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Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor
Author’s Accepted Manuscript Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor M.H. Choopan Dastjerdi, H. Khalafi, Y. Kasesaz, S.M. Mirvakili, J. Emami, H. Ghods, A. Ezzati www.elsevier.com/locate/nima
PII: DOI: Reference:
S0168-9002(16)00207-2 http://dx.doi.org/10.1016/j.nima.2016.02.040 NIMA58603
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 20 November 2015 Revised date: 8 February 2016 Accepted date: 13 February 2016 Cite this article as: M.H. Choopan Dastjerdi, H. Khalafi, Y. Kasesaz, S.M. Mirvakili, J. Emami, H. Ghods and A. Ezzati, Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.02.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor M. H. Choopan Dastjerdi1,2,*, H. Khalafi1, Y. Kasesaz1, S. M. Mirvakili1, J. Emami1, H Ghods1, A. Ezzati1 1
Reactor research school, Nuclear Science and Technology Research Institute, Atomic Energy Organization of Iran, Tehran, Iran 2
Department of Energy Engineering and Physics, Amirkabir University of Technology, Tehran, Iran
Abstract To obtain a thermal neutron beam for neutron radiography applications, a neutron collimator has been designed and implemented at the Tehran Research Reactor (TRR). TRR is a 5 MW open pool light water moderated reactor with seven beam tubes. The neutron collimator is implemented in the E beam tube of the TRR. The design of the neutron collimator was performed using MCNPX Monte Carlo code. In this work, polycrystalline bismuth and graphite have been used as a gamma filter and an illuminator, respectively. The L/D parameter of the facility was chosen in the range of 150-250. The thermal neutron flux at the image plane can be varied from 2.26 × 106 to 6.5 × 106 n cm-2 s-1. Characterization of the beam was performed by ASTM standard IQI and foil activation technique to determine the quality of neutron beam. The results show that the obtained neutron beam has a good quality for neutron radiography applications. Keywords: Neutron radiography; Collimator; MCNP; Tehran Research Reactor; ASTM standard 1. Introduction Neutron radiography (NR) is a useful technique in non-destructive testing and evaluation (NDT & E) for industrial application and research [1]. Utilization of neutrons makes it possible to inspect bulk of specimen. NR gives both visual and quantitative information about the spatial distribution of different elements and isotopes in the investigated samples [2, 3]. NR is complementary to X and gamma radiography and finds application in diverse areas such as the examination of nuclear fuels and the detection of explosives [4]. The neutron beams from research reactors and spallation neutron sources are extensively and successfully used for NR [5-13]. Nuclear reactors are the preferred thermal neutron source in general, since high neutron fluxes are available and exposures can be made in a relatively short time span. An appropriate neutron beam for NR must have maximum thermal neutron intensity (nth > 106 n cm-2 s-1) and uniformity with lowest gamma contamination (N/G > 105 n cm-2 mrem-1) at the image plane [11]. For this purpose a neutron collimator and neutron/gamma filters are used [12]. An NR system was installed at the Tehran Research Reactor (TRR) about three decades ago as the first and the only national NR system [14]. As seen in Fig. 1, this system has a neutron collimator that was installed in H beam tube of TRR. H beam tube is the north part of the trough tube. The system efficiency has been reduced due to the replacement of the high enriched uranium (HEU) fuel with the low enriched uranium (LEU) fuel and the changes in reactor core configuration. Indeed relatively low neutron flux (2.4×104 n cm-2 s-1) and high gamma content of the beam revealed an unfavorable effect on NR image. 1
Since that time, the system has lost its performance and currently does not work and there was not another attempt to make a new NR system. Perhaps it can be attributed to the lack of national demand for NR exams. Now, with national improvements in some industries and sciences like nuclear industries, materials science and engineering, and aerospace and their demands to perform some Non-Destructive Testing (NDT) such as NR exam for quality assurance, the need for an NR system was felt. Therefore, it was decided to create a new NR facility by using TRR [15]. In this study, a neutron collimator as an important part of an NR facility has been designed, installed and characterized to meet the appropriate neutron beam parameters according to the other world-wide NR facilities. Design calculations of the collimator components (position, dimensions and materials) and investigation of beam parameters have been carried out by MCNPX Monte Carlo transport code [15, 16]. The neutron flux and the gamma dose rate at the image plane have been measured using indium foil activation and TLD700 dosimeter respectively. ASTM standard E545 that includes both the beam purity indicator (BPI) and sensitivity indicator (SI) has been used to determine the quality of the obtained neutron beam. Also, a rose flower and a fuel rod have been imaged as the first use of the obtained beam. 2. Collimator design and simulation 2.1. TRR and the E beam tube TRR is a 5 MW pool type research reactor. Light water acts as both the coolant and moderator. Its fuel assemblies contain low enriched uranium (with 20 percent concentration of U-235) fuel plates in the form of U3O8Al alloy. The TRR pool consist of two sections, one section being called stall-end contains seven beam tubes as well as thermal column and other section known as open pool is used for bulk irradiation studies [17]. The layout of the TRR is shown in Fig. 2. Among the all beam tubes, E beam tube was chosen to implement the development project due to the various practical considerations. E beam tube is comprised of an aluminum chamber and stainless steel housing with cylindrical structures. It is a radial beam tube with an overall length of 304.8 cm and has three sections: the first section with the length of about 176.2 cm and the diameter of 16.6 cm, the second part with the length of about 93.3 cm and the diameter of 20.5 cm, and the third part with the length of about 35.3 cm and the diameter of 35.7 cm (see Fig. 3). The dimensions and geometrical shape of the beam tube is one of the intrinsic constrains on collimator design. 2.2. Design calculations The collimator design will affect the properties of the neutron beam. Hence collimator designs at the other facilities were reviewed prior to its design for the present facility [5-13, 18, 19]. An appropriate neutron beam for neutron radiography is a Category-I beam quality as designated by the American Society of Testing and Materials (ASTM) Standards [20]. The parameters of design were considered to obtain a thermal neutron flux of nth>106 n cm-2 s-1 at the image plane and L/D ratio of >130 and the neutron beam divergence of under 4˚. Moreover, the uniform neutron beam was designed with a neutron to gamma ratio of >105 n cm-2 mrem-1 at image plane of >20 cm in diameter. To obtain an appropriate neutron beam with those parameters, several models of collimator were designed with respect to constrains and available materials [15]. The collimator design has been based on the 2
divergent collimator because a divergent beam collimator produces the highest resolution [19]. In this case, a fundamental design parameter is the L/D ratio, where D is the diameter of the aperture and L is the distance from the aperture to the image plane. The L/D ratio determines the geometric unsharpness and links the neutron flux at the aperture with that at the image plane through an inverse square relationship. In order to increase the thermal neutron content (TNC) of the beam and to provide a source of neutrons that is approximately uniform, a cylindrical slab of graphite illuminator with the length of 10 - 15 cm and the diameter of 16 cm (equal to the diameter of the beam tube in its first section) was placed on the beam tube near the core. To reduce the gamma content of the beam a cylindrical slab of polycrystalline bismuth with the length of 10 cm and the diameter of 10 cm has been used. Since the gamma filter is a scattering source, it is important that the filter be placed between the illuminator and the defining aperture. The aperture must prevent thermal neutrons from entering the beam except through the hole. A 3 cm thick boral disk as aperture with a 2 cm hole in its center and several cylindrical lead parts as the gamma shield with cadmium lining as collimator absorbing wall have been used. The aperture has been placed at a distance of 84 cm from the beam tube inlet surface, therefore for an L/D of 130 and a minimum beam line length of 344 cm, the aperture diameter has been taken to be 2 cm with a circular cross section. The location of the aperture determines a beam divergence angle of about 2.670. Simulations have been done in two stages. In the first stage, the TRR core and its surroundings have been simulated to obtain the neutron flux and gamma dose rate at the E beam tube entry. In the second stage, the performance of the designed collimator and the beam parameters has been investigated. The MCNP model of the TRR core and collimator design model are shown in Fig.4 and Fig.5 respectively. The total and thermal neutron fluxes at the entrance of the beam tube have been calculated 7.12×1012 n cm-2 s-1 and 4.52×1012 n cm-2 s-1 respectively. The TNC of the beam (nth/ntotal) is 63% which indicate that this beam tube has a good thermal neutron flux for NR purpose. There is also a large amount of gamma radiation due to the fact that E beam tube has a direct view of the TRR core. The gamma dose rate at the entrance of the E beam tube has been calculated 88.54 Mrem/hr. Thermal neutron flux and gamma dose rate along the beam tube up to the exit have been calculated and are shown in Fig. 6 and Fig.7 respectively, once for empty beam tube and once with collimator components (e.g. graphite and bismuth). After installing the collimator components in beam tube, neutrons have very different transport in beam tube than before installation. Indeed these materials can scatter and absorb neutrons, for example the aperture material is boral (Al + B4C) and is a strong neutron absorber (because the aperture must prevent thermal neutrons from entering the beam except through the hole). The absences of these materials reduce the neutron flux more than the reduction in accordance with the 1/R2 low. As seen, the graphite slab and polycrystalline bismuth increase the TNC of the beam at the aperture position from 52% to 79%. The purpose of the illuminator is to provide an approximately uniform neutron source [5, 18] and 10 cm is the optimized thickness and thicker graphite will reduce the thermal neutron flux available to the collimator and increase gamma ray content. By changing the position of the image plane relative to the aperture (L), thermal neutron flux and L/D ratio can be varied. The thermal neutron flux at the 3-m image plane (L/D = 150) and 5-m image plane (L/D = 250) has been calculated 6.5×106 n cm-2 s-1 and 2.26 × 106 n cm-2 s-1respectively. In the case of bare beam tube the thermal neutron content of the beam is lesser than the case that the collimator is placed in. From Fig. 7, it seems that the existence of the bismuth as the gamma ray filter and lead parts of the collimator 3
components reduced the gamma ray content at the exit from 9.08 rem s-1 to 13.4 mrem s-1 and improved the N/G ratio to 4.85×105 n cm-2 mrem-1 at the 3-m image plane (L/D = 150) The calculated neutron flux spatial distribution at the 3-m image plane (L/D = 150) is shown in Fig. 8. It shows that the flux is uniform over a diameter of >25 cm. The cadmium lining of the collimator wall absorbs the scattered and off trajectory neutrons and causes the beam uniformity at the image plane. To ensure the safety of installing the collimator components into the beam tube, a stress and deformation analysis of the beam tube due to its own weight and collimator components weights has been done. The maximum stress is 2.16 MPa and happens at the outer layer of the beam chamber. The yield stress of aluminum is about 110 MPa which is greater than the maximum stress. The maximum deformation is 0.11 mm that is negligible in comparison with the total length of the beam tube. Therefore the beam tube will be safe after the installation of the collimator. 3. Construction and experimental characterization The mechanical design and construction of the collimator has been done in a modular fashion )as shown in Fig. 9(. After emptying the E beam tube, the collimator parts have been installed (Fig. 10). The modular construction eases the modification of beam characteristics by changing collimator components individually without requiring the change of the entire collimator. Primary characterization of the beam has been performed after inserting the collimator in the beam tube. Neutron flux measurements have been performed using indium foil activation. Also, the TNC of the beam has been measured by two simultaneous exposures of bare and cadmium covered indium foils. The measured fluxes were approximately 7.3×1012 ± 9% n cm-2 s-1 with a TNC of the beam of about 61% at the inlet of E beam tube and about 6.1×106 ± 7% n cm-2 s-1 with a TNC of the beam of about 70% at the 3-m image plane (L/D = 150) respectively. Gamma ray dose rate has been measured using TLD700 dosimeter. The measured gamma dose rate at the 3-m image plane (L/D = 150) was about 13.9 mrem s-1. Therefore the measured N/G ratio was about 4.82×105 n cm-2 mrem-1. As seen, the measured and the calculated neutron fluxes and gamma dose rate and N/G ratio is in good agreement. Determination of the beam quality has been done with the beam quality indicators according to the ASTM standard E545 [21]. This standard describes the beam quality indicators and the use of them to determine the quality of the neutron beam for thermal neutron radiography. These beam quality indicators include Beam Purity Indicator (BPI) and Sensitivity Indicator (SI). ASTM standards E2023 and E2003 provide guidelines for constructing a SI and a BPI respectively. Densitometric measurements of a radiograph image of a BPI permit determination of the thermal neutron content (NC), gamma content (γ), pair production content (P), and scattered neutron content (S) of the beam. Visual inspection of the image of the SI provides qualitative information of the sensitivity of detail visible (gaps and holes) on a neutron radiograph. Table 1 provides a rating system based on the obtained information from the BPI and SI [21]. A 25 µm gadolinium foil mated with single-coated D3 film in a light tight cassette has been used to radiograph the BPI and SI. The position of the cassette was at the 3-m image plane (L/D = 150) and the exposure time was about 2.5 minutes. Fig. 11 shows the locations of the BPI and SI on the cassette, and the obtained radiograph of them respectively. The results of the analyzing of the BPI and SI radiograph image are presented in Table 2.
4
Based on the analysis of the BPI and SI radiographs the TRR is a category I radiographic facility, corresponding to the highest of five possible radiographic categories in Table 1. A comparison of the developed neutron beam at TRR with some other facilities and also the old NR facility at TRR is given in Table 3. As the first use of this neutron beam, a rose flower and three nuclear fuel rods have been radiographed. These fuel rods have different U-235 enrichments but pellets in each rod have the equal U-235 enrichment. The position of the image plane was at the 3-m image plane (L/D = 150) and the exposure time was about 2.5 minutes. Fig. 12 shows the obtained neutron radiographs of these objects. In the radiographic image of the rose, the structure of the pedicel and the layered petals are well visible. The internal components and details of fuel rods like springs, pellets and the gaps between pellets are visible in the radiographic image of the fuel rods. Also, the differences in the gray levels of pellets due to their differences in enrichment are visible, as the darker pellets have more U-235 enrichment. 4. Conclusion In order to expand the neutron radiography applications, a new neutron collimator as an important part of a neutron imaging facility has been designed, installed and experimentally characterized at the Tehran research reactor. The design calculations have been done using the MCNP Monte Carlo code. Preliminary experimental characterization of the beam shows a thermal neutron flux of about 6.1×106 n cm2 s-1 and a N/G ratio of about 4.82×105 n cm2 mrem-1 at the 3-m image plane (L/D=150). Furthermore, the obtained neutron beam has been characterized using the ASTM BPI and SI indicators and measurements indicate that the radiograph image obtained at this beam is of Category-I, as defined in ASTM E545. It will be planned to facilitate this new neutron beam with required equipment in the future as shown in Fig. 13. References [1] IAEA, TECDOC-1604, Neutron Imaging: A Non-Destructive Tool for Materials Testing, Report of the coordinated research project, (2008). [2] M. Basturk , H. Tatlisu, H. Bock, Nondestructive inspection of fresh WWER-440 fuel assemblies, Journal of Nuclear Materials 350 (2006) 240–245. [3] M. Grosse, E. Lehmann, P. Vontobel, M. Steinbrueck, Quantitative determination of absorbed hydrogen in oxidised zircaloy by means of neutron radiography, Nuclear Instruments and Methods in Physics Research A 566 (2006) 739–745. [4] E.H. Lehmann, P. Vontobel, A. Hermann, Non-destructive analysis of nuclear fuel by means of thermal and cold neutrons, Nuclear Instruments and Methods in Physics Research A 515 (2003) 745–759. [5] J. C. Domanus, Practical Neutron Radiography, Kluwer Academic Publishers, Dordrecht, Holland, (1992). [6] E. Lehmann, H. Pleinert, L. Wiezel, Status of the installation of a new neutron radiography facility at the spallation neutron source SINQ, Proceedings of the Fifth World Conference on Neutron Radiography, Berlin, (1996), p. 444.
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[7] D.S. Hussey, D.L. Jacobson, M. Arif, P.R. Huffman, R.E. Williams, J.C. Cook, New neutron imaging facility at the NIST, Nuclear Instruments and Methods in Physics Research A 542 (2005) 9-15. [8] B. Schillinger, E. Calzada, F. Grunauer, E. Steichele, The design of the neutron radiography and tomography facility at the new research reactor FRM-II at Technical University Munich, Applied Radiation and Isotopes, vol. 61, issue 4, in: Proceedings of the Fourth International Topical Meeting on Neutron Radiography, (2004), pp. 653–657. [9] U. Garbe, T. Randall, C. Hughes, The new neutron radiography/tomography/imaging station DINGO at OPAL, Nuclear Instruments and Methods in Physics Research A 651 (2011) 42–46. [10] R. Rosa, F. Andreoli, M. Mattoni, M. Palomba, Neutron collimator for neutron radiography applications at tangential port of the TRIGA RC-1 reactor, Nuclear Instruments and Methods in Physics Research A 605 (2009) 57–61. [11] Kaushal K. Mishra, Ayman I. Hawari, and Victor H. Gillette, Design and Performance of a Thermal Neutron Imaging Facility at the North Carolina State University PULSTAR Reactor, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 6, DECEMBER (2006). [12] M. Dinca,M. Pavelescu, C. Iorgulis, Collimated neutron beam for neutron radiography, Rom. J. Phys (2006) 51 (3–4), 435–441. [13] F. Kharfi, L. Boukerdja, A. Attari, M. Abbaci, A. Boucenna, Implementation of neutron tomography around the Algerian Es-Salam research reactor: preliminary studies and first steps, Nuclear Instruments and Methods in Physics Research A 542 (2005) 213–218. [14] K. K. Moghadam, A. Tabatabaeian, Neutron Radiography facility for AEOI nuclear research center, Proceedings of the Second World Conference on Neutron Radiography, Paris, (1986), pp. 25-32. [15] M.H.C. Dastjerdi, H. Khalafi, Design of a thermal neutron beam for a new neutron imaging facility at Tehran research reactor, Physics Procedia 69 (2015) 92-95. [16] Pelowitz, D. B., MCNPX user’s manual, Los Alamos National Laboratory, Los Alamos, (2005). [17] Safety Analysis Report of Tehran Research Reactor, Atomic Energy Organization of Iran, (2009). [18] J. F. W. Markgraf, Collimators for Thermal Neutron Radiography, D. Reidel, Dordrecht, (1987). [19] J. P. Barton, Material Evaluation 25 (1967) 45A. [20] Metals test methods and analytical procedures vol. 03.03, sec. 3, Ann. Book of ASTM Stand., Nondestructive Testing, (1999). [21] ASTM International, Standard Test Method for Determining Image Quality in Direct Thermal Neutron Radiographic Examination, ASTM Standard E545-10, (2010).
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Figure Captions Fig. 1: Sketch of the previous (old) NR facility at TRR [14] Fig. 2: A schematic of the TRR showing the various sections such as pools and beam tubes Fig. 3: Cross section view of the E beam tube Fig. 4: Three-dimensional MCNP model of the TRR core and beam tubes Fig. 5: The designed collimator and its components Fig. 6: Calculated thermal neutron content of the beam along the E beam tube before and after installing the collimator Fig. 7: Calculated gamma dose rate along the E beam tube before and after installing the collimator Fig. 8: Calculated spatial distribution of the thermal neutron flux at the 3-m image plane (L/D=150) Fig. 9: The modular structure and the constructed components of the collimator are shown. Fig. 10: The empty beam tube (left). Collimator components are installed in the E beam tube and conical shape inside the collimator is visible (right). Fig. 11: A photo of the BPI and SI (left). The radiographic image of the indicators used in ASTM standard E545 for determining the image quality in direct thermal neutron radiographs (right). (a) BPI; clearly visible in the beam purity indicator image are the BN disks, the cadmium wires and the density difference between the teflon block and the open region in the center. By comparing the density of the BN disks, the Teflon block, the open central region and the lead disks (which are not visible in this image), the thermal neutron content of the beam is N=67.51%. (b) SI; in the image of the sensitivity indicator all of the aluminum shims are visible giving G=7. Also, one can observe the six consecutive hole, giving H=6 (Note: Images have been adjusted to emphasize details). These three measures indicate that the typical radiograph image obtained at E beam tube is of Category-I, as defined in ASTM E545. Fig. 12: Radiographic image of the Rose (left) and nuclear fuel rods (right) (Note: Images have been adjusted to emphasize details) Fig. 13: Conceptual sketch of the new NR facility at TRR
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Table captions Table 1: Values of characterization parameters corresponding to radiographic category [21] Table 2: Radiographic category designation of the obtained beam Table 3: A comparison of the new neutron radiography facility at TRR with other facilities and also the old NR facility at TRR
Tables Table 1: Radiographic category I II III IV V
NC
H
G
S
γ
P
65 60 55 50 45
6 6 5 4 3
6 6 5 5 5
5 6 7 8 9
3 4 5 6 7
3 4 5 6 7
NC
H
G
S
γ
P
67.51
6
7
0.57
0.78
0.87
TABLE 2:
8
Radiographic category I
Table 3: Imaging facility TRR new neutron radiography facility (current work)
Neutron source
Neutron flux (n cm-2 s-1)
Beam filters
L/D
6
5 MW reactor (flux ~7.12x1012 n cm-2 s-1)
2.26x10 to 6.5x106
10 cm bismuth
150 to 250
5 MW reactor (flux ~ 1012 n cm-2 s-1)
2.4×104
10 cm bismuth
114
North Carolina State University (NCSU) [11]
1 MW reactor (flux ~2.5x1012 n cm-2 s-1)
1.8x106 to 7x106
4 inch bismuth and 6 inch sapphire
100 to 150
National Institute of Standards and Technology (NIST) [7]
20 MW reactor (flux ~4x1014 n cm-2 s-1)
4.75x106 to 1.84x107
10 cm bismuth
280 to 560
Paul Scherrer Institute (PSI) [6]
Spallation source (flux ~1014 n cm-2 s-1)
3.96x106 to 2.82x107
5 cm bismuth
200 to 550
TRR old neutron radiography facility [14]
9
Figures Fig. 1:
Fig. 2:
Fig. 3:
Fig. 4:
Fig. 5:
Fig. 6:
1.E+13 Before installation After installation
Thermal neutron flux (n cm-2 s-1)
1.E+12
1.E+11
1.E+10
1.E+09
1.E+08
1.E+07
1.E+06 0
50
100
150 Distance (cm)
200
250
300
Fig. 7:
1.E+08 Befor installation
Gamma dose rate (mrem h-1)
1.E+07
After installation
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01 0
50
100
150 Distance (cm)
200
250
300
Fig. 8:
Fig. 9:
Fig. 10:
Fig. 11:
Fig. 12:
Fig. 13:
PII: DOI: Reference:
S0168-9002(16)00207-2 http://dx.doi.org/10.1016/j.nima.2016.02.040 NIMA58603
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 20 November 2015 Revised date: 8 February 2016 Accepted date: 13 February 2016 Cite this article as: M.H. Choopan Dastjerdi, H. Khalafi, Y. Kasesaz, S.M. Mirvakili, J. Emami, H. Ghods and A. Ezzati, Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.02.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor M. H. Choopan Dastjerdi1,2,*, H. Khalafi1, Y. Kasesaz1, S. M. Mirvakili1, J. Emami1, H Ghods1, A. Ezzati1 1
Reactor research school, Nuclear Science and Technology Research Institute, Atomic Energy Organization of Iran, Tehran, Iran 2
Department of Energy Engineering and Physics, Amirkabir University of Technology, Tehran, Iran
Abstract To obtain a thermal neutron beam for neutron radiography applications, a neutron collimator has been designed and implemented at the Tehran Research Reactor (TRR). TRR is a 5 MW open pool light water moderated reactor with seven beam tubes. The neutron collimator is implemented in the E beam tube of the TRR. The design of the neutron collimator was performed using MCNPX Monte Carlo code. In this work, polycrystalline bismuth and graphite have been used as a gamma filter and an illuminator, respectively. The L/D parameter of the facility was chosen in the range of 150-250. The thermal neutron flux at the image plane can be varied from 2.26 × 106 to 6.5 × 106 n cm-2 s-1. Characterization of the beam was performed by ASTM standard IQI and foil activation technique to determine the quality of neutron beam. The results show that the obtained neutron beam has a good quality for neutron radiography applications. Keywords: Neutron radiography; Collimator; MCNP; Tehran Research Reactor; ASTM standard 1. Introduction Neutron radiography (NR) is a useful technique in non-destructive testing and evaluation (NDT & E) for industrial application and research [1]. Utilization of neutrons makes it possible to inspect bulk of specimen. NR gives both visual and quantitative information about the spatial distribution of different elements and isotopes in the investigated samples [2, 3]. NR is complementary to X and gamma radiography and finds application in diverse areas such as the examination of nuclear fuels and the detection of explosives [4]. The neutron beams from research reactors and spallation neutron sources are extensively and successfully used for NR [5-13]. Nuclear reactors are the preferred thermal neutron source in general, since high neutron fluxes are available and exposures can be made in a relatively short time span. An appropriate neutron beam for NR must have maximum thermal neutron intensity (nth > 106 n cm-2 s-1) and uniformity with lowest gamma contamination (N/G > 105 n cm-2 mrem-1) at the image plane [11]. For this purpose a neutron collimator and neutron/gamma filters are used [12]. An NR system was installed at the Tehran Research Reactor (TRR) about three decades ago as the first and the only national NR system [14]. As seen in Fig. 1, this system has a neutron collimator that was installed in H beam tube of TRR. H beam tube is the north part of the trough tube. The system efficiency has been reduced due to the replacement of the high enriched uranium (HEU) fuel with the low enriched uranium (LEU) fuel and the changes in reactor core configuration. Indeed relatively low neutron flux (2.4×104 n cm-2 s-1) and high gamma content of the beam revealed an unfavorable effect on NR image. 1
Since that time, the system has lost its performance and currently does not work and there was not another attempt to make a new NR system. Perhaps it can be attributed to the lack of national demand for NR exams. Now, with national improvements in some industries and sciences like nuclear industries, materials science and engineering, and aerospace and their demands to perform some Non-Destructive Testing (NDT) such as NR exam for quality assurance, the need for an NR system was felt. Therefore, it was decided to create a new NR facility by using TRR [15]. In this study, a neutron collimator as an important part of an NR facility has been designed, installed and characterized to meet the appropriate neutron beam parameters according to the other world-wide NR facilities. Design calculations of the collimator components (position, dimensions and materials) and investigation of beam parameters have been carried out by MCNPX Monte Carlo transport code [15, 16]. The neutron flux and the gamma dose rate at the image plane have been measured using indium foil activation and TLD700 dosimeter respectively. ASTM standard E545 that includes both the beam purity indicator (BPI) and sensitivity indicator (SI) has been used to determine the quality of the obtained neutron beam. Also, a rose flower and a fuel rod have been imaged as the first use of the obtained beam. 2. Collimator design and simulation 2.1. TRR and the E beam tube TRR is a 5 MW pool type research reactor. Light water acts as both the coolant and moderator. Its fuel assemblies contain low enriched uranium (with 20 percent concentration of U-235) fuel plates in the form of U3O8Al alloy. The TRR pool consist of two sections, one section being called stall-end contains seven beam tubes as well as thermal column and other section known as open pool is used for bulk irradiation studies [17]. The layout of the TRR is shown in Fig. 2. Among the all beam tubes, E beam tube was chosen to implement the development project due to the various practical considerations. E beam tube is comprised of an aluminum chamber and stainless steel housing with cylindrical structures. It is a radial beam tube with an overall length of 304.8 cm and has three sections: the first section with the length of about 176.2 cm and the diameter of 16.6 cm, the second part with the length of about 93.3 cm and the diameter of 20.5 cm, and the third part with the length of about 35.3 cm and the diameter of 35.7 cm (see Fig. 3). The dimensions and geometrical shape of the beam tube is one of the intrinsic constrains on collimator design. 2.2. Design calculations The collimator design will affect the properties of the neutron beam. Hence collimator designs at the other facilities were reviewed prior to its design for the present facility [5-13, 18, 19]. An appropriate neutron beam for neutron radiography is a Category-I beam quality as designated by the American Society of Testing and Materials (ASTM) Standards [20]. The parameters of design were considered to obtain a thermal neutron flux of nth>106 n cm-2 s-1 at the image plane and L/D ratio of >130 and the neutron beam divergence of under 4˚. Moreover, the uniform neutron beam was designed with a neutron to gamma ratio of >105 n cm-2 mrem-1 at image plane of >20 cm in diameter. To obtain an appropriate neutron beam with those parameters, several models of collimator were designed with respect to constrains and available materials [15]. The collimator design has been based on the 2
divergent collimator because a divergent beam collimator produces the highest resolution [19]. In this case, a fundamental design parameter is the L/D ratio, where D is the diameter of the aperture and L is the distance from the aperture to the image plane. The L/D ratio determines the geometric unsharpness and links the neutron flux at the aperture with that at the image plane through an inverse square relationship. In order to increase the thermal neutron content (TNC) of the beam and to provide a source of neutrons that is approximately uniform, a cylindrical slab of graphite illuminator with the length of 10 - 15 cm and the diameter of 16 cm (equal to the diameter of the beam tube in its first section) was placed on the beam tube near the core. To reduce the gamma content of the beam a cylindrical slab of polycrystalline bismuth with the length of 10 cm and the diameter of 10 cm has been used. Since the gamma filter is a scattering source, it is important that the filter be placed between the illuminator and the defining aperture. The aperture must prevent thermal neutrons from entering the beam except through the hole. A 3 cm thick boral disk as aperture with a 2 cm hole in its center and several cylindrical lead parts as the gamma shield with cadmium lining as collimator absorbing wall have been used. The aperture has been placed at a distance of 84 cm from the beam tube inlet surface, therefore for an L/D of 130 and a minimum beam line length of 344 cm, the aperture diameter has been taken to be 2 cm with a circular cross section. The location of the aperture determines a beam divergence angle of about 2.670. Simulations have been done in two stages. In the first stage, the TRR core and its surroundings have been simulated to obtain the neutron flux and gamma dose rate at the E beam tube entry. In the second stage, the performance of the designed collimator and the beam parameters has been investigated. The MCNP model of the TRR core and collimator design model are shown in Fig.4 and Fig.5 respectively. The total and thermal neutron fluxes at the entrance of the beam tube have been calculated 7.12×1012 n cm-2 s-1 and 4.52×1012 n cm-2 s-1 respectively. The TNC of the beam (nth/ntotal) is 63% which indicate that this beam tube has a good thermal neutron flux for NR purpose. There is also a large amount of gamma radiation due to the fact that E beam tube has a direct view of the TRR core. The gamma dose rate at the entrance of the E beam tube has been calculated 88.54 Mrem/hr. Thermal neutron flux and gamma dose rate along the beam tube up to the exit have been calculated and are shown in Fig. 6 and Fig.7 respectively, once for empty beam tube and once with collimator components (e.g. graphite and bismuth). After installing the collimator components in beam tube, neutrons have very different transport in beam tube than before installation. Indeed these materials can scatter and absorb neutrons, for example the aperture material is boral (Al + B4C) and is a strong neutron absorber (because the aperture must prevent thermal neutrons from entering the beam except through the hole). The absences of these materials reduce the neutron flux more than the reduction in accordance with the 1/R2 low. As seen, the graphite slab and polycrystalline bismuth increase the TNC of the beam at the aperture position from 52% to 79%. The purpose of the illuminator is to provide an approximately uniform neutron source [5, 18] and 10 cm is the optimized thickness and thicker graphite will reduce the thermal neutron flux available to the collimator and increase gamma ray content. By changing the position of the image plane relative to the aperture (L), thermal neutron flux and L/D ratio can be varied. The thermal neutron flux at the 3-m image plane (L/D = 150) and 5-m image plane (L/D = 250) has been calculated 6.5×106 n cm-2 s-1 and 2.26 × 106 n cm-2 s-1respectively. In the case of bare beam tube the thermal neutron content of the beam is lesser than the case that the collimator is placed in. From Fig. 7, it seems that the existence of the bismuth as the gamma ray filter and lead parts of the collimator 3
components reduced the gamma ray content at the exit from 9.08 rem s-1 to 13.4 mrem s-1 and improved the N/G ratio to 4.85×105 n cm-2 mrem-1 at the 3-m image plane (L/D = 150) The calculated neutron flux spatial distribution at the 3-m image plane (L/D = 150) is shown in Fig. 8. It shows that the flux is uniform over a diameter of >25 cm. The cadmium lining of the collimator wall absorbs the scattered and off trajectory neutrons and causes the beam uniformity at the image plane. To ensure the safety of installing the collimator components into the beam tube, a stress and deformation analysis of the beam tube due to its own weight and collimator components weights has been done. The maximum stress is 2.16 MPa and happens at the outer layer of the beam chamber. The yield stress of aluminum is about 110 MPa which is greater than the maximum stress. The maximum deformation is 0.11 mm that is negligible in comparison with the total length of the beam tube. Therefore the beam tube will be safe after the installation of the collimator. 3. Construction and experimental characterization The mechanical design and construction of the collimator has been done in a modular fashion )as shown in Fig. 9(. After emptying the E beam tube, the collimator parts have been installed (Fig. 10). The modular construction eases the modification of beam characteristics by changing collimator components individually without requiring the change of the entire collimator. Primary characterization of the beam has been performed after inserting the collimator in the beam tube. Neutron flux measurements have been performed using indium foil activation. Also, the TNC of the beam has been measured by two simultaneous exposures of bare and cadmium covered indium foils. The measured fluxes were approximately 7.3×1012 ± 9% n cm-2 s-1 with a TNC of the beam of about 61% at the inlet of E beam tube and about 6.1×106 ± 7% n cm-2 s-1 with a TNC of the beam of about 70% at the 3-m image plane (L/D = 150) respectively. Gamma ray dose rate has been measured using TLD700 dosimeter. The measured gamma dose rate at the 3-m image plane (L/D = 150) was about 13.9 mrem s-1. Therefore the measured N/G ratio was about 4.82×105 n cm-2 mrem-1. As seen, the measured and the calculated neutron fluxes and gamma dose rate and N/G ratio is in good agreement. Determination of the beam quality has been done with the beam quality indicators according to the ASTM standard E545 [21]. This standard describes the beam quality indicators and the use of them to determine the quality of the neutron beam for thermal neutron radiography. These beam quality indicators include Beam Purity Indicator (BPI) and Sensitivity Indicator (SI). ASTM standards E2023 and E2003 provide guidelines for constructing a SI and a BPI respectively. Densitometric measurements of a radiograph image of a BPI permit determination of the thermal neutron content (NC), gamma content (γ), pair production content (P), and scattered neutron content (S) of the beam. Visual inspection of the image of the SI provides qualitative information of the sensitivity of detail visible (gaps and holes) on a neutron radiograph. Table 1 provides a rating system based on the obtained information from the BPI and SI [21]. A 25 µm gadolinium foil mated with single-coated D3 film in a light tight cassette has been used to radiograph the BPI and SI. The position of the cassette was at the 3-m image plane (L/D = 150) and the exposure time was about 2.5 minutes. Fig. 11 shows the locations of the BPI and SI on the cassette, and the obtained radiograph of them respectively. The results of the analyzing of the BPI and SI radiograph image are presented in Table 2.
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Based on the analysis of the BPI and SI radiographs the TRR is a category I radiographic facility, corresponding to the highest of five possible radiographic categories in Table 1. A comparison of the developed neutron beam at TRR with some other facilities and also the old NR facility at TRR is given in Table 3. As the first use of this neutron beam, a rose flower and three nuclear fuel rods have been radiographed. These fuel rods have different U-235 enrichments but pellets in each rod have the equal U-235 enrichment. The position of the image plane was at the 3-m image plane (L/D = 150) and the exposure time was about 2.5 minutes. Fig. 12 shows the obtained neutron radiographs of these objects. In the radiographic image of the rose, the structure of the pedicel and the layered petals are well visible. The internal components and details of fuel rods like springs, pellets and the gaps between pellets are visible in the radiographic image of the fuel rods. Also, the differences in the gray levels of pellets due to their differences in enrichment are visible, as the darker pellets have more U-235 enrichment. 4. Conclusion In order to expand the neutron radiography applications, a new neutron collimator as an important part of a neutron imaging facility has been designed, installed and experimentally characterized at the Tehran research reactor. The design calculations have been done using the MCNP Monte Carlo code. Preliminary experimental characterization of the beam shows a thermal neutron flux of about 6.1×106 n cm2 s-1 and a N/G ratio of about 4.82×105 n cm2 mrem-1 at the 3-m image plane (L/D=150). Furthermore, the obtained neutron beam has been characterized using the ASTM BPI and SI indicators and measurements indicate that the radiograph image obtained at this beam is of Category-I, as defined in ASTM E545. It will be planned to facilitate this new neutron beam with required equipment in the future as shown in Fig. 13. References [1] IAEA, TECDOC-1604, Neutron Imaging: A Non-Destructive Tool for Materials Testing, Report of the coordinated research project, (2008). [2] M. Basturk , H. Tatlisu, H. Bock, Nondestructive inspection of fresh WWER-440 fuel assemblies, Journal of Nuclear Materials 350 (2006) 240–245. [3] M. Grosse, E. Lehmann, P. Vontobel, M. Steinbrueck, Quantitative determination of absorbed hydrogen in oxidised zircaloy by means of neutron radiography, Nuclear Instruments and Methods in Physics Research A 566 (2006) 739–745. [4] E.H. Lehmann, P. Vontobel, A. Hermann, Non-destructive analysis of nuclear fuel by means of thermal and cold neutrons, Nuclear Instruments and Methods in Physics Research A 515 (2003) 745–759. [5] J. C. Domanus, Practical Neutron Radiography, Kluwer Academic Publishers, Dordrecht, Holland, (1992). [6] E. Lehmann, H. Pleinert, L. Wiezel, Status of the installation of a new neutron radiography facility at the spallation neutron source SINQ, Proceedings of the Fifth World Conference on Neutron Radiography, Berlin, (1996), p. 444.
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[7] D.S. Hussey, D.L. Jacobson, M. Arif, P.R. Huffman, R.E. Williams, J.C. Cook, New neutron imaging facility at the NIST, Nuclear Instruments and Methods in Physics Research A 542 (2005) 9-15. [8] B. Schillinger, E. Calzada, F. Grunauer, E. Steichele, The design of the neutron radiography and tomography facility at the new research reactor FRM-II at Technical University Munich, Applied Radiation and Isotopes, vol. 61, issue 4, in: Proceedings of the Fourth International Topical Meeting on Neutron Radiography, (2004), pp. 653–657. [9] U. Garbe, T. Randall, C. Hughes, The new neutron radiography/tomography/imaging station DINGO at OPAL, Nuclear Instruments and Methods in Physics Research A 651 (2011) 42–46. [10] R. Rosa, F. Andreoli, M. Mattoni, M. Palomba, Neutron collimator for neutron radiography applications at tangential port of the TRIGA RC-1 reactor, Nuclear Instruments and Methods in Physics Research A 605 (2009) 57–61. [11] Kaushal K. Mishra, Ayman I. Hawari, and Victor H. Gillette, Design and Performance of a Thermal Neutron Imaging Facility at the North Carolina State University PULSTAR Reactor, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 6, DECEMBER (2006). [12] M. Dinca,M. Pavelescu, C. Iorgulis, Collimated neutron beam for neutron radiography, Rom. J. Phys (2006) 51 (3–4), 435–441. [13] F. Kharfi, L. Boukerdja, A. Attari, M. Abbaci, A. Boucenna, Implementation of neutron tomography around the Algerian Es-Salam research reactor: preliminary studies and first steps, Nuclear Instruments and Methods in Physics Research A 542 (2005) 213–218. [14] K. K. Moghadam, A. Tabatabaeian, Neutron Radiography facility for AEOI nuclear research center, Proceedings of the Second World Conference on Neutron Radiography, Paris, (1986), pp. 25-32. [15] M.H.C. Dastjerdi, H. Khalafi, Design of a thermal neutron beam for a new neutron imaging facility at Tehran research reactor, Physics Procedia 69 (2015) 92-95. [16] Pelowitz, D. B., MCNPX user’s manual, Los Alamos National Laboratory, Los Alamos, (2005). [17] Safety Analysis Report of Tehran Research Reactor, Atomic Energy Organization of Iran, (2009). [18] J. F. W. Markgraf, Collimators for Thermal Neutron Radiography, D. Reidel, Dordrecht, (1987). [19] J. P. Barton, Material Evaluation 25 (1967) 45A. [20] Metals test methods and analytical procedures vol. 03.03, sec. 3, Ann. Book of ASTM Stand., Nondestructive Testing, (1999). [21] ASTM International, Standard Test Method for Determining Image Quality in Direct Thermal Neutron Radiographic Examination, ASTM Standard E545-10, (2010).
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Figure Captions Fig. 1: Sketch of the previous (old) NR facility at TRR [14] Fig. 2: A schematic of the TRR showing the various sections such as pools and beam tubes Fig. 3: Cross section view of the E beam tube Fig. 4: Three-dimensional MCNP model of the TRR core and beam tubes Fig. 5: The designed collimator and its components Fig. 6: Calculated thermal neutron content of the beam along the E beam tube before and after installing the collimator Fig. 7: Calculated gamma dose rate along the E beam tube before and after installing the collimator Fig. 8: Calculated spatial distribution of the thermal neutron flux at the 3-m image plane (L/D=150) Fig. 9: The modular structure and the constructed components of the collimator are shown. Fig. 10: The empty beam tube (left). Collimator components are installed in the E beam tube and conical shape inside the collimator is visible (right). Fig. 11: A photo of the BPI and SI (left). The radiographic image of the indicators used in ASTM standard E545 for determining the image quality in direct thermal neutron radiographs (right). (a) BPI; clearly visible in the beam purity indicator image are the BN disks, the cadmium wires and the density difference between the teflon block and the open region in the center. By comparing the density of the BN disks, the Teflon block, the open central region and the lead disks (which are not visible in this image), the thermal neutron content of the beam is N=67.51%. (b) SI; in the image of the sensitivity indicator all of the aluminum shims are visible giving G=7. Also, one can observe the six consecutive hole, giving H=6 (Note: Images have been adjusted to emphasize details). These three measures indicate that the typical radiograph image obtained at E beam tube is of Category-I, as defined in ASTM E545. Fig. 12: Radiographic image of the Rose (left) and nuclear fuel rods (right) (Note: Images have been adjusted to emphasize details) Fig. 13: Conceptual sketch of the new NR facility at TRR
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Table captions Table 1: Values of characterization parameters corresponding to radiographic category [21] Table 2: Radiographic category designation of the obtained beam Table 3: A comparison of the new neutron radiography facility at TRR with other facilities and also the old NR facility at TRR
Tables Table 1: Radiographic category I II III IV V
NC
H
G
S
γ
P
65 60 55 50 45
6 6 5 4 3
6 6 5 5 5
5 6 7 8 9
3 4 5 6 7
3 4 5 6 7
NC
H
G
S
γ
P
67.51
6
7
0.57
0.78
0.87
TABLE 2:
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Radiographic category I
Table 3: Imaging facility TRR new neutron radiography facility (current work)
Neutron source
Neutron flux (n cm-2 s-1)
Beam filters
L/D
6
5 MW reactor (flux ~7.12x1012 n cm-2 s-1)
2.26x10 to 6.5x106
10 cm bismuth
150 to 250
5 MW reactor (flux ~ 1012 n cm-2 s-1)
2.4×104
10 cm bismuth
114
North Carolina State University (NCSU) [11]
1 MW reactor (flux ~2.5x1012 n cm-2 s-1)
1.8x106 to 7x106
4 inch bismuth and 6 inch sapphire
100 to 150
National Institute of Standards and Technology (NIST) [7]
20 MW reactor (flux ~4x1014 n cm-2 s-1)
4.75x106 to 1.84x107
10 cm bismuth
280 to 560
Paul Scherrer Institute (PSI) [6]
Spallation source (flux ~1014 n cm-2 s-1)
3.96x106 to 2.82x107
5 cm bismuth
200 to 550
TRR old neutron radiography facility [14]
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Figures Fig. 1:
Fig. 2:
Fig. 3:
Fig. 4:
Fig. 5:
Fig. 6:
1.E+13 Before installation After installation
Thermal neutron flux (n cm-2 s-1)
1.E+12
1.E+11
1.E+10
1.E+09
1.E+08
1.E+07
1.E+06 0
50
100
150 Distance (cm)
200
250
300
Fig. 7:
1.E+08 Befor installation
Gamma dose rate (mrem h-1)
1.E+07
After installation
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01 0
50
100
150 Distance (cm)
200
250
300
Fig. 8:
Fig. 9:
Fig. 10:
Fig. 11:
Fig. 12:
Fig. 13: