Plasma and Fusion Research: Regular Articles

Volume 14, 3405048 (2019)

Neutronics Analyses for Shield Upgrading of the Compact NeutralParticle Analyzer for LHD Deuterium Plasma Experiments∗)
Takeo NISHITANI1) , Tetsuo OZAKI1) , Kenji SAITO1) , Shuji KAMIO1) , Kunihiro OGAWA1,2) ,Mitsutaka ISOBE1,2) and Masaki OSAKABE1,2)
1)

National Institute for Fusion Science, National Institutes of Natural Sciences, 322-6 Oroshi-cho, Toki 509-5292, Japan
2)
SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi-cho, Toki 509-5292, Japan(Received 18 December 2018 / Accepted 4 February 2019)
The Compact Neutral Particle Analyzer (CNPA) is an E//B type neutral particle energy analyzer using permanent magnets. CNPA is installed on the diagnostic platform and connected to the outside horizontal port.
Based on the preliminary shielding design, 15 cm thick borated polyethylene shields were installed before thefast deuterium plasma campaign. However, neutron induced noises could not be ignored in plasmas with theneutron emission rate higher than ∼1 × 1014 n/s. We carried out the detailed neutronics analyses for the originalneutron shield by using the MCNP6 code, which takes into account the detailed structure. The neutron streamingfrom the beam inlet and the penetration of the bulk shield are approximately 60% and 40% contribution to theneutron flux at the detector array, respectively. Consequently, 5 cm borated polyethylene plates are added to thebulk shield and an extension shield of borated polyethylene cylinder shield with 20 cm in diameter and 35 cm inlength is added around the beam inlet, where the neutron flux is expected to be 1.9 × 106 n/cm2 · s at the detector
array for the plasma with the neutron emission rate of 1.9 × 1016 n/s.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: compact neutral particle analyzer, shielding design, MCNP, neutron streaming, deuterium plasma,LHD
DOI: 10.1585/pfr.14.3405048

1. Introduction
The deuterium plasma experiment was initiated inMarch 2017 in the Large Helical Device (LHD) [1]. The
licensed annual neutron budget is 2.1 × 1019 neutrons/year.
The maximum neutron production rate is expected tobe 1.9 × 1016 n/s [2]. In such a radiation environment,
many diagnostic and control components, such as a programmable logic controller (PLC) and detectors, have possibilities of radiation induced noises, errors, and damage.
Therefore, relocation to the basement or the peripheral region of the torus hall, and shielding was undertaken forcontrol and diagnostic systems before the first deuterium
plasma campaign. Some diagnostic equipment must remain in the torus hall, mainly because that equipment must
face the plasma directly. Radiation shielding is required forsuch diagnostics equipment in order to avoid radiation induced noises and errors. In this paper, shielding designfor the compact neutral particle analyzer (CNPA) [3, 4]
is presented as one such example. Before the first deuterium plasma campaign, 15 cm thick borated polyethylene
shields were installed based on the preliminary neutronics
design. However, it was found that neutron induced noises
could not be ignored in plasmas with the neutron emission
rate higher than ∼ 1 × 1014 n/s. This paper concentrates on

the neutronics analyses for the original neutron shield, andthe upgraded shielding design to improve the shielding performance.

2. Compact Neutral Particle Analyzer(CNPA)
The Compact Neutral Particle Analyzer (CNPA) is an
E//B type neutral particle energy analyzer using permanent
magnets as shown in Fig. 1.
This analyzer was developed by the Ioﬀe Institute,
Russia [4]. Neutral particles from the plasma are ionizedon an electron stripping foil. After the ionization, the ionorbits are bent by the permanent magnet, and those ionsare detected by the channeltron array. The incident neutral
particle energy is analyzed by the position of the channel-author’s e-mail: nishitani.takeo@nifs.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

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Fig. 1 Schematic view of the CNPA.
c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3405048 (2019)

Fig. 2 Three-dimensional illustration of the CNPA on LHD. The
CNPA is installed on the diagnostic platform and connected to the O-port.

Fig. 3 Schematic view of the MCNP calculation model for LHD
and the experimental building.

tron array.
CNPA is installed on the diagnostic platform and connected to the outside horizontal port (O-port) via a flighttube. A 3-dimentional view of CNPA without the neutron
shield is shown in Fig. 2. In LHD, CNPA measures the radial high energy particle distribution combining with theimpurity pellet measurement.

3. Radiation Environment AroundLHD
The special distribution and the energy spectra of neutrons and gamma-rays in the LHD torus hall have been
calculated by a General Monte Carlo N-Particle Transport
Code version 6 (MCNP-6) [5] with the nuclear data libraryof ENDF/B-VII.1 [6], where LHD and the LHD experimental building are modeled manually based on the CADdrawings [7]. The schematic view of the MCNP calculation model is shown in Fig. 3, which is drawn by Super MC [8]. In the torus hall, the LHD and the interfermeter support are modeled. The other apparatuses suchas neutral beam injectors and vacuum pumps, are not included. Detailed modeling of LHD itself is described inRef. [8, 9]. The source neutron energy is assumed to be
99.5% of 2.45 MeV and 0.5% of 14.1 MeV. The 14.1 MeVneutron is generated by the triton burnup process [10].
Figure 4 shows neutron and gamma-ray spectra on thecenter line of the O-port with several radial distances fromthe center of the torus for the maximum neutron emission
rate of 1.9 × 1016 n/s. Virgin neutrons of 2.45 MeV and
14.1 MeV are clearly seen not only in the torus hall. It
can be considered that several peaks and gaps in the keV
region are corresponding to resonances of 56 Fe and otherstructural materials. The thermal neutron peak is formatted in all spectra. There is not significant diﬀerence in theneutron flux and spectrum among the major radii of 10,15 and 20 m in the torus hall. This is due to the reflectioneﬀect of the concrete wall.
In the gamma-ray spectra, prompt gamma-rays fromconcrete components, such as H(n, γ), 56 Fe(n, n ), 29 Si(n,γ), and 56 Fe(n, γ) are identified. Also, there is not a large

Fig. 4 (a) Neutron and (b) gamma-ray spectra at the neutron
emission rate of 1.9 × 1016 n/s in the torus hall on theequatorial port (O-port) level (Z = 5.5 m).

diﬀerence in the gamma-ray flux and spectrum among themajor radius of 10, 15 and 20 m.

4. Neutronics Analyses of the CNPA
Shield4.1 Analyses of the original shield
A photograph of the original shield made of 15 cm
thick borated polyethylene in shown in Fig. 5. The original

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Fig. 5 Photograph of the original shield for the CNPA.

Fig. 6 MCNP calculation model of the original shield for
CNPA. Frame of stainless steel is modeled, but not shownin this figure.

shield is based on the preliminary neutronics design [9],but is not the same as the preliminary neutronics design
itself. In the preliminary shielding design, the shield ismodeled by a simple box of borated polyethylene. However, the original shield has a detailed structure such as ableholes and support frames. Also, a shield for electronics isadded under the main shield for the CNPA.
As described above, this shield is insuﬃcient forhigher neutron yield plasma shots. At first, neutronics
analysis is carried out for the original shield taking into account such detailed structures. The original shield is modeled as precise as possible for the MCNP calculation asshown in Fig. 6.
In order to identify the neutron and gamma-ray transport roots to the detectors of CNPA, we carried out the neutronics calculations for three models: the original shield,
the original model without cable holes, and the originalmodel without cable holes and the beam inlet. In theMCNP calculations, these three models of the CNPA areincorporated with the LHD whole model shown in Fig. 3.
In order to improve the statistics of the calculation, the gridmesh type weight window technique is employed. The
typical number of source particles is 5 × 108 . Figures 7Fig. 7 Neutron flux distribution on the horizontal plane in theCNPA at the neutron emission rate of the neutron emission rate of 1.9 × 1016 n/s for (a) the original shield, (b)without cable holes, and (c) without cable holes and thebeam inlet hole.
Table 1 Neutron and gamma-ray flux at the detector position in the CNPA for the neutron emission rate of
1.9 × 1016 n/s.

and 8 show calculated neutron and gamma-ray distributions, respectively, in the CNPA on the horizontal planeat the neutron emission rate of the neutron emission rate of
1.9 × 1016 n/s for those three models. Also, Table 1 summarized neutron and gamma-ray flux at the detector position at the neutron emission rate of 1.9 × 1016 n/s for thosethree models.
The neutron streaming from the cable holes is not significant contribution to the neutron flux at the detectors. On3405048-3

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Fig. 9 MCNP calculation model of the upgraded shield for theCNPA.

4.2 Upgraded shield and the analysis

Fig. 8 Gamma-ray flux distribution in the CNPA at the neutron emission rate of the neutron emission rate of
1.9 × 1016 n/s for (a) the original shield, (b) without cable holes, and (c) without cable holes and the beam inlethole.

other hand, the contribution of neutron streaming from thebeam inlet is approximately 60% to the neutron flux at thedetector position. Therefore, reducing the beam inlet diameter is very eﬀective for reducing the neutron flux at thedetector position. Even though we close cable holes andthe beam inlet, the neutron flux at the detector position isstill 1.0 × 107 n/cm2 · s, which is the contribution from theneutron transmission through the bulk shield. Therefore,adding the bulk shield thickness is also required to reduce
the neutron flux at the detector position in order to be lower
than 1 × 107 n/cm2 · s.
There is no significant diﬀerence of the gamma-ray
flux at the detector position among those three models.
Gamma-rays are generated inside the bulk shield near thesurface facing to LHD. However, generated gamma-rays
are absorbed by the bulk shield itself. We installed a lead
block to protect the detectors from the gamma-rays generated inside the bulk shield. However, it seems that the lead
block is not eﬀective for reducing the gamma-ray flux atthe detector position.

By the neutronics analyses for the original shield, wecarried out the design of the upgrade shield as shown inFig. 9. The borated polyethylene with 5 cm thickness isadded to the bulk shield. The diameter of the beam inlet
is reduced from 124 mm to 65 mm. Furthermore, an extension shield of 20 cm in diameter and 35 cm in length isadded at the beam inlet. The neutron streaming from thecable holes is not significant. However, the cable holes will
be filled with plastic putty to enhance the shielding performance. The thickness of the plastic putty is assumed to beequivalent to the 5 cm polyethylene in the neutronics analysis.
The MCNP calculation is performed for the upgradeshield for the CNPA shown in Fig. 9. Figures 10 and 11
show neutron and gamma-ray distributions in the CNPA
at the neutron emission rate of 1.9 × 1016 n/s for the upgraded shield on horizontal and vertical planes. It is clearlyobserved that the neutron flux at CNPA is reduced remarkably. The neutron flux at the detector position is
1.9 × 106 n/cm2 · s, which is low enough for the requiredspecifications. Also, the gamma-ray flux at the detector
position is 2.6 × 108 n/cm2 · s, which is approximately 2/3of that of the original shield.
Figure 12 shows neutron and gamma-ray spectra at
the detector position in the CNPA. In the neutron spectrum, virgin neutrons of 2.45 MeV and the flat spectrum
generated by the slowing down process are clearly seen. In
these calculations, the source neutron energy in the plasmais assumed to be 2.45 MeV only. The 14 MeV source neutron is not token into account, because the fraction of the14 MeV source neutron is typically 0.1-0.2% of the total
neutron in many shots of the first deuterium plasma campaign. Because the boron in the borated polyethylene absorbs thermal neutrons, the thermal peak lower than 0.1 eV
is not observed.

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Fig. 10 Neutron flux distribution on the horizontal and verticalplanes in CNPA at the neutron emission rate of the neutron emission rate of 1.9 × 1016 n/s.

Fig. 12 (a) Neutron and (b) gamma-ray spectra at the detector position in CNPA for the neutron emission rate of
1.9 × 1016 n/s.

the gamma-ray eﬀect on the channeltron. However, thegamma-ray may generate secondary electrons on the active area of the channeltron. Therefore, it is consideredthat gamma-rays also will make noises in the CNPA measurement. Gamma-rays lower than 0.5 MeV are relatively
shielded easily by a tungsten sheet or similar material. Itis proposed that the channeltron detector array should becovered by the tungsten sheet locally.

5. SummaryFig. 11 Gamma-ray flux distribution on the horizontal and vertical planes in CNPA at the neutron emission rate of theneutron emission rate of 1.9 × 1016 n/s.

The gamma-ray spectrum is softer than that of theenvironment in the torus hall shown in Fig. 4 (b). In theCNPA, gamma-rays lower than 0.5 MeV are dominant.
The prompt gamma-rays from neutron capture reactionsof 56 Fe(n, γ)57 Fe, H(n, γ)D and annihilation gamma-raysof 0.511 MeV are identified. There is no guide line ofNeutronics design for the upgraded shield of CNPA
has been performed using the MCNP6 code to prevent theneutron-induced noise in the higher neutron emission rateshots of LHD. At first, we carried out the detailed neutronics analyses for the original shield with taking accountof the detailed structure such as cabling holes and support frames in order to identify the neutron transport rootto the detectors. It is found that contributions of the neutron streaming from the beam inlet and the penetration of
the bulk shield are approximately 60% and 40%. Consequently, 5 cm borated polyethylene plates are added to thebulk shield and the borated polyethylene cylinder shieldof 20 cm in diameter and 35 cm in length is added aroundthe flight tube. In the upgraded shield, the neutron flux is3405048-5

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expected to be 1.7 × 106 n/cm2 · s at the detector array at
the neutron emission rate of 1.9 × 1016 n/s, which is low
enough for the required specifications.

Acknowledgments
This work was supported by the LHD project budgets(ULHH003 and ULGG801).
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[4] Ioﬀe institute, Neutral particle analyzer,
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[6] M.B. Chadwick et al., Nucl. Data Sheets 112, 2887 (2011).
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[9] T. Nishitani et al., forthcoming in Progress in Nucl. Sci.
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[10] M. Isobe et al., IEEE Trans. Plasma Sci. 46, 2050 (2018).

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