 GPa 0.28 4.4 e−5 /C
6.11 g/cm3 58 GPa 0.34 1.48 e−5 /C
2.69 g/cm3 71 GPa 0.33 2.3 e−5 /C
0.163 W/mm C
0.181 W/mm C
0.237 W/mm C
Fig. 3. Simulated deformation image based on the thermal energy for plate-type nuclear fuel having a core plate of tungsten.
Fig. 4. Simulated deformation image based on the thermal energy for practical plate-type nuclear fuel having a core plate of U3 Si2 –Al.
the thicknesses of the aluminum alloy at the front and backside were 0.5 mm, respectively. Here, the typical thickness of each component widely used in the field was selected. Three plates were bonded using metal paste except the two areas of delaminationtype defects, D and E, as shown in Fig. 2. The sizes of the two internal defects were 3.0 mm × 3.0 mm and 5.0 mm × 5.0 mm, respectively. The dotted rectangle in A of Fig. 2 is the viewing area of the CCD camera in Fig. 1. 3. Simulation of thermal deformation and experiments on detecting internal defects by using an active optical noncontact imaging inspection A computer simulation was conducted by using ANSYS software to see the pattern of thermal deformation and the deformation difference between the prepared test specimen and practical platetype nuclear fuel. Two test specimens having a plate core of tungsten and U3 Si2 –Al, respectively, were designed using software. The structures and dimensions were the same as the test specimen of Fig. 2. The material properties used for the simulation are shown in Table 1. Here, the material properties of U3 Si2 –Al were selected from the published test data (Wang and Xu, 2003; Williams et al., 1985). To induce the thermal deformation of the specimen, a heat flow of 80 W was applied to the front aluminum plate for 0.5 s from an initial temperature of 40 ◦ C. The simulated deformation images for the plate-type nuclear fuel having a tungsten and U3 Si2 –Al core are shown in Fig. 3 and Fig. 4, respectively. The relative deformations from the minimum surface value for the delamination defects, 5 mm × 5 mm in size as shown in Figs. 3 and 4, are 1.41 m and 6.52 m, respectively. The relative deformations for the defects 3 mm × 3 mm in size are 1.04 m and 4.80 m, respectively. Here, the deformation amplitude from the U3 Si2 –Al
Fig. 5. Simulated deformation image for fixed plate-type nuclear fuel having a core plate of tungsten.
Fig. 6. Simulated deformation image for fixed plate-type nuclear fuel having a core plate of U3 Si2 –Al.
specimen is larger than the tungsten specimen, and the deformation difference between the two specimens is about several micrometers on the internal defects. From the simulation results, we can see that an optical interferometer can measure the thermal deformation image of plate-type nuclear fuel since the practical measurement resolution of the laser interferometer is usually about 0.01–0.05 m. Deformation images acquired by a computer simulation for fixed plate-type nuclear fuels having a tungsten core and U3 Si2 –Al core are shown in Figs. 5 and 6, respectively. Here, the length direction, left and right sides, of the Z-axis for each simulation sample was fixed as shown in Figs. 5 and 6. The relative deformations from the minimum surface value for the delamination defects 5 mm × 5 mm in size are 7.24 m and 10.24 m, respectively. The relative deformations for the defects 3 mm × 3 mm in size are 3.81 m and 7.01 m, respectively. The image distinctness of the internal defects from the acquired image was improved by fixing the length direction of the Z-axis. A periodic thermal wave with sine pattern intensities was applied to the specimen for the experiments. The shape of the specimen was periodically varied according to the energy of thermal wave. The applied pattern of the thermal wave and the signal processing flowchart of the developed noncontact imaging inspection system are shown in Figs. 7 and 8, respectively. To obtain a relatively deformed image, a reference phase image was extracted using a digital laser speckle interferometer at the user-set voltage time. In this experiment, the user-set time was selected as the lowest thermal energy time of (0), as shown in Fig. 7. Here, the reference phase image was extracted from four speckle images with phases of 0◦ , 90◦ , 180◦ , and 270◦ at time (0) using a 4-bucket algorithm (Cheng and Wyant, 1985). After extracting a reference phase image, the control computer extracted the current deformation image by subtracting a reference phase image from a current phase image, which was also calculated from the four phase-shifted speckle images at the current time. In this experiment, the control computer acquired about 30 deformation images in one period of 10 s. The information regarding the internal defects can be extracted after the signal processing to remove the noise components, such as the shape variation of the whole specimen surface according to the thermal energy, as shown in Fig. 8. This function was carried out by using a band-pass filter operated in the frequency domain since the frequency band of the whole shape variation is usually
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Fig. 7. Thermal wave pattern of a developed noncontact imaging inspection system.
Fig. 9. Wrapped phase maps during the 4th period of thermal energy from a flexible holder.
Fig. 8. Signal processing flowchart of the developed noncontact imaging inspection system.
low. Next, a low-pass filter in the time domain was carried out to remove impulse-type noise, which rarely exists in real environments. The computer can only display the inspected images when the input voltage of the heat is within the user set level band. This viewing technique can supply a more distinct defect image since defect information is well detected in a particular time band, such as the lowest energy time of each period. Wrapped phase maps during the 4th period of the thermal energy shown in Fig. 7 from a flexible holder supported by spring force and a one-sided free holder are shown in Figs. 9 and 10, respectively. Here, the one-sided free holder had a soft sponge holder, which was replaced on one side (B2) of the flexible holder shown in Fig. 1. The unwrapped phase images corresponding to those in Figs. 9 and 10 are shown in Figs. 11 and 12, respectively. As shown in Figs. 11 and 12, the flexible holder provided more distinct defect images than the one-sided free holder. The deformation images after signal processing for the images in Figs. 11 and 12 are shown in Figs. 13 and 14, respectively. The distinctness in the defect images was improved after removing the low-frequency components, which mainly contain the whole shape variation of the specimen surface according to the thermal energy. Herein, the applied band-passing range was 5–200 pixels
Fig. 10. Wrapped phase maps during the 4th period of thermal energy from a onesided free holder.
for the band-pass filter in the frequency domain. The image size was 512 × 512 pixels. As shown in Figs. 13 and 14, the defects were more distinct at around the lowest energy time of the thermal wave by virtue of the remaining strains in the defect areas. The normalized images for the internal defect area of Figs. 13 and 14 are shown in Figs. 15 and 16, respectively. By virtue of the strain concentration at the interior of the specimen, the flexible holder provided more distinct defect images than the one-sided free holder.
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Fig. 11. Unwrapped images for the wrapped phase maps during the 4th period of thermal energy from a flexible holder.
Fig. 12. Unwrapped images for the wrapped phase maps during the 4th period of thermal energy from a one-sided free holder.
The unwrapped deformation images at the lowest energy time of the thermal wave from the flexible holder and the one-sided free holder are shown in Figs. 17 and 18, respectively. Here, the images are from the 1st through 14th periods, and the 17th through 30th periods. As shown in Figs. 17 and 18, more distinct defect images were provided by the flexible holder. As the experimental results, internal defects were visually shown after the 2nd period of applied periodic thermal wave using
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Fig. 13. Deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a flexible holder.
Fig. 14. Deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a one-sided free holder.
a digital speckle pattern laser interferometer. A band-pass filter operated in the frequency domain improved the detection capability of defects by removing the noise of the whole body variation caused by a sinusoidal thermal wave, except for the defect information. In addition, the adopted flexible holder supported by spring force improved the detection efficiency of defects by concentrating the strains at the interior.
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Fig. 15. Normalized deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a flexible holder.
Fig. 16. Normalized deformation images after signal processing for the unwrapped images during the 4th period of thermal energy from a one-sided free holder.
Fig. 17. Unwrapped deformation images at the lowest energy time of the thermal wave from a flexible holder.
Fig. 18. Unwrapped deformation images at the lowest energy time of the thermal wave from a one-side free holder.
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4. Conclusion In this paper, a nondestructive testing technique using an active digital speckle pattern interferometer was preliminary investigated to visually detect internal defects for plate-type nuclear fuel. The developed nondestructive noncontact imaging inspection system was used to detect delamination defects of a similar test specimen, where such defects should be detected during the production process for safety assurance. The developed speckle interferometer system showed vivid defect images after the second period of the applied periodic thermal wave. The adopted flexible holder in the system improved the detection efficiency of the internal defects by concentrating the strains. In addition, the developed filtering technique, which removes the whole-body variation of the specimen surface according to the thermal energy except the defect components, was effective in improving the capability of defect detection. As the preliminary results of the computer simulation and experiments, the developed noncontact imaging technique using an active laser interferometer can be a useful nondestructive inspection tool for plate-type nuclear fuel. Acknowledgements This work was supported by the Ministry of Education, Science and Technology of Korea (grant No. WCI 2011-001). References Barna, B.A., 1979. The suitability of X-ray paper as an inspection tool for flat plate nuclear fuel. In: ANS Winter ‘79 Meeting, November 11-16, San Francisco, CA. Borring, J., Gundtoft, H.E., Borum, K.K., Toft, P., 2002. Non-destructive evaluation of the cladding thickness in leu fuel plates by accurate ultrasonic scanning technique. In: 1994 International Meeting on Reduced Enrichment for Research and Test Reactors, Williamsburg, VA (US), September 18–23, pp.178–190. Cheng, Y.Y., Wyant, J.C., 1985. Phase shifter calibration in phase-shifting interferometry. Applied Optics 24 (18), 3049–3052. Dolinko, A.E., Kaufmann, G.H., 2007. Enhancement in flaw detectability by means of lockin temporal speckle pattern interferometry and thermal waves. Optics and Lasers in Engineering 45, 690–694.
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