Plasma and Fusion Research: Regular Articles

Volume 14, 3405082 (2019)

Neutronics Assessment for the Thailand Tokamak Upgrade∗)
Siriyaporn SANGAROON, Jiraporn PROMPING1) , Apiwat WISITSORASAK2) ,Boonyarit CHATTHONG3) , Ponkris KLAYWITTAPHAT4) , Roppon PICHA1)
and Thawatchai ONJUN1)
Department of Physics, Mahasarakham University, Mahasarakham, Thailand
Thailand Institute of Nuclear Technology (Public Organization), Bangkok, Thailand
2)
Department of Physics, King Mongkut University of Technology Thonburi, Bangkok, Thailand
3)
Department of Physics, Prince of Songkla University, Hat Yai, Songkhla, Thailand
4)Faculty of engineering, Thaksin University, Phatthalung Campus, Phatthalung, Thailand
1)(Received 27 December 2018 / Accepted 15 March 2019)
Shielding configuration studies for the fusion plasma experiment of the Thailand tokamak upgrade are presented in this work. The neutron rate of 1014 n/s of DT fusion is determined to assess the next step operation inphase II of the Thailand tokamak upgrade in the future. In order to optimize the materials and dimensions of theshielding, a series of MCNP simulation was performed to assess the neutron streaming in order to enhance understanding of neutron and gamma transport in the plasma torus, hall and the pathways shielding. Eﬀect of threecommonly available concrete composition (ordinary concrete, barite concrete and boron frits-baryte concrete)
have been investigated as a torus wall shielding and entrance maze. Four orders of magnitude of fast neutron flux
reduction was observed at outside torus hall. The thermal neutron was significantly reduced using the labyrinth
structure. The conceptual design of the biological radiation shielding is assessed and presented. This study
provides support for the future neutron and gamma radiation safety at the Thailand tokamak upgrade facility.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: Thailand tokamak, shielding, neutron, MCNP
DOI: 10.1585/pfr.14.3405082

1. Introduction
It is important to understand the neutron and gamma
radiation distributions in the thermonuclear fusion device
study [1–4]. Neutrons are uncharged particles which escape the magnetic field and interact with the surrounding
structure through the elastic and inelastic scattering, neutron capture and other events. Gamma rays are mainly
produced by the interaction of the neutron and surrounding structure of the tokamak. The information of radiation
flux distribution in fusion device allows the prediction ofother quantities such as radiation dose, radiation heating,radiation damage, neutron activation and other parameter
necessary for safety studies [1]. For the radiation safety,
many parameters in building construction become important to protect against radiation damage to operating andcasual personnel. The design of biological shielding andbuilding construction depends on the operating parameters, physics and engineering requirements specified forthe machine. Many types of concrete, ordinary, heavyweight, barite, etc., have been widely used for radiation
protection in nuclear power plants, medical field and otherapplication where radioactive shielding is required [5, 6].
For Thailand, the Thailand tokamak will be conauthor’s e-mail: siriyaporn.s@msu.ac.th∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

structed based on the HT-6 M tokamak and installedat Thailand Institute of Nuclear Technology (TINT),
Ongkharak site in Nakorn Nayok under the guidance ofInstitute of Plasma Physics, Chinese Academy of Sciences(ASIPP) and the collaboration of the Center for Plasma andNuclear Fusion Technology (CPaF) in 2019-2020. Themachine was previously developed and operated by theASIPP in Hefei [7]. It was originally designed for thestudy of auxiliary heating (neutral beam injection and ionand electron cyclotron resonance heating), transport process and diﬀusion process of the impurity [8–10]. In phase
I operation at TINT, hydrogen plasma will be used. However, the neutron flux is determined to assess the next step
operation in phase II of the Thailand tokamak which can beupgraded to fusion plasma. For such small tokamak, total
neutron up to 1014 n/s is estimated to be generated. With
such high neutron flux, there is potential hazard to workers and environment. Therefore, the emitted neutron andgamma-rays must be shielded more eﬀectively in the tokamak vessel and the building. In this work, the design of theThailand tokamak upgrade biological shielding is investigated. A series of neutronics analysis have been performed
for Thailand tokamak upgrade based on the existing HT6 M geometry and parameters as shown in Table 1. The
calculations are carried out using the Monte Carlo method
based on Monte Carlo N-Particle (MCNP5) [11] and nu-3405082-1

c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles
Table 1

Volume 14, 3405082 (2019)

Main parameters of HT-6 M [7].

Parameter
Major radius, R (cm)Minor radius, a (cm)
TF on axis, Bt (kG)
Plasma current, I p (kA)
Flat-top time, t f t (ms)
Ion temperature, T i (eV)Electron temperature, T e (eV)
Plasma density, n (cm−3 )Confinement time, τE (s)Value652015150 (q = 3)1504008001 × 1014
0.01

clear library ENDF-VI for the simple torus geometry andshielding for the torus hall. The results of neutron andgamma fluences around the tokamak device have been estimated. The conceptual design of the biological radiation
shielding is investigated. The study provides results thatcan be used for neutron and gamma radiation safety in theThailand tokamak upgrade commission and operation in
the future. This paper consists of the following sections:section 2 represents the tokamak and shielding geometry,section 3 represents the results and section 4 representssummary of this work.

Fig. 1 3-D calculation model of the Thailand tokamak upgradefor estimating neutron streaming at the plasma torus andsurrounding structure.

2. Tokamak and Shielding Geometry
In this work, the 3-D transport method calculations
based on the MCNP code are performed to estimate ofradiation streaming at the plasma torus and surrounding
structure of the Thailand tokamak upgrade. Detailed geometrical model of Thailand tokamak upgrade and torus
hall have been created and described in this section. The
model consists of approximately 40 cells defined using
about 70 surfaces. The vacuum vessel and magnetic fieldcoil have been modeled using the existing parameters ofHT-6 M tokamak [7].

2.1Thailand tokamak upgrade model

Thailand tokamak upgrade is set to be developed fromthe existing tokamak chamber of HT-6 M tokamak. The
main parameters are shown in Table 1. To prepare forthe installation and operation, J. Promping et al. [12] havestudied the relevant plasma scenarios that will be operatedusing this tokamak by using turbulent transport modelse.g. Multi mode model (MMM95) and Mixed Bohm/gyroBohm (Mixed B/gB) with BALDUR code. The results
provide the background plasmas in range of ne (0) ∼ 1.2 ×
1019 m−3 , ne (a) ∼ 0.6 × 1018 m−3 , T e (0) ∼ 600 eV and T e (a)∼ 10 eV where a is the minor radius [12].
In phase II, the Thailand tokamak is aimed toward operation using plasma which is estimated to generate total
neutron up to 1014 n/s. Therefore, the machine requires theconstruction of radiation shielding. The 3-D simple torus
geometry is shown in Fig. 1. The model consists of plasma
facing components, vacuum vessel made of stainless steel,Fig. 2 A cross-section view of the Thailand tokamak with TF
and PF coils and neutron source.

toroidal field coils made of copper, poloidal field coils(PF), including of equilibrium field coils and displacement
feedback control coil, made of copper and concrete shielding walls. Figure 2 shows a cross-sectional view of thetorus with the major radius (R) of 65 cm and minor radius(a) of 20 cm, stainless steel vessel (SS), 16 toroidal fieldcoils (TF) and 9 poloidal field coils (CS, OH1-OH4, EF1EF2, Pc1-Pc2). In the hydrogen plasma operation stage
in phase I, the existing vacuum vessel of stainless steel
welding with two insulation gaps with the major radius of
0.65 m, minor radius of 0.25 m and thickness of 5 mm isused. In the upgraded design for the fusion plasma, thevacuum vessel wall is implemented with a thick layer ofstainless steel and iron wall in order to minimize the fastneutron fluence and displacement damage to the coil. Dueto the limitation space between vacuum vessel and the TF
coil, the combination of stainless steel and iron is limited
at 10 cm thick toward the TF coil direction.
The uniform 14.1 MeV neutron source fromdeuterium-tritium (D-T) fusion plasma with the minor radius of 20 cm by assuming toroidal and poloidal
symmetries has been developed in the model as shown in
the poloidal cross section view in Fig. 2. The statistical

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Volume 14, 3405082 (2019)weight of the particle is set as 1 × 1014 n/s. The scrape-oﬀ
layer (SOL) has been set to 5 cm.

2.2

Torus hall

The biological shielding and building construction
have been implemented to quantify radiation streaming
during the fusion plasma operation. The purpose is to ensure a low level external radiation flux. Aim of the design in this work is to reduce worker exposure in the Thailand tokamak upgrade building and to reduce the displacement damage to the various equipment. The toroidal andpoloidal view of the torus hall is shown in Fig. 3. The torus
hall layout occupies area of 10 m × 10 m including freespace for the additional systems (diagnostic system, heating system, various equipments, etc.). The entrance mazeis located at the corner of the torus hall. One of the design requirements for the Thailand tokamak is that neutron
flux outside the torus hall be low enough to permit humanworking activities. For this purpose the torus model wascomplemented by the design of ordinary concrete structure all around the torus facility. The thickness of the ordinary concrete wall, floor, roof and entrance maze is 30 cm
as depicted in green in Fig. 3. Additionally, the eﬀect ofthe concrete composition (ordinary concrete with densityof 2.35 g/cm3 , barite concrete with density of 3.35 g/cm3
and boron frits-baryte concrete with density of 3.10 g/cm3 )is determined for diﬀerent neutron and gamma attenuation
purpose and is presented in this paper.

Fig. 3 MCNP geometry of the torus hall with ordinary concretewall.

3. Results
3.1 Vacuum vessel shieldIn the present work, the neutron transport in the torus
and neutron streaming through the TF coils, PF coils
and vacuum vessel are evaluated. The neutron streaming
through the torus, torus hall and entrance maze is shown in
Fig. 4. It is clearly seen in the figure that the neutron stream
is attenuated by the designed ordinary concrete wall in themodel. The neutron rate of ∼ 5 × 108 n/s is observed at
the plasma core and is reduced by two orders of magnitude
approximately outside the concrete wall.
Figure 5 shows the midplane neutron fluence profile(at Z = 200 cm) in radial direction between the torus centre(X = 0 cm) and outside the torus hall (X ≥ 500 cm). Theneutrons that are directed towards the thick vacuum vessel wall are scattered back to the plasma. It can be seen
the increasing peak of the neutron flux in the plasma corewhen the thickness of the stainless steel (SS) and iron (Fe)is increased. There is no significant reduction of the neutron streaming in the torus hall for diﬀerent vacuum vessel
shielding thickness and material. However, it can be seenthat the neutron flux in the torus hall is lower than in thevacuum vessel about two orders of magnitude.

3.2 Torus hall shield
Originally, 30 cm-thick ordinary concrete structure(wall, floor, roof and entrance maze) was modeled asshown in green in Fig. 3. The neutron fluence was deter-Fig. 4 Top: top view, at Z = 200 cm, of neutron streaming
through the torus, torus hall and entrance maze. Bottom:
poloidal cross section view of neutron streaming throughthe torus and torus hall.

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Fig. 5 Midplane neutron flux profile (Z = 200 cm) with diﬀerent
vacuum vessel wall thickness and material.

Fig. 6 Neutron flux at M1-M6 tallies position (as shown inFig. 3) with the 30 cm-thick ordinary concrete wall.

Fig. 7 Neutron flux at the M1-M6 positions with and without
the 30 cm-thick barite concrete covering the 30 cm-thickordinary concrete wall.

through the torus hall. The barite concrete has been considered as a heavyweight concrete shielding. The most important part of the design needs to determine fast neutron
streaming and reducing through the biological shielding.
Figure 7 shows the comparison of thermal and fastneutron at diﬀerent tally position by using the ordinary
concrete wall with and without barite concrete layer. The
thermal neutron flux reduction along the radial direction
and pathways from position M1 to M6 was observed. Thefast neutron flux of 3 × 105 n/cm2 approximately are observed at position M6. Due to the high fast neutron absorption cross-section of barite, the fast neutron is attenuated by the barite concrete by one order of magnitude.
There are no significant epithermal neutron flux reduction
by using the heavyweight concrete wall.

3.3 Entrance mazemined in the air using the MCNP track length estimationtallies of the neutron flux (F4:n). The tallies is set up using
the spherical cells of 5 cm in radius and positioned at M1M6 (depicted in Fig. 3) along the radial direction of torus
and pathways of the entrance maze.
In Fig. 6, the neutron flux is presented in three energy
groups: i) thermal neutron (E < 0.5 eV); ii) epithermal neutron (0.5 eV ≤ E ≤ 0.1 MeV); and iii) fast neutron (E >0.1 MeV) for diﬀerent positions of cell tallies (M1-M6). It
can be seen that the neutron fluxes are attenuated by the ordinary concrete shield (between position M1, M2 and M6)by three orders of magnitude and by maze (between position M3, M4 and M5) by two orders of magnitude.
At horizontal position outside concrete wall (M6),the neutron flux is dominated by fast neutron (as shownin Fig. 6) therefore the additional fast neutron shield isneeded. For the further design, the internal surface of theordinary concrete wall is covered by 30 cm-thick layer ofbarite concrete in order to minimize the neutron streaming

As can be seen on the red-circle-dashed line in Fig. 7that the neutron flux in the entrance maze (at position M4and M5) is dominated by thermal neutron. A further attenuation of neutron fluence can be achieved by using alayer of thermal neutron absorption material. The concrete, boron frits-baryte was implemented in the model asshown in blue in Fig. 8. The thickness of boron frits-baryte
concrete is varied between 10 - 20 cm due to the limitation
space of entrance maze. The pathway width is between
60 - 70 cm.
The results show that thermal neutron flux was attenuated by two orders of magnitude at the labyrinth position(M4) and by one order of magnitude at the exit maze (M5)
as can be seen in Fig. 9.
Moreover, one of the most important issue of the neutronics calculation is to study the neutron capture gammaray stream because it causes heat in the tokamak components. The gamma-ray flux is generated due to thermal neutron capture in a surrounding structure (torus and3405082-4

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Fig. 8 Cross section view of the torus hall entrance maze.

Fig. 10 Gamma flux at M1-M6 tallies position with and without
the implementation of boron frits-baryte concrete wall on
the ordinary and barite concrete.

of torus structure including the vacuum vessel, TF coils,
PF coils, torus hall and biological shielding by using thediﬀerent concrete compositions. The various thickness ofstainless steel and iron were complimented to the HT-6 M
existing vacuum vessel wall. The results show that the neutron flux is not significantly reduced in the torus hall when
the layer of stainless steel and iron is added. The neutronand gamma streaming can be attenuated by the combination of diﬀerent type concrete compositions wall. The radiation fluence along the labyrinth structure is significantly
reduced. The design of biological shielding presented inthis paper contributes to the installation, commissioning
and upgrade design stages of the Thailand tokamak project.
Fig. 9 Thermal neutron flux at M1-M6 tallies position with andwithout the implementation of boron frits-baryte concrete
wall on the ordinary and barite concrete.

shielding). In this work, the calculation of the neutron capture gamma-ray flux due to neutron capture in a concrete
combination shield have been concerned and presented inFig. 10. The red-square-dashed shows the gamma flux
generated by captured neutron around the torus structure
and ordinary and barite concrete wall. About ∼ 5 × 108
photon/cm2 is observed near the torus (M1). The gamma
fluxes reduce by one order of magnitude in radial direction in the torus hall (M2 and M3) and significantly decrease in the maze and outside torus wall (M4-M6). However, it can be seen that by covering the entrance maze withboron frits-baryte concrete, the gamma flux induced at thelabyrinth position (M4) and exit maze (M5) reduce by one
order of magnitude.

4. Summary
The neutron flux of the Thailand tokamak upgrade
was evaluated using the MCNP code. The model consists

Acknowledgement
This research was financially supported by Facultyof Science (Grant year 2019). The authors would like toexpress sincere thanks to Mahasarakham University andThailand Institute of Nuclear Technology (TINT). This
work is part of a collaborative research project under theCenter for Plasma and Nuclear Fusion Technology (CPaF)
and the International Atomic Energy Agency (IAEA) under Contract No. 22785. The authors acknowledge theHigh Performance Computing Server of the TINT for providing computing resources that have contributed to the results in this paper.
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