Simulation of a collimator and sapphire filter for PGAA facility of the Moroccan TRIGA MARK II research reactor
A. Jalil, A. Chetaine, H. Amsil, K. Embarch, K. Laraki, H. Marah
PII: S0969-8043(19)30089-2
DOI: https://doi.org/10.1016/j.apradiso.2019.04.042 Reference: ARI 8714
To appear in: Applied Radiation and Isotopes Received Date: 30 January 2019
Revised Date: 30 April 2019 Accepted Date: 30 April 2019
Please cite this article as: Jalil, A., Chetaine, A., Amsil, H., Embarch, K., Laraki, K., Marah, H.,
Simulation of a collimator and sapphire filter for PGAA facility of the Moroccan TRIGA MARK II research reactor, Applied Radiation and Isotopes (2019), doi: https://doi.org/10.1016/j.apradiso.2019.04.042.
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Simulation of a collimator and sapphire filter for PGAA facility of the Moroccan TRIGA MARK II Research Reactor
A. Jalil, A. Chetaine
Mohammed V University of Rabat (Rabat, Morocco) [email protected]
H. Amsil, K. Embarch, K. Laraki, H. Marah
Nuclear Centre of Energy, Science and Nuclear Techniques, Morocco [email protected]
ABSTRACT
The Prompt Gamma Neutron Activation Analysis (PGAA) facility will be commissioned at the NB1 beam tube of the Moroccan TRIGA MARK II research reactor. PGAA is a non-destructive analytical technique used for multi-elemental analysis and especially for light element analysis. For better performances, PGAA requires small and focused beam, and high Фth/Фep and Фth/Фfast ratios, and low background gamma radiation. For this purpose, fast neutron filters are commonly used to reduce fast and epithermal components of neutron flux. In this paper, the impact of sapphire filter, introduced, within the collimator hole, at the beam port on neutron spectra was evaluated. Monte Carlo, MCNP6.2, code has been used with ENDF VII.1 neutron cross section library to perform this study. An approximate TRIGA MARK II reactor MCNP model has been used to generate neutron spectra at the entrance of the collimator. Then an independent model for both collimator and sapphire filter has been brought into calculation to fulfil our purpose. Results showed an increase of about 8 times for both Фth/Фep and Фth/Фfast
ratios using a 10cm length of sapphire.
Key Words: MCNP, TRIGA Mark II, PGAA, sapphire, neutrons,
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1- Introduction
The Moroccan TRIGA (Training, Research, Isotopes, General Atomics) MARK II research reactor (TMRII) can be operated at a maximum power of 2MW. TMRII was designed mainly for research purposes and it is used for isotope production, neutron activation analysis, neutron radiography, neutron diffraction and as a resource for training students and professionals. TMRII is an open pool light water-cooled reactor with graphite as a moderator and reflector. Including 96 fuel elements, TMRII’s core includes five fuel control rods, seventeen graphite reactor elements, one central thimble, and one pneumatic system [1]
TMRII is equipped with four horizontal beam tubes and one thermal column to install new nuclear facilities. Tangential (NB1) beam tube is dedicated to the PGAA facility which is widely applied for quantitative and qualitative material characterization especially for light element determination. The PGAA facility consists of several main components namely: i- collimator device and sapphire filter, ii- primary beam shutter, iii- super mirror neutron guide, iv- beam shaper, v- high purity germanium detector, and vi- beam stop.
Installed inside the beam port, the collimator device eliminates a stream of diverging gamma rays so that only those traveling quasi-parallel to a specified direction (in our case the axis of the beam tube) are collected. It consists of a series of mounted carbon steel rings, connected to Borated Polyethylene (5% boron) rings and a disk of carbon steel as a closing disc. The inner radius of the collimator entrance is 3.41 cm, whilethe end of the collimator at the primary beam shutter side is 2.5 cm inner radius. The overall collimator length is 140cm.
The primary beam shutter consists of two sections dedicated to open and close the neutron beam. The first section consists of neutron and gamma absorbing materials and acts like a beam stop. The second one is dotted with an open aperture designed for both to minimize the neutron leakage and to absorb the remaining diverging neutrons. The supermirror neutron guide consists mainly of multilayer coated mirrors (glasses) treated with Ti and Ni. Using such multilayer coatings in neutron guides allows to improve significantly the transport of neutrons. The beam shaper is installed after the supermirror guide to control the beam size depending on sample size.
The input beam size is 2.5cm x 10cm and the three available output sizes are namely: 2.5cm x 2.5cm, 2cm x 2cm and 1cm x 1cm. In order to collect prompt gamma rays emitted from irradiated samples, a 25 % high purity semiconductor Germanium detector (HPGe) is used. The beam stop consists of alternated layers of neutron and gamma absorbing materials to stop the beam crossing the sample. Figure 1 describes the whole facility with different components.
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3/9 Figure 1: Horizontal cross section view of the PGNAA facility at TRIGA MARK II CENM, including collimators, primary shutter, supermirror guide neutron, beam shaper, shielding, sample area, detector HPGe
and beam stop.
This study is restricted to determine the sapphire effect on the neutron flux components by keeping the thermal one more than 105 n.cm-2.s-2 at sample position [2]. In fact, any increase between the fast to thermal and epithermal to thermal ratios should be as optimal as possible.
The present work aims at presenting the modeling results of PGAA collimator for the TRIGA Mark II research reactor for different lengths of the sapphire filter and investigating their effect on neutron flux components. Moreover, the effects of the collimator structure on both beam shape and neutron background at the exit of the beam tube have been also evoked.
2- Methodology and components description
To achieve our objective, the TRIGA Mark II reactor calculations were performed with MCNP6.2 Monte Carlo code. The calculation results were used to generate the neutron spectra at the NB1 beam tube especially at the entrance of the collimator. In order to achieve an appropriate accuracy, the MCNP output spectra were divided into 7 bins (0°-1°,1°-2°, 2°-3° …) Subsequently, the reported spectra were used as an input for an independent PGAA collimator model for more reducing time-consuming calculations. Four different configurations have been investigated: beam tube without collimator, for an empty collimator and for 5 cm and 10cm length sapphire filter inserted inside the collimator.
Neutron source used in this study was generated using a TRIGA MARK II reactor MCNP6.2 [3][4] model with fresh fuel. This methodology is in keeping with previous similar studies e.g. [5]. A set of 7 neutron spectra was generated for 32 energy groups with 1-degree angular binning with a maximum angle of 7 degrees to be used as an input for this calculation. Surface neutron
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source of 14cm diameter located at 10cm from the collimator inlet was used as input for each angle bin with neutron spectra presented in (Figure 2).
Figure 2: Spectrum of neutron flux per energy per angle of emission
Collimator
The collimator consists of two main components, each of which plays a special role, namely, 5 cm holed carbon steel rings assembled with a total length of 100 cm (carbon steel plug) and borated high-density polyethylene (B4C-HDPE) with the same outer dimension and 35 cm in length (see Figure 3). The carbon steel plug reduces fast neutrons background and the B4C-HDPE thermalizes escaping fast neutrons and absorbs slow neutrons. A sapphire crystal filter with 5 cm diameter was used respectively for 5 cm and 10 cm length at the exit of the carbon steel plug. These two lengths were used to optimize sapphire filter length in order to obtain the best thermal to epithermal and thermal to fast neutron flux ratios.
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09
1.00E-091.00E-081.00E-071.00E-061.00E-051.00E-041.00E-031.00E-021.00E-011.00E+001.00E+011.00E+02
Flux (n cm-2.s-1)
Energy (MeV)
0°-1°
1°-2°
2°-3°
3°-4°
4°-5°
5°-6°
6°-7°
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Figure 3: Neutron beam collimator with sapphire (Al2O3) filter
Sapphire filter
Sapphire has a rhombohedral unit cell belonging to the R3c space group with a = 5.128 Å and α =55.28° [2]. The phonon frequency of Al2O3 , as shown in Figure 4, was calculated by PhonoPy [3] based of the force constant Phonon database [4].
Figure 4: Phonon frequency of Al2O3
Based on the data of the phonon frequency (Figure 4), the LEAPR module of the NJOY code [9]
was used to generate the Sapphire , thermal neutron scattering cross section at 300K temperature. This approach is consistent with previous similar works [10], [11]. For Sapphire crystal, the thermal neutron scattering cross sections calculated by NJOY code are closer to experimental data [12] (see Figure 5). The MT card was associated with material for loading of the corresponding , data from the thermal data file. In general, , effects are most significant below 2eV [3]. Generated sapphire filter cross sections data have been used in the MCNP6.2model.
-0.5 0 0.5 1 1.5 2 2.5 3
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
Distribution (arbitrary unit)
E (eV)
Al(Al2O3) O(Al2O3) Total(Al2O3) Reactor side
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Figure 5: Comparison of reference (300K) and the calculated total cross section for Al2O3
3- Results and Discussion
Figure 6 show the MCNP6.2 model of the collimator with 10cm filter of the Sapphire length. Neutron spectrum calculation results for the four configurations are presented in Figure 7 and summarized in Table 1.
Figure 6: MCNP model of the collimator with 10cm filter of the Sapphire length
0.1 1 10 100
1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02
Total Cross section [barn]
Energy [eV]
Calculate 300k Ref 300k
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7/9 Figure 7: Neutron spectra calculated for the empty beam tube, collimator without filter and for collimator with sapphire filter for 5cm and 10cm length, respectively, at the exit of the beam tube.
Table 1: ratio rate of thermal to epithermal and thermal to fast neutron flux Sapphire
length
0 cm 5 cm 10 cm
Фth/Фep 4.17 9.59 32.36
Фth/Фfast 2.65 6.43 21.35
The results exhibit that the collimator without filter has no effect on the neutron flux. In contrast, the Sapphire filter show a visible impact on neutron flux particularly in configuration with 10 cm filter length. Фth/Фep and Фth/Фfast ratios increased about 7.5 and 8 times for 10 cm sapphire filter, respectively, and about 2.3 and 2.4 times for 5 cm filter length, respectively. A decrease of thermal flux of about 53% was observed for the use of 10 cm compared to 22% for 5 cm sapphire filter length. This decrease may be related to the cross section of the Sapphire that has a large resonant part in the energies 33 keV and 530 keV and hence has an impact in the neutron spectra in exit of the collimator.
Scattered neutrons background and neutron beam flux are illustrated by mesh tallies mapping for four neutron energy ranges: i: 0eV-0.5eV, ii: 0.5eV-1keV, iii: 1keV-1MeV and iv:
1MeV-17.2MeV. These results were calculated with 10 cm filter length.
The background of scattered neutrons is reduced to more than 1000 times by the collimator as
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09
1.00E-09 1.00E-07 1.00E-05 1.00E-03 1.00E-01 1.00E+01
Flux n cm-2.s-1
Energy MeV
without collimator collimator without filter collimator with 5cm filter collimator with 10cm filter
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shown in Figure 8. Neutron flux decrease is very low (2-3 times) for slow neutrons compared to that of epithermal and fast neutrons (from 1 to 2 orders of magnitude). The divergence of the beam is evaluated to be about 2.15° at the exit of the beam tube.
Figure 8: - i (left top): Mesh tally results for neutron flux range of 0eV and 0.5eV energy -ii (left bottom): Mesh tally results for neutron flux range of 0.5eV and 1keV
-iii (right top): Mesh tally results for neutron flux range of 1keV and 1MeV –iv (right bottom): Mesh tally results for neutron flux range of 1MeV and 17.2MeV
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CONCLUSIONS
This study evaluates three aspects of neutron beam collimator and sapphire filter for the Moroccan PGAA facility. These aspects regard mainly Фth/Фep and Фth/Фfast ratios, neutron background and neutron divergence at dedicated beam tube. It is concluded that 10 cm length filter sapphire could achieve a good increase in Фth/Фep and Фth/Фfast ratios, while keeping a sufficiently optimal thermal neutron flux value to keep good performances of the PGAA facility.
The resulting beam divergence at the collimator exit is determined to be 2.15°. The collimator reduces the neutron background to about more than three orders of magnitude at the exit of neutron beam tube.
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- Simulation of the collimator with Monte Carlo N-Particle code MCNP6.2 - Generate thermal neutron treatment file ( , ) for Sapphir (Al2O3) using
LEAPR/NJOY2016
