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ScienceDirect Physics Procedia 88 (2017) 361 – 368
8th International Topical Meeting on Neutron Radiography, Beijing, China, 4-8 September 2016
Monte Carlo simulation for designing collimator of the neutron radiography facility in Malaysia Rafhayudi Jamroa*, Nikolay Kardjilovb, Mohamad HairieRabira, Mohamed Rawi Mohd Zaina, Abdul Aziz Mohamedc, NurSazwani Mohd Alid, Faridah Idrisa, Megat Harun Al Rashid Megat Ahmada, Khairiah Yazida,Hafizal Yazida, Azraf Azmana, Mohd Rizal Mamata a
Malaysian Nuclear Agency, 43000 Kajang, Malaysia b HZB, Berlin, Germany c Center for Nuclear Energy, UNITEN, Kajang, Selangor, Malaysia d University Technology Malaysia (UTM)
Abstract Neutron collimator is the most important component in a neutron radiography facility set-up, which defines the neutron beam characteristic at the object plane. The neutron radiography facility in Malaysia was built at one of the radial beam ports of TRIGA MARK II PUSPATI research reactor (RTP).At present, the facility has low thermal neutron intensity at the sample position, which leads to long irradiation times; it gives many limitations for the industrial applications. The collimator used for this facility is based on step divergent collimator type. The aim of this research is to design the best geometry and to choose materials for thermal neutron collimator so as to obtain a uniform beam, high L/D ratio and a maximum thermal neutron flux at the object plane. In order to achieve this aim new collimator geometry has been designed to improve the existing radiography facility by using Monte-Carlo simulation codes of SIMRES and MCNPX. The new design results are compared with those of the existing facility. Our simulation result may be of help in the design of new collimator for neutron radiography facility. © Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2017 2017The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of ITMNR-8. Peer-review under responsibility of the organizing committee of ITMNR-8 Keywords:Monte Carlo Simulation, neutron and gamma flux, collimator, NUR II and neutron radiography
* Corresponding author. Tel.: +60389250510; fax: +60389250907. E-mail address:
[email protected]
1875-3892 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ITMNR-8 doi:10.1016/j.phpro.2017.06.049
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1. Introduction Neutron radiography (NR) is an important tool in non-destructive testing which has been widely used especially in industrial, medical, metallurgical, nuclear and explosive inspection. In order to enlarge the range of applications of NR technique, it is necessary to design and optimized high thermal neutron intensity at least 10 5 ncm-2s-1. However, these systems must be properly shielded and the radiography rules and regulations must be fulfilled. In addition the size, weight and operational conditions of the system must be optimized. Generally, the collimator is a beam forming assembly, which determines the geometric properties of the beam. In addition, it may contain filter to modify the energy spectrum or to reduce gamma contaminations of the beam. Various effective parameters on the image quality are needed to be studied to achieve a neutron radiography system with a good resolution. The geometry of collimator must be selected based on maximum intensity and uniformity of a neutron flux at the image plane at the end of collimator outlet. In practice, it is intended to have an experimental arrangement which is accomplishing neutron beam parameters as close as possible to the ideal ones. For that reason the neutron collimator should be optimized in respect to neutron and gamma radiation using MCNPX and SIMRES codes based on Monte Carlo Method. 2. Neutron Radiography Facility and Neutron Collimator Since the 1MW Reactor TRIGA MARK II PUSPATI (RTP) was commissioned in June 1982, a lot of developmental work was performed in order to establish the neutron radiography facility as a tool for research and development purposes. The neutron radiographic facility system in Malaysian Nuclear Agency was ready for use in 1984. The imaging system was based on film as an image recorder. Many improvements have been made to the system since then including the installation of the step divergence collimator type at the radial piercing beam port, the installation of the beam catcher in front of the radiographic film holder and construction of the biological shielding surrounding the irradiation facility. RTP is a swimming pool-type light water research reactor using enriched uranium-zirconium-hydride fuel with 8.5, 12 and 20wt% of 19.9% enrichment of U235 (SAR, 1985) and equipped with graphite reflector. There are four beam ports, three radial beam ports and one tangential beam port, and one thermal column as shown in Figure 1. Biological shielding
BP2 (Radial)
BP1 (radial)
Graphite reflector
Thermal column Core
Reactor tank BP3 (Radial piercing)
BP4 (tangential)
Figure 1: Cut-away view of the MINT TRIGA MARK II research reactor
In late 1983, neutron radiographic facility system known as NUR-1 was constructed and tested. The data collected from this test facility enabled the design and construction of the permanent facility, later known as NUR
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II.The construction of NUR II took place in November 1984 and was completed in February 1985. In January 1987, a new collimator was installed at the beam port of NUR II as shown in Figure 2. At 750kW the neutron flux at the outlet of the NUR II collimator is about 1.04x107 ncm-2s-1. However, due to a few problems related to very long exposure time, higher dose rate around the facility and limited working spaces, the NUR II facility was upgraded permanently in order to improve its performance. In year 2014, current NUR II facility has been dismantled and a design study for a facility upgrade and in particular design of a new neutron radiography collimator at RTP has been started. This upgrade project includes a re-configuration of NUR II exposure room, extraction of the current neutron radiography collimator from the reactor building, removal of the beam trap and rearrangement of shielding materials, designing and purchasing selected materials and etc. Since then, several materials have been chosen to meet the shielding requirements at the facility. Biological shielding
Water
Reactor Core
Collimator
Beam trap
Object
Figure 2: The side view of current NUR II facility
Generally, the propagating direction of neutron is only altered by interactions with atom nuclei. Collimation is the process of producing a beam of neutrons with directions that are within a few degrees of being parallel (Danyal J et. al. 2012). Collimator for neutron radiography must be designed with the requirement of the emergent neutron beam. In turn, the requirement placed upon the beam depends strongly upon the details of the target application (M. Dinca et.al. 2006). This paper provides necessary information using Monte Carlo simulations to demonstrate that a new neutron radiography collimator is capable to increase the thermal neutron flux and minimize gamma dose at the end of the NR collimator outlet and to achieve high resolution image for applications in non-destructive evaluation of industries and cultural heritage objects. 3. Materials and Methods Radiation transport and dosimetry calculation inside the new neutron radiography collimator were carried out using the MNCPX computer code and a SIMRES. The interactions of nuclear particles such as neutrons and photons with matter can be described through statistical means. The stochastic modelling of this nuclear interaction can be best simulated using Monte Carlo approach. This Monte Carlo (MC) method is a computational algorithm that can provide approximate solutions to a variety of nuclear and also other physical problems by the simulation of random quantities (Rafhayudi et al. 2008). In the case of neutron fluxes inside a neutron radiography collimator, a variety of MC called Monte-Carlo N-Particle (MCNP) is well known to provide reasonable agreement on radiations transport
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and dosimetry calculation. This is because of occupational radiation protection requires detailed knowledge of the dose and the photon flux in the accessible areas surrounding sources of the radiation facility. In that instance it may be advantageous to use Monte Carlo code to model the source and to calculate dose throughout the facility (Leone et al. 2005). The radial NUR II collimator has an overall length of 232 cm (Figure 3) and has two sections. Step divergence collimator type which first part has a length of about 100 cm and the diameter of 15 cm, and the second part has the length of about 117cm and the diameter of 20 cm respectively. A Bi filter is placed at the entrance of the beam collimator with a 15 cm length of and 15 cm in diameter. Furthermore, lead and flexi boron are used as lining inside a collimator.
Figure 3: Recent neutron radiography collimator at NUR II Facility
The major goal of this work consists of the design and the simulations of a new step divergent collimator at the NR facility at Malaysian Nuclear Agency in order to achieve a lower gamma contamination and higher thermal neutron flux. The beam for neutron radiography purposes should provide large field of viewin order to allow the investigation of large objects. An ideal neutron beam should be parallel, mono-energetic, with high intensity, free of contamination by other radiation and uniform on its cross section. In this paper, there is some modification of the design and the materials selection in respect to the current collimator design. For instance the following materials have been chosen: High Density Polyethylene (HDPE), Bismuth (Bi), Sapphire crystal (Al 2O3), Ferro Boron and Lead (Pb). The following filters were suggested for the new collimator design: Sapphire crystal (Al2O3) with a length of 7.62 cm surrounded by Ferro Boron material; Bismuth (Bi) with a length of 15 cm surrounded by Pb material and followed by a cone of lead (Pb) until at the end of collimator as shown in Figure 4 and Table 1.
Figure 4: New proposal for neutron radiography collimator design
On the other hand, neutron beam homogenity, beam profile at the image plane and colimator design done by neutron ray-tracing simulation program can be viewed in 3D view. This is performed by the neutron ray-tracing
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simulation program SIMRES. The neutron ray-tracing simulation program SIMRES has been develop since 1990from a simulator of resolution functions for three-axis spectrometers (RESTRAX) to the present general propose software for modelling and optimization of neutron scattering instrument (J.Saroun, 2015). Table 1. Material used in simulation of the collimator for the neutron imaging facility at the TRIGA MARK IInuclear reactor .
Materials Bismuth Lining Lead Lining Sapphire Crystal (with 3 different diameter) Lead lining Bismuth Lead Lead lining
Length (cm) 10.0 cm 10.0 cm 7.62 cm 7.62 cm 15.0 cm 15.0 cm Till the end of collimator
It’s seemed to be an alternative and in part complementary to the other MC simulation like McStas, VITESS or NISP. The SIMRES Monte Carlo code was used as well for optimization of the tangential neutron collimator at the neutron diffractometer. Moreover, SIMRESS software provides to the instrument scientists more flexibilityfor the design of new or upgrading old instruments and optimization of their configurations. This version permits to simulate neutron flux and its distribution in real and momentum subspaces as well as using resolution functions. In contrast to previous versions, SIMRES can now simulate any user-defined instrument layout, which can be built from available neutron optical components. This newly designed code still includes highly efficient sampling strategy, so that the high speed of simulations carried out with SIMRES is preserved. Flexibility of the code is further enhanced by Java based graphical user interface, which includes instrument layout editor, 3D visualization of the instrument model, graphs representing various types of results, as well as a number of commands useful for optimization of instrument parameters. In particular, SIMRES became the basic tools for design and optimization of the neutron instrument and related. 4. Result and discussion Following the final modification of the new neutron radiography design for neutron radiography facility at TRIGA MARK II PUSPATI with opening window of 7.62 cm length of Single Sapphire Crystal (50mm, 40mm and 30mm slits respectively) surrounded by Ferro Boron material. After that, all the streaming radiations were blocked by using 15 cm length of Bismuth where the Bismuth will be covered with lead. Then, the lead will be used as a lining until the end of the collimator. In addition MCNPX was used to simulate all radiations transported from the reactor core until the end of the neutron collimator. The profiles of the thermal neutron/gamma fluxes and simulation results for the new neutron radiography layout starting from the reactor core obtained by the simulation are shown in the figure 5 and 6 respectively. This isa typical output of 2 dimensional map of neutron and gamma fluxes distribution obtained the from MCNPX code. Figure 5 shows a final new collimator design successfully done by using theMCNPX code at the beam port #3 (radial beam port). All the materials and dimension are modelled as close as possible to the proposed one. The position of the inlet section which is simulated with varied materials is shown in Figure 2. Main considerations for the material selection are high absorption properties for gammas and fast neutrons and good transmission for thermal neutrons (Muhammad Rawi Mohamed Zin et al. 2015). More than five materials have been selected for the inlet section in the MCNPX model. The reactor core is placed in the center of the model. The radial beam line is facing directly to the reactor core; hence incoming neutrons enter the inlet section of the collimator. The collimator is designed as a step divergent type with lowest step of 15cm in diameter and upper step of 20 cm in diameter. From the results above it is clearly seen that neutron and photon fluxes have almost the same value of 5.0 x 10 8 ncm-2s-1at the entrance of the neutron radiography collimator. The figure remains the same until the radiations reach the sapphire filter (Al2O3) where the fluxes slightly increase until they reach the peak at 8.0 x 10 8 ncm-2s-1. Sapphire (Al2O3) is used commonly as a beam filter in order to get rid of the gamma ray and fast neutron content. In this way
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the signal to noise ratio for many NDT technique can be improved. After the radiations pass through 15 cm of Bi, the pattern of both fluxes rapidly decreases to 5.0 x 10 7 ncm-2s-1and 2.0 x 105 ncm-1s-2 for neutron and photon respectively. Bismuth is not a leading candidates as neutron filter material as it has undesirable high value of σ inel due to high density of low frequency phonon state (Nieman, 1980). However, due to high density which is 9.78 g/cm3 and atomic number (Z=83), bismuth is very effective as shielding against gamma rays. Meanwhile the pattern of neutron and photon fluxes is remaining almost constant until the end of the neutron beam collimator where it is about 4.54 x 104 ncm-2s-1 for the photon and 3.5 x 105 ncm-2s-1 for the neutron flux.
Figure 5: Final collimator design MCNP geometry
Meanwhile, Figure 7 shows a contour plot of the neutron and the photon dose measurements inside the new neutron radiography collimator at the NUR II facility. The simulation results showed that the neutron dose shows a high reading of 10Sv/hr at the first 25 cm of the collimator while the gamma dose shows a low dose of approximately 1Sv/hr at the same distance. After 40 cm away from main entrance, the neutron dose is about 1Sv/hr and only 0.01Sv/hr for the gamma dose. The results indicate after 100 cm away from the collimator inlet, that the neutron and the gamma dose distributions decrease steadily down to 0.01Sv/hr and 0.0001Sv/hr respectively. Finally, at the end of the collimator the dose for neutron indicates approximately 0.1Sv/hr and 0.0001Sv/hr for gamma dose. From the simulation result, it can be concluded that the gamma dose rate distribution inside the new neutron radiography collimator at the NUR II facility is within a safely working condition and safely to handle. 4.1.SIMRES The neutron beam homogenity, the beam profile at the image plane and the colimator design done by neutron ray-tracing simulations can be viewed in 3D mode by the neutron ray-tracing simulation program SIMRES Version 6.3. The simulation was performed from the edge of the neutron radiography collimator. The results are shown in Figure 8 and Figure 9 respectively. The profile analysis in Figure 8 shows that the beam intensity has a maximum at the centre of the beam. There is also indication that the new neutron beam size is about 20 cm in diameter. Meanwhile, from the Figure 9 it can be seen that the neutron beam of the neutron radiography is almost homogenous. It is clearly seen that all beams are almost separated homogenously indicating low internal scattering from the collimator inner wall. It is also shown that the beam is not immediately diverged upon exit from the collimator.
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flux particle/cm2/s
1.0E+09 1.0E+08
1.0E+07
neutron flux gamma flux
1.0E+06 1.0E+05 1.0E+04 0 20 40 60 Collimator length (cm)
80
100
120
140
160
180
200
220
Figure 6: Contour plot of neutron and photon fluxes distribution inside new neutron radiography collimator
Figure 7: Contour plot of neutron and photon doses distribution inside new neutron radiography collimator
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Figure 8: Beam intensity at the end of collimator by SIMRES Simulation
Figure 9: Neutron beam homogenity at the end of the collimator by SIMRES
5. Conclusion The collimation of the neutron beam at the radial beam port of the RTP PUSPATI reactor is simulated successfully for the new NUR II imaging facility by using MCNPX and SIMRES computer code. This work presented the results of the optimization study of the collimator at Malaysia neutron radiography system. The results obtained here are expected to be useful for the construction of the new radiography system. Acknowledgment The author would like to acknowledge the help and assistance of Jan Saroun (Nuclear Physics Institute CAS, Rez) and all members with support and all of contributions. References SAR Report For PuspatiTrigaMarkII Reactor Facility, PusatPenyelidikan Atom Tun Dr Ismail (Puspati), Bangi, Selangor, 1983 Danyal J. Turkoglu. Design, Donstruction and Characterization of an external Neutron Beam Facility at The Ohio State University Nuclear Reactor Laboratory. MSc Thesis. The Ohio State University. 2012 G.M. MacGillivray. Neutron Radiography Collimator Design. www.researchgate.net/publictopics.PublicPostFileLoader.html%3Fid%3D54086ae5d4c118db108b45c7%26key%3D188afd12-df74-4e618e8f-5aa78a50a8d6+&cd=1&hl=en&ct=clnk. 2015 RafhayudiJamro, DaniealHergenreder,Carlos Lecot,RedzuwanYahaya, Abdul Aziz Mohamed, Megat Harun Al Rashid Megat Ahmad, Julia Abd Karim, IkkiKurniawan, HafizalYazid&ShukriMohd. 2008. Calculation of neutron flux in PUSPATI’S TRIGA MARK II Research reactor using Monte Carlo N-Particles Approach. Seminar R&D 2008.26-29 Ogos 2008.AgensiNuklear Malaysia. Leone, J., Furler, M., Oakley, M., Caracappa, P., Wang, B., &Xu., X.G. 2005. Dose Mapping Using MCNP5 Mesh Tallies.The Radiation Safety Journal.Vol.88. M. Dinca, M. Pavelescu and C. Iorgulis. Collimated Neutron Beam for Neutron Radiography.Rom. Journ. Phys., Volume 51, Nos. 3-4, p. 435411, Bucharest, 2006 J. Saroun, Monte Carlo simulation package RESTRAX, http://neutron.ujf.cas.cz/restrax/ Muhammad Rawi Mohamed Zin, RafhayudiJamro, KhairiahYazid, Hishammuddin Hussain, HafizalYazid, MegatHarul Al-Rashid Megat Ahmad, AzrafAzman, Glam HadzirPatai Mohamad, Nai’imSyaugiHamzah and Mohamad Puad Abu. Simulation of Collimator for Neutron Imaging Facility of TRIGA MARK II PUSPATI Reactor. Physics Procedia 69(2015) 138-142. 10 World Conference on Neutron Radiography 2014 Nieman, H.T. (1980). Single Crystal filters for neutron spectrometry. Review of Scientific Instrumens, 1299-1303.
