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Study of hydride blisters in Zr-alloy using neutron tomography
Journal of Nuclear Materials 421 (2012) 47–53
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Study of hydride blisters in Zr-alloy using neutron tomography Ashish Agrawal a,⇑, Yogesh Kashyap a, P.S. Sarkar a, A.N. Behra b, M. Shukla a, R.N. Singh b, Amar Sinha a, J.K. Chakravartty b a b
Neutron & X-ray Physics Facilities, Bhabha Atomic Research Centre, Trombay 400 085, Mumbai India Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Trombay 400 085, Mumbai India
a r t i c l e
i n f o
Article history: Received 1 June 2011 Accepted 26 October 2011 Available online 15 November 2011
a b s t r a c t Formation of hydride blisters in Zircaloy pressure tubes of pressurized heavy water reactor (PHWR) is a major life limiting factor which hinders the safe and uninterrupted operation of the reactor. Nondestructive detection and evaluation of location and size of these blisters as well as hydride distribution in the matrix surrounding them may help in damage quantification and residual life extension. In this article we present the neutron tomography studies carried out on simulated hydride blister samples grown on Zircaloy tubes. Characterization on samples having various levels of hydrogen concentrations were also carried out for quantification of the detectability of our neutron tomography system. We could identify the spatial in-homogeneity of hydride concentration present in the samples. Quantitatively hydrogen concentration difference up to 25 wppm has been observed experimentally and calibrated against image intensity in the reconstructed image. This study establishes neutron tomography as a potential nondestructive evaluation tool for the estimation of the severity of damage in the integrity of the pressure tubes and provides valuable information about kinetics of blister formation. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Zr–2.5Nb-alloy is used as a fabrication material for pressure tube in pressurized heavy water reactor (PHWR). In PHWR, the pressure tubes are maintained at 253–293 °C and are surrounded by concentrically located cooler calandria tube. Garter springs are provided at regular interval in the annulus space to prevent contact between hot pressure tube and cooler calandria tube. However, either due to segregation of garter springs during installation or due to its movement during operation, large span of pressure tube remains unsupported, which can make contact with cooler calandria tube and thereby setting up temperature gradient. Hydrogen is known to migrate down the thermal gradient and once the solid solubility is exceeded at the cold spot, low density hydride phase precipitates out. The hydride thus formed at the cold spot has blistery appearance and is called hydride blister. Though an individual blister is unlikely to defy leak before break criteria of pressure tube design, an array of blisters joined together by delayed hydride cracking (DHC) may form a crack larger than the critical crack length, which can cause catastrophic failure of the pressure tube [1,2]. Since pressure tube is the primary containment for the hot pressurized coolant, catastrophic failure of pressure tube will result in complete loss of coolant, which if unabated for long period may eventually lead to release of activity.
⇑ Corresponding author. E-mail address: [email protected] (A. Agrawal). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.10.047
In view of this, it is important to study and examine the blister formation process in pressure tubes and quantification of the hydride concentration so that the severity of the problem can be identified. Various techniques have been used for this purpose such as optical imaging, ultrasonic imaging, X-ray and neutron radiography. While optical imaging provides quite a good resolution for feature observation, it is a destructive technique and most of the information regarding shape, size, concentration, etc. is lost or altered during sample preparation. Moreover due to its destructive nature, the sample cannot be reutilized hence this technique is not suitable for onsite investigation [1,2]. Ultrasonic detection is blind about the shape and size of the blister. Apart from this the sensitivity of this method is limited to concentration difference of several thousands of wppm [3]. Since neutrons possess high scattering and absorption cross sections for elements like Cd, Gd and most of the low Z materials specially hydrogen (H), neutron imaging is the most suitable probe for investigating materials containing H and other low-atomic-weight materials. Recently neutron radiography using neutron sensitive imaging plate has been used for quantitative characterization of hydrogen concentration in Zircaloy tubes from pressurized heavy water reactor [4,5]. Although neutron radiography is quite versatile technique, the information extracted is limited because of the fact that this technique integrates the information in the object along the beam path. For this reason, localized information cannot be achieved using neutron radiography. In this work, neutron tomography has been used to characterize the hydride blisters formed in Zr–2.5Nb PHWR pressure tubes.
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A. Agrawal et al. / Journal of Nuclear Materials 421 (2012) 47–53
Hydride blisters were grown under controlled thermal boundary condition in Zr–2.5Nb pressure tube spools of length 200 mm. Three dimensional images of blisters formed have been obtained through neutron tomography. Shape, size, concentration of hydrogen, spatial in-homogeneity of concentration distribution in the vicinity of blisters has been determined. Hydrogen concentration calibration with respect to image intensity in the reconstructed planes was carried out separately using Zr–2.5Nb coupons charged with hydrogen in the range of 25–100 wppm in steps of 25 wppm. The hydrogen concentration up to 25 wppm has been distinguished along with the inhomogeneity in its spatial distribution.
2. Neutron tomography Neutron tomography is a well known non-destructive imaging technique having widespread applications in bio-medical sciences, agriculture, engineering, industries and material research [6–11]. In this technique the sample is rotated about its axis with a given angular step and radiographic projections are taken for each rotational step. In the next step a complex mathematical algorithm is used to reconstruct the various cross sectional slices of the object from the projection data [12]. The 3D volume of the object is produced by stacking of these slices. Using various image processing techniques, desired volume and features in the reconstructed volume can be highlighted. This volume image characterizes internal composition of materials, detects voids and cracks and determines shape and size of various components in the sample.
3. Experimental set-up The experimental set-up consists of a neutron source, a sample holder, an imaging area detector with image acquisition and control software system. This software was developed in-house for this experiment. The experiments were carried out at E12 beamhole in CIRUS reactor (India). A specially designed collimator (pinhole diameter 16 mm and length 2000 mm) with L/D 125 was used to collimate the beam into a cone of diameter 120 mm at the sample position in Fig. 1. The neutron flux of the order of 3 106 n/s and thermal to fast neutron flux ratio of 10 has been measured at the sample position (3000 mm from pinhole). The measured divergence of beam was close to 1°. Sample stage used for this experiment has two translational stages for remotely placing the sample at the suitable location in front of the beam and a rotational stage which can rotate the sample with a precision of 0.25°.
Fig. 1. Neutron radiography and tomography facility at CIRUS reactor, INDIA. (1) Neutron beam-hole E12, (2) shielded neutron sensitive digital imaging area detector and (3) sample position.
A neutron imaging area detector for this application was specially designed and developed in house and has been used for image acquisition for tomography. It consists of a neutron sensitive scintillator screen LiF–ZnS (Ag), a front coated (Aluminum) mirror and a 16 bit cooled charge coupled device (CCD) camera. The spatial resolution of imaging unit was 100 lm. In tomography experiment the sample is placed over the rotational stage. The rotations are synchronized with the image acquisition software. All the tomographic projection data are transferred to a PC and stored. The imaging unit has been shielded properly with lead and borated rubber for safety of CCD and also to reduce background. The schematic of the experimental setup for the neutron tomography experiment has been shown in Fig. 2. 4. Preparation of sample: simulated blisters in Zircaloy pressure tube Zr–2.5Nb pressure tube sections of length 200 mm were gaseously charged with 100 wppm of hydrogen [1]. Three hydride blisters 120° apart from each other were grown on the outer circumference of tube sections using a fixture as shown in Fig. 11 of Ref. [1]. The hydrogen charged tube sections were first heated to a temperature of 300 °C. The tube sections were then soaked for 2 h to attain thermal equilibrium. Subsequently, water cooled conical copper fingers were brought in contact with the tube surface to impose thermal gradient, which was maintained for 3 months. The imposition of thermal gradient resulted in migration of hydrogen towards the cold spot. Since at the cold spot the solid solubility of hydrogen is very little, the hydrogen arriving at the cold spot as a result of thermal migration precipitates out as hydride, which due its low density bulges out of surface resulting in blistery appearance [1,2]. Size of blisters formed was measured to be 2 mm approximately. 5. Neutron tomography of blisters in PHWR pressure tube Neutron tomography of the above pressure tube has been carried out using the experimental setup described in Section 3. The sample (internal diameter 81.5 mm, external diameter 88 mm and length 150 mm) was placed over a rotational stage and it was rotated with and angular step 0.25°. Radiographic projections were collected for 20 s at each rotational position. A typical radiographic image of such sample with the blisters formed in a cylindrical pressure tube has been shown in Fig. 3. The three hydride blisters are clearly identified along with information about their relative location and size. For tomography total 1440 projection were collected along with reference and background images. A full tomographic scan took around 8 h. All projections were normalized using reference and background images the reconstruction was carried out using FDK algorithm of filtered back projection [12]. The 3D volume of the object was obtained showing spatial distribution of neutron attenuation coefficients of the materials present in the object. Fig. 4 shows the various views of reconstructed volume of the object. The hydride blisters in the sample containing hydrogen concentration of 16,000 wppm (estimated from stoichiometric formulae ZrH1.66) can be clearly seen in the reconstructed volume in Fig. 4a–c. The image gray values represent the neutron attenuation coefficient of the materials. The images have been pseudo colored, green for hydride blister and gray for Zircaloy. Due to presence of excess hydrogen concentration blisters formed in the pressure tube attenuate and scatter larger number of neutron as compared to its surrounding. The relative concentration of these blisters can also be distinguished from the reconstructed slice image in Fig. 4b. This
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Fig. 2. Schematic of experimental set-up used for neutron tomography experiment in the present study.
Fig. 3. A typical neutron radiograph of the cylindrical pressure tube sample of PHWR showing three hydride blisters 120° apart.
distribution also shows the shape and size of the blister formed in its three dimensional perspective. It has been found and shown in enlarged image of blister in Fig. 5 that the blister has a shape of an elongated ellipsoid while it was measured to approximately 2 mm. Information about size of blister, its shape, its depth, presence of
cracks inside blister and orientation of crack with respect to hoop stress are required for safety assessment of the pressure tubes. Since the shape and size of the blisters formed depends upon shape and size of the contact tip, thermal boundary condition and tube section dimensions, varying these, the dependence over various parameters can be calibrated for their effect on the shape and size of the blister. It is important to note that the catastrophic failure of the pressure tube in the event of delayed hydride cracking (DHC) in which an array of such blisters join together and form a crack larger than the critical crack length, will strongly depend upon the shape and sizes of the blisters formed. As can be seen from image and plot profile in Fig. 5 gray values suddenly decreases in the vicinity of blister. Also from the enlarged image of the blister shown in Fig. 6 it is seen that the hydride distribution is anisotropic in region surrounding the blister. The plot profile near the blister at two different regions clearly shows the non-uniformity in the hydrogen depletion in this region (Fig. 6). As it is known that before blister formation the sample consists
Fig. 4. (a) View of the reconstructed 3D volume of the sample pressure tube along with the blisters present in it, (b) XY slice of the reconstructed volume and (c) YZ slice of the reconstructed volume (the images have been pseudo colored, green for hydride blister and gray for Zircaloy). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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A. Agrawal et al. / Journal of Nuclear Materials 421 (2012) 47–53
Fig. 5. Slice image of a blister and its plot profile showing a clear depletion of hydrogen concentration in the vicinity of blister.
Fig. 6. Enlarged view of 3D volume image of a blister formed in the pressure tube showing the hydride distribution along with the region of depletion. Plot profile at two different places in the image shows that the hydrogen has been depleted more in one region as compared to the other region.
Fig. 7. Various views of 3D rendered volume image of the homogenized sample coupons with different hydride concentrations (a and b). The concentration difference can be clearly identified from the volume images.
of a uniform spatial distribution of hydrogen, the lack of hydrogen in these areas clearly show that hydrogen from these regions has migrated to the region where blister is formed. The distribution of hydrogen concentration in these regions becomes much lower as compared to blister formation regions. Evaluation of hydrogen concentration and its spatial variation in these samples along may provide the information about the kinetics of the hydrogen migration in the sample under various thermal boundary conditions. To quantify the concentration of hydride in the surrounding region of blister, tomography experiments on samples having various proportions of hydride have been carried out. Calibration has
been performed with respect to gray values in the reconstructed images. The sensitivity of our imaging system for detecting low level hydride concentration could be found out with these experiments. 6. Quantitative analysis of hydrogen concentration Since hydrogen has a relatively higher attenuation coefficient, its small presence in zirconium alloy affects the resulting gray values in the image significantly. Hence concentration of hydrogen can be calibrated against the gray values in the image. In order
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Fig. 8. Views of the sample slices. (a) XZ slice, (b) YZ slice and (c) XY slice. All these images clearly show the existence of a non-homogeneous distribution of hydrogen in vertical direction while practically uniform distribution in XY horizontal plane.
Fig. 9. Histogram distributions of the gray values in the reconstructed images of various coupons with different concentration of hydrogen.
Table 1 Concentration of hydrogen in Zircaloy coupons and corresponding mode of histogram distribution of gray values in the reconstructed image of the samples. Serial no.
Concentration of hydrogen in Zircaloy coupons
Mode of histogram distribution
1 2 3 4
25 50 75 100
15,300 15,420 15,850 15,960
to get the quantitative information about the concentration and spatial distribution of hydrogen inside the blister as well as its surrounding regions, the quantification of concentration was carried out using neutron tomography. Various samples of Zr–2.5 Niobium charged with different concentration of hydrogen (0 wppm, 25 wppm, 50 wppm, 75 wppm, 100 wppm) were prepared and investigated using neutron tomography which is described in this section. Quantitative values of hydrogen distribution in the sample may provide additional information regarding blister formation
like its sensitivity for catastrophic failure through DHC, its dependence over thermal boundary condition and depletion of hydrogen in near about region along with its spatial distribution. 6.1. Preparation of the samples The samples to be studied were made up from Zr–2.5Nb gaseously charged at 363 °C with various concentrations of hydrogen 25 wppm, 50 wppm, 75 wppm, 100 wppm using a modified Seivert’s apparatus. Samples were sealed in Pyrex glass capsules at 75 torr of Helium. These capsules were loaded in a resistance heated furnace maintained at 400 °C and soaked for 24 h followed by furnace cooling. The temperature of 400 °C was chosen so as to dissolve all the hydrides. During furnace cooling hydrides precipitate out uniformly in the sample. 6.2. Experimental results Neutron tomography experiments were carried out in these coupons of Zr–2.5Nb. A set of four samples with hydrogen concentration of 25, 50, 75, 100 wppm were mounted on the rotational
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Fig. 10. Calibration curve: showing the linear proportionality between the mode of gray value distribution in reconstructed images and hydrogen concentration in the coupons.
stage and a total 720 projections were acquired rotating the sample with angular step 0.5°. After flat field correction the various slices of the coupons were reconstructed along with its 3D volume. Various slices of the reconstructed planes and 3D volume showing various concentration of hydrogen distributed in it has been shown in Fig. 7. It is clearly seen that the concentration difference of 25 wppm can be clearly distinguished in the reconstructed 3D images. The concentration of higher gray values in the image (Fig. 7a) increases with the increase in hydrogen concentration. In the next section a relation between image gray values and hydrogen concentration has been established. Various slice images, XY, YZ, XZ planes of the reconstructed volume has been shown in Fig. 8a–c. It is clear from this image that distribution of hydrogen is not homogeneous in the sample and there exist a certain degree of inhomogeneity in its spatial distribution.
7. Calibration To calibrate the concentration of hydrogen in the samples against the image intensities (gray values) in the reconstructed image, histogram of the 3D volume image of all the samples were calculated separately. Equal area was selected in the reconstructed image of each sample. Fig. 9 shows the plot of histograms of each sample. It is seen from the histograms that the most probable gray value (mode of the distribution) in the reconstructed images keeps on increasing with the increment of hydrogen concentration in the volume. This is because with more and more hydrogen in the sample, larger number of voxel in the image possesses higher gray values hence the mode of the histogram distribution shifts towards higher values. Table 1 shows the hydrogen concentration in the Zircaloy coupons and corresponding values of mode of this histogram distribution. A variation on peak heights in the histogram is attributed to the non-uniform distribution of hydrogen in the sample as described in the previous section. A graph showing the variation of hydrogen concentration value with respect to mode of the distribution has been plotted in Fig. 10 which shows the linear proportionality between the two quantities.
8. Conclusion In this study the application of neutron tomography has been explored as a tool for nondestructive investigation of blister formation process in the pressurized heavy water reactors (PHWRs) pressure tubes. Neutron tomography has been used to visualize and analyze shape, size and concentration distribution of hydrogen in the surrounding volume of the blister. A region with much lesser hydrogen concentration showing depletion of hydrogen to form the blister has been observed. A further study for calibration of hydrogen concentration in Zr–2.5Nb has been carried out and it has been found that this technique can detect concentration difference of hydrogen as low as 25 wppm. An in-homogeneity in spatial distribution of hydrogen with such a low concentration distribution has also been detected. The linear relation between Mode of histogram distribution of gray values and hydrogen concentration has been established which can be used to predict the unknown concentration of hydrogen from the reconstructed image. Future efforts in this study will concentrate on the application of this technique to study the dependence of the blister formation process over various system parameters like thermal boundary conditions and effects of shape and size of blister over the catastrophic failure of the pressure tube. Acknowledgements We acknowledge the constant support from the staff of Reactor Operation and maintenance divisions of CIRUS reactor particularly from Dr. A.K. Sahu, Sri N. Ramesh, Sri Anil bhatnagar and Sri N.K. Mondal. We acknowledge the constant support and guidance of Dr. S. Banerjee Chairman AEC, and Dr. S. Kailas Director Physics group. References [1] R.N. Singh, R. Kishore, T.K. Sinha, B.P. Kashyap, J. Nucl. Mater. 301 (2002) 153– 164. [2] R.N. Singh, R. Kishore, T.K. Sinha, S. Banerjee, B.P. Kashyap, Mater. Sci. Eng., A 339 (2003) 17–28. [3] J.L. Singh, Sunil Kumar, R. Keswani, S. Muralidhar, A.K. Sengupta, H.N. Singh, K.C. Sahoo, in: Proceedings of 14th World conference on NDT New Delhi, 1996, pp. 107–110.
A. Agrawal et al. / Journal of Nuclear Materials 421 (2012) 47–53 [4] Ryou Yasuda, Masahito Nakata, Masahito Matsubayashi, Katsuya Harada, Yuichi Hatakeyama, Hidetoshi Amino, J. Nucl. Mater. 320 (2003) 223–230. [5] E. Sváb, Gy. Mészáros, Z. Somogyvári, M. Balaskó, F. Körösi, Appl. Radiat. Isot. 61 (4) (2004) 471–477. [6] F.C. de Beer, J.J. Le Roux, E.P. Kearsley, Nucl. Instrum. Methods Phys. Res. A 542 (2005) 226–231. [7] Winkler Björn, Knorr Karsten, Kahle Andreas, Vontobel Peter, Lehmann Eberhard, Hennion Bernard, Bayon Guy, Eur. J. Miner. 14 (2002) 349–354. [8] Leanne G. Tumlinson, Hungyuan Liu, Wendy K. Silk, Jan W. Hopmans, Soil Sci. Soc. Am. J. 72 (2008) 1234–1242.
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[9] T.M. Nakanishi, J. Furukawa, M. Matsubayashi, Nucl. Instrum. Methods Phys. Res. A 424 (1999) 136–141. [10] Eberhard H. Lehmann, P. Vontobel, G. Frei, C.Br. onnimann, Nucl. Instrum. Methods Phys. Res. A 531 (2004) 228–237. [11] M. Michaloudaki, E. Lehmann, D. Kosteas, Int. J. Adhes. Adhes. 25 (2005) 257– 267. [12] A.C. Kak, M. Slaney, Principles of Computerized Tomographic Imaging, IEEE Press, New York, 1988.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Study of hydride blisters in Zr-alloy using neutron tomography Ashish Agrawal a,⇑, Yogesh Kashyap a, P.S. Sarkar a, A.N. Behra b, M. Shukla a, R.N. Singh b, Amar Sinha a, J.K. Chakravartty b a b
Neutron & X-ray Physics Facilities, Bhabha Atomic Research Centre, Trombay 400 085, Mumbai India Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Trombay 400 085, Mumbai India
a r t i c l e
i n f o
Article history: Received 1 June 2011 Accepted 26 October 2011 Available online 15 November 2011
a b s t r a c t Formation of hydride blisters in Zircaloy pressure tubes of pressurized heavy water reactor (PHWR) is a major life limiting factor which hinders the safe and uninterrupted operation of the reactor. Nondestructive detection and evaluation of location and size of these blisters as well as hydride distribution in the matrix surrounding them may help in damage quantification and residual life extension. In this article we present the neutron tomography studies carried out on simulated hydride blister samples grown on Zircaloy tubes. Characterization on samples having various levels of hydrogen concentrations were also carried out for quantification of the detectability of our neutron tomography system. We could identify the spatial in-homogeneity of hydride concentration present in the samples. Quantitatively hydrogen concentration difference up to 25 wppm has been observed experimentally and calibrated against image intensity in the reconstructed image. This study establishes neutron tomography as a potential nondestructive evaluation tool for the estimation of the severity of damage in the integrity of the pressure tubes and provides valuable information about kinetics of blister formation. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Zr–2.5Nb-alloy is used as a fabrication material for pressure tube in pressurized heavy water reactor (PHWR). In PHWR, the pressure tubes are maintained at 253–293 °C and are surrounded by concentrically located cooler calandria tube. Garter springs are provided at regular interval in the annulus space to prevent contact between hot pressure tube and cooler calandria tube. However, either due to segregation of garter springs during installation or due to its movement during operation, large span of pressure tube remains unsupported, which can make contact with cooler calandria tube and thereby setting up temperature gradient. Hydrogen is known to migrate down the thermal gradient and once the solid solubility is exceeded at the cold spot, low density hydride phase precipitates out. The hydride thus formed at the cold spot has blistery appearance and is called hydride blister. Though an individual blister is unlikely to defy leak before break criteria of pressure tube design, an array of blisters joined together by delayed hydride cracking (DHC) may form a crack larger than the critical crack length, which can cause catastrophic failure of the pressure tube [1,2]. Since pressure tube is the primary containment for the hot pressurized coolant, catastrophic failure of pressure tube will result in complete loss of coolant, which if unabated for long period may eventually lead to release of activity.
⇑ Corresponding author. E-mail address: [email protected] (A. Agrawal). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.10.047
In view of this, it is important to study and examine the blister formation process in pressure tubes and quantification of the hydride concentration so that the severity of the problem can be identified. Various techniques have been used for this purpose such as optical imaging, ultrasonic imaging, X-ray and neutron radiography. While optical imaging provides quite a good resolution for feature observation, it is a destructive technique and most of the information regarding shape, size, concentration, etc. is lost or altered during sample preparation. Moreover due to its destructive nature, the sample cannot be reutilized hence this technique is not suitable for onsite investigation [1,2]. Ultrasonic detection is blind about the shape and size of the blister. Apart from this the sensitivity of this method is limited to concentration difference of several thousands of wppm [3]. Since neutrons possess high scattering and absorption cross sections for elements like Cd, Gd and most of the low Z materials specially hydrogen (H), neutron imaging is the most suitable probe for investigating materials containing H and other low-atomic-weight materials. Recently neutron radiography using neutron sensitive imaging plate has been used for quantitative characterization of hydrogen concentration in Zircaloy tubes from pressurized heavy water reactor [4,5]. Although neutron radiography is quite versatile technique, the information extracted is limited because of the fact that this technique integrates the information in the object along the beam path. For this reason, localized information cannot be achieved using neutron radiography. In this work, neutron tomography has been used to characterize the hydride blisters formed in Zr–2.5Nb PHWR pressure tubes.
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Hydride blisters were grown under controlled thermal boundary condition in Zr–2.5Nb pressure tube spools of length 200 mm. Three dimensional images of blisters formed have been obtained through neutron tomography. Shape, size, concentration of hydrogen, spatial in-homogeneity of concentration distribution in the vicinity of blisters has been determined. Hydrogen concentration calibration with respect to image intensity in the reconstructed planes was carried out separately using Zr–2.5Nb coupons charged with hydrogen in the range of 25–100 wppm in steps of 25 wppm. The hydrogen concentration up to 25 wppm has been distinguished along with the inhomogeneity in its spatial distribution.
2. Neutron tomography Neutron tomography is a well known non-destructive imaging technique having widespread applications in bio-medical sciences, agriculture, engineering, industries and material research [6–11]. In this technique the sample is rotated about its axis with a given angular step and radiographic projections are taken for each rotational step. In the next step a complex mathematical algorithm is used to reconstruct the various cross sectional slices of the object from the projection data [12]. The 3D volume of the object is produced by stacking of these slices. Using various image processing techniques, desired volume and features in the reconstructed volume can be highlighted. This volume image characterizes internal composition of materials, detects voids and cracks and determines shape and size of various components in the sample.
3. Experimental set-up The experimental set-up consists of a neutron source, a sample holder, an imaging area detector with image acquisition and control software system. This software was developed in-house for this experiment. The experiments were carried out at E12 beamhole in CIRUS reactor (India). A specially designed collimator (pinhole diameter 16 mm and length 2000 mm) with L/D 125 was used to collimate the beam into a cone of diameter 120 mm at the sample position in Fig. 1. The neutron flux of the order of 3 106 n/s and thermal to fast neutron flux ratio of 10 has been measured at the sample position (3000 mm from pinhole). The measured divergence of beam was close to 1°. Sample stage used for this experiment has two translational stages for remotely placing the sample at the suitable location in front of the beam and a rotational stage which can rotate the sample with a precision of 0.25°.
Fig. 1. Neutron radiography and tomography facility at CIRUS reactor, INDIA. (1) Neutron beam-hole E12, (2) shielded neutron sensitive digital imaging area detector and (3) sample position.
A neutron imaging area detector for this application was specially designed and developed in house and has been used for image acquisition for tomography. It consists of a neutron sensitive scintillator screen LiF–ZnS (Ag), a front coated (Aluminum) mirror and a 16 bit cooled charge coupled device (CCD) camera. The spatial resolution of imaging unit was 100 lm. In tomography experiment the sample is placed over the rotational stage. The rotations are synchronized with the image acquisition software. All the tomographic projection data are transferred to a PC and stored. The imaging unit has been shielded properly with lead and borated rubber for safety of CCD and also to reduce background. The schematic of the experimental setup for the neutron tomography experiment has been shown in Fig. 2. 4. Preparation of sample: simulated blisters in Zircaloy pressure tube Zr–2.5Nb pressure tube sections of length 200 mm were gaseously charged with 100 wppm of hydrogen [1]. Three hydride blisters 120° apart from each other were grown on the outer circumference of tube sections using a fixture as shown in Fig. 11 of Ref. [1]. The hydrogen charged tube sections were first heated to a temperature of 300 °C. The tube sections were then soaked for 2 h to attain thermal equilibrium. Subsequently, water cooled conical copper fingers were brought in contact with the tube surface to impose thermal gradient, which was maintained for 3 months. The imposition of thermal gradient resulted in migration of hydrogen towards the cold spot. Since at the cold spot the solid solubility of hydrogen is very little, the hydrogen arriving at the cold spot as a result of thermal migration precipitates out as hydride, which due its low density bulges out of surface resulting in blistery appearance [1,2]. Size of blisters formed was measured to be 2 mm approximately. 5. Neutron tomography of blisters in PHWR pressure tube Neutron tomography of the above pressure tube has been carried out using the experimental setup described in Section 3. The sample (internal diameter 81.5 mm, external diameter 88 mm and length 150 mm) was placed over a rotational stage and it was rotated with and angular step 0.25°. Radiographic projections were collected for 20 s at each rotational position. A typical radiographic image of such sample with the blisters formed in a cylindrical pressure tube has been shown in Fig. 3. The three hydride blisters are clearly identified along with information about their relative location and size. For tomography total 1440 projection were collected along with reference and background images. A full tomographic scan took around 8 h. All projections were normalized using reference and background images the reconstruction was carried out using FDK algorithm of filtered back projection [12]. The 3D volume of the object was obtained showing spatial distribution of neutron attenuation coefficients of the materials present in the object. Fig. 4 shows the various views of reconstructed volume of the object. The hydride blisters in the sample containing hydrogen concentration of 16,000 wppm (estimated from stoichiometric formulae ZrH1.66) can be clearly seen in the reconstructed volume in Fig. 4a–c. The image gray values represent the neutron attenuation coefficient of the materials. The images have been pseudo colored, green for hydride blister and gray for Zircaloy. Due to presence of excess hydrogen concentration blisters formed in the pressure tube attenuate and scatter larger number of neutron as compared to its surrounding. The relative concentration of these blisters can also be distinguished from the reconstructed slice image in Fig. 4b. This
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49
Fig. 2. Schematic of experimental set-up used for neutron tomography experiment in the present study.
Fig. 3. A typical neutron radiograph of the cylindrical pressure tube sample of PHWR showing three hydride blisters 120° apart.
distribution also shows the shape and size of the blister formed in its three dimensional perspective. It has been found and shown in enlarged image of blister in Fig. 5 that the blister has a shape of an elongated ellipsoid while it was measured to approximately 2 mm. Information about size of blister, its shape, its depth, presence of
cracks inside blister and orientation of crack with respect to hoop stress are required for safety assessment of the pressure tubes. Since the shape and size of the blisters formed depends upon shape and size of the contact tip, thermal boundary condition and tube section dimensions, varying these, the dependence over various parameters can be calibrated for their effect on the shape and size of the blister. It is important to note that the catastrophic failure of the pressure tube in the event of delayed hydride cracking (DHC) in which an array of such blisters join together and form a crack larger than the critical crack length, will strongly depend upon the shape and sizes of the blisters formed. As can be seen from image and plot profile in Fig. 5 gray values suddenly decreases in the vicinity of blister. Also from the enlarged image of the blister shown in Fig. 6 it is seen that the hydride distribution is anisotropic in region surrounding the blister. The plot profile near the blister at two different regions clearly shows the non-uniformity in the hydrogen depletion in this region (Fig. 6). As it is known that before blister formation the sample consists
Fig. 4. (a) View of the reconstructed 3D volume of the sample pressure tube along with the blisters present in it, (b) XY slice of the reconstructed volume and (c) YZ slice of the reconstructed volume (the images have been pseudo colored, green for hydride blister and gray for Zircaloy). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Slice image of a blister and its plot profile showing a clear depletion of hydrogen concentration in the vicinity of blister.
Fig. 6. Enlarged view of 3D volume image of a blister formed in the pressure tube showing the hydride distribution along with the region of depletion. Plot profile at two different places in the image shows that the hydrogen has been depleted more in one region as compared to the other region.
Fig. 7. Various views of 3D rendered volume image of the homogenized sample coupons with different hydride concentrations (a and b). The concentration difference can be clearly identified from the volume images.
of a uniform spatial distribution of hydrogen, the lack of hydrogen in these areas clearly show that hydrogen from these regions has migrated to the region where blister is formed. The distribution of hydrogen concentration in these regions becomes much lower as compared to blister formation regions. Evaluation of hydrogen concentration and its spatial variation in these samples along may provide the information about the kinetics of the hydrogen migration in the sample under various thermal boundary conditions. To quantify the concentration of hydride in the surrounding region of blister, tomography experiments on samples having various proportions of hydride have been carried out. Calibration has
been performed with respect to gray values in the reconstructed images. The sensitivity of our imaging system for detecting low level hydride concentration could be found out with these experiments. 6. Quantitative analysis of hydrogen concentration Since hydrogen has a relatively higher attenuation coefficient, its small presence in zirconium alloy affects the resulting gray values in the image significantly. Hence concentration of hydrogen can be calibrated against the gray values in the image. In order
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Fig. 8. Views of the sample slices. (a) XZ slice, (b) YZ slice and (c) XY slice. All these images clearly show the existence of a non-homogeneous distribution of hydrogen in vertical direction while practically uniform distribution in XY horizontal plane.
Fig. 9. Histogram distributions of the gray values in the reconstructed images of various coupons with different concentration of hydrogen.
Table 1 Concentration of hydrogen in Zircaloy coupons and corresponding mode of histogram distribution of gray values in the reconstructed image of the samples. Serial no.
Concentration of hydrogen in Zircaloy coupons
Mode of histogram distribution
1 2 3 4
25 50 75 100
15,300 15,420 15,850 15,960
to get the quantitative information about the concentration and spatial distribution of hydrogen inside the blister as well as its surrounding regions, the quantification of concentration was carried out using neutron tomography. Various samples of Zr–2.5 Niobium charged with different concentration of hydrogen (0 wppm, 25 wppm, 50 wppm, 75 wppm, 100 wppm) were prepared and investigated using neutron tomography which is described in this section. Quantitative values of hydrogen distribution in the sample may provide additional information regarding blister formation
like its sensitivity for catastrophic failure through DHC, its dependence over thermal boundary condition and depletion of hydrogen in near about region along with its spatial distribution. 6.1. Preparation of the samples The samples to be studied were made up from Zr–2.5Nb gaseously charged at 363 °C with various concentrations of hydrogen 25 wppm, 50 wppm, 75 wppm, 100 wppm using a modified Seivert’s apparatus. Samples were sealed in Pyrex glass capsules at 75 torr of Helium. These capsules were loaded in a resistance heated furnace maintained at 400 °C and soaked for 24 h followed by furnace cooling. The temperature of 400 °C was chosen so as to dissolve all the hydrides. During furnace cooling hydrides precipitate out uniformly in the sample. 6.2. Experimental results Neutron tomography experiments were carried out in these coupons of Zr–2.5Nb. A set of four samples with hydrogen concentration of 25, 50, 75, 100 wppm were mounted on the rotational
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Fig. 10. Calibration curve: showing the linear proportionality between the mode of gray value distribution in reconstructed images and hydrogen concentration in the coupons.
stage and a total 720 projections were acquired rotating the sample with angular step 0.5°. After flat field correction the various slices of the coupons were reconstructed along with its 3D volume. Various slices of the reconstructed planes and 3D volume showing various concentration of hydrogen distributed in it has been shown in Fig. 7. It is clearly seen that the concentration difference of 25 wppm can be clearly distinguished in the reconstructed 3D images. The concentration of higher gray values in the image (Fig. 7a) increases with the increase in hydrogen concentration. In the next section a relation between image gray values and hydrogen concentration has been established. Various slice images, XY, YZ, XZ planes of the reconstructed volume has been shown in Fig. 8a–c. It is clear from this image that distribution of hydrogen is not homogeneous in the sample and there exist a certain degree of inhomogeneity in its spatial distribution.
7. Calibration To calibrate the concentration of hydrogen in the samples against the image intensities (gray values) in the reconstructed image, histogram of the 3D volume image of all the samples were calculated separately. Equal area was selected in the reconstructed image of each sample. Fig. 9 shows the plot of histograms of each sample. It is seen from the histograms that the most probable gray value (mode of the distribution) in the reconstructed images keeps on increasing with the increment of hydrogen concentration in the volume. This is because with more and more hydrogen in the sample, larger number of voxel in the image possesses higher gray values hence the mode of the histogram distribution shifts towards higher values. Table 1 shows the hydrogen concentration in the Zircaloy coupons and corresponding values of mode of this histogram distribution. A variation on peak heights in the histogram is attributed to the non-uniform distribution of hydrogen in the sample as described in the previous section. A graph showing the variation of hydrogen concentration value with respect to mode of the distribution has been plotted in Fig. 10 which shows the linear proportionality between the two quantities.
8. Conclusion In this study the application of neutron tomography has been explored as a tool for nondestructive investigation of blister formation process in the pressurized heavy water reactors (PHWRs) pressure tubes. Neutron tomography has been used to visualize and analyze shape, size and concentration distribution of hydrogen in the surrounding volume of the blister. A region with much lesser hydrogen concentration showing depletion of hydrogen to form the blister has been observed. A further study for calibration of hydrogen concentration in Zr–2.5Nb has been carried out and it has been found that this technique can detect concentration difference of hydrogen as low as 25 wppm. An in-homogeneity in spatial distribution of hydrogen with such a low concentration distribution has also been detected. The linear relation between Mode of histogram distribution of gray values and hydrogen concentration has been established which can be used to predict the unknown concentration of hydrogen from the reconstructed image. Future efforts in this study will concentrate on the application of this technique to study the dependence of the blister formation process over various system parameters like thermal boundary conditions and effects of shape and size of blister over the catastrophic failure of the pressure tube. Acknowledgements We acknowledge the constant support from the staff of Reactor Operation and maintenance divisions of CIRUS reactor particularly from Dr. A.K. Sahu, Sri N. Ramesh, Sri Anil bhatnagar and Sri N.K. Mondal. We acknowledge the constant support and guidance of Dr. S. Banerjee Chairman AEC, and Dr. S. Kailas Director Physics group. References [1] R.N. Singh, R. Kishore, T.K. Sinha, B.P. Kashyap, J. Nucl. Mater. 301 (2002) 153– 164. [2] R.N. Singh, R. Kishore, T.K. Sinha, S. Banerjee, B.P. Kashyap, Mater. Sci. Eng., A 339 (2003) 17–28. [3] J.L. Singh, Sunil Kumar, R. Keswani, S. Muralidhar, A.K. Sengupta, H.N. Singh, K.C. Sahoo, in: Proceedings of 14th World conference on NDT New Delhi, 1996, pp. 107–110.
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