Plasma and Fusion Research: Regular Articles

Volume 11, 2405057 (2016)

Radiation Field Estimation for the Diagnostic and Control
Components by Monte Carlo Neutronics Calculations with LHD
3-Dimensional Modeling∗)
Takeo NISHITANI1) , Kunihiro Ogawa1,2) , Kiyohiko Nishimura1) and Mitsutaka Isobe1,2)1)2)National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, JapanThe Graduate University for Advanced Studies, 322-6 Oroshi-cho, Toki 509-5292, Japan(Received 24 November 2015 / Accepted 9 March 2016)

The precise estimation of the radiation field is required to evaluate the soundness of the diagnostic andcontrol components for the LHD deuterium experiment. Monte Carlo neutron calculations are carried out withthree-dimensional modeling of LHD using MCNP-6 code with the cross-section library of ENDF B-VI. Threedimensional map of the total neutron flux, neutron and gamma-ray spectra, and the dose rate profile are obtained.
Neutron and gamma fluxes by MCNP-6 code are about 2/3 of those by DORT code, which is probably due to thetwo-dimensional modeling of the DORT calculation. The gamma-ray absorbed dose for Si is 20 - 70 Gy duringnine years operation, where high integrated electric devices such as the programmable logic controller (PLC)
may survive if we consider the gamma-ray eﬀect only.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: LHD, deuterium experiment, radiation field, neutronics, MCNP-6, three-dimensional modeling, neutron spectrum, gamma-ray spectrum, dose rate
DOI: 10.1585/pfr.11.2405057

1. Introduction
LHD plans to start the deuterium experiment in thespring of 2017. The deuterium experiment is nine-years
campaign, in which the total neutron budget is 2.1 × 1019
neutrons for the first six years, and 3.2 × 1016 neutrons
for the next three years. One of the most important issues of the deuterium experiment is irradiation eﬀect on
diagnostics components and control devices. The precise
estimation of the radiation (neutron and gamma) field isnecessary for responding to the influence of irradiation effects on those components. Previously, the radiation fieldin the LHD torus hall was calculated by two-dimensional
neutron transport cord DORT [1] mainly for the radiation
safety analysis and the licensing of the deuterium experiment. However, LHD has a structure too complicated to betreated by the two-dimensional model. Here, we evaluatedthe radiation field in the LHD torus hall by Monte Carloneutrons calculation with three-dimensional modeling ofLHD.

2. Calculation Method
In this calculation, the MCNP Monte Carlo neutronics
code (MCNP-6) [2] is used with the cross-section libraryof ENDF B-VI [3]. The model is divided by cells in theMCNP calculation, where all surfaces of cells are definedby quadratic (or less) equations or torus. However, the heauthor’s e-mail: nishitani.takeo@nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

lical surface could not be represented. In the MCNP calculation geometry, the LHD components within the support
structure are divided by small toroidal angle pitch, and thecomponents are assumed to be toroidally symmetric in a
toroidal pitch angle. The geometry in one toroidal pitch
angle is modeled based on the CAD drawing with some
simplification. As the first step, the toroidal pitch angle
is selected to be six degrees. Figure 1 shows the poloidal
cross sections of the LHD calculation model, where thetoroidal angle of φ = 0 is defined by the center of the U/Lport. A poloidal angle θ of a center of one helical coil ispresented by θ = φ (m/l) + α sin{φ (m/l)}, where m = 10,l = 2, and α = 7.8 degrees. Surface equations of the helical structure are automatically generated by EXCEL basedon this formula for each toroidal pitch angle. The LHD
plasma is modeled by a simple torus with the major radius of 3.9 m and the minor radius of 0.5 m. The neutron
source is isotropic and homogeneous in the torus with theenergy of 2.45 MeV. The secondarily-generated 14 MeV
neutron by the triton burn-up process is ignored. The helical and poloidal coils are modeled by homogenized materials respectively, based on each composition such as superconducting coils, electric insulators and liquid helium.
The material of the vacuum vessel and the support structure is stainless steel (SS) 316L, and the cryostat is SS304.
This model includes the concrete torus hall wall, the floor,and the ceiling, with cylindrical geometry. This 36-degrees
model consists of approximately 150 cells.

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c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 2405057 (2016)

Fig. 1 Poloidal cross sections of the LHD calculation model at φ = 0◦ , 6◦ 12◦ , and 18◦ , where the toroidal angle φ = 0◦ is defined by thecenter of the U/L port. Also poloidal cross sections at φ = −6◦ , −12◦ , and −18◦ are similar.

Fig. 2 Detailed neutron flux distributions in the poloidal cross-sections on the O-port and the U/L port radial directions, and in the toroidal
cross-section on the mid plane for the plasma with the total neutron yield of 1.9 × 1016 neutrons/s.

3. Results
3.1

Neutron and gamma-ray flux distributions

Three-dimensional maps of neutron and gamma-ray
fluxes in the LHD torus hall were obtained. Figure 2
shows the detailed neutron distributions in the poloidal
cross-sections on the O-port and U/L port radial directions, and in the horizontal cross-section on the mid planefor the plasma with the total neutron yield of 5.7 × 1016
neutrons/3s (1.9 × 1016 neutrons/s), which is the expected
maximum neutron yield. The neutron flux decreased dramatically outside the cryostat. The neutron streaming fromO-port or U/L ports is not as distinguished as we expected,
which is probably due to the shielding eﬀect of rather thickport flanges. The neutron flux in the center column is aslarge as that in the cryostat.
Figure 3 shows the neutron and gamma-ray flux profiles in the torus hall for the same neutron yield plasma asFig. 2 compared with the DORT calculation [4]. Neutronand gamma fluxes by MCNP are about 2/3 of those by theDORT calculation where the neutron and gamma fluxes arederived by averaging those fluxes by two cylinder models
based on the O-port and the U/L port cross-section without

port flanges, as shown in Fig. 4. Even though averaging
two models, the eﬀective port area is excessive, which may
cause overestimation of the neutron and gamma fluxes inthe torus hall. However, this DORT calculation is adequate
for the radiation safety analysis, because it provides safety
side estimation of the radiation field.
The neutron flux in the O-port line at R = 7.3 m isabout 40% larger than that in the U/L-port line, which isdue to the neutron streaming eﬀect from the O-port. However, the neutron fluxes in the O-port and U/L-port lines
are almost identical for R > 9 m.
The gamma-ray flux profile by MCNP is rather flat for
R > 9 m compared with that by DORT. The reason is underdiscussion at present.

3.2 Neutron and gamma-ray energy spectra
Figure 5 shows neutron and gamma-ray spectra in thetorus hall in front of the O-port outlet (R = 8 m). In the neutron spectrum, 2.45 MeV neutrons from D-D reactions areidentified, however, scattered neutrons lower than 2 MeV
are dominant. The averaged neutron energy is derived tobe 0.54 MeV from this spectrum, which is important forthe shielding design of the equipments in the torus hall and2405057-2

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Volume 11, 2405057 (2016)

Fig. 3 Neutron and gamma-ray flux profiles in the torus hall forthe plasma with the total neutron yield of 1.9 × 1016 neutrons/s compared with the DORT calculation [4].

Fig. 5 Neutron and gamma-ray spectra in the torus hall in frontof the O-port outlet (R = 8 m).

3.3 Dose rate and dose distributions

Fig. 4 Cylindrical models of LHD for DORT calculation basedon (a) the O-port line and (b) the U/L-port line crosssections.

also for the sky shine dose estimation.
On the gamma-ray spectrum, a peak of 7.8 MeV isthe prompt gamma-ray from 56 Fe which is a major component of stainless steels. The averaged gamma-ray energy is
1.4 MeV. This gamma-ray spectrum does not include delayed gamma-rays from activated materials of LHD andthe torus hall concrete, which are considered to be muchsmaller than prompt gamma-rays.

From the engineering point of view such as the irradiation eﬀect on the components for diagnostics and control
systems, not only neutron and gamma-ray fluxes but alsodose rate and/or total dose are important. Organic materials for electric insulator and highly integrated electronic
devices are considered to be rather irradiation sensitive.
Figure 6 shows the dose profiles for organic material in thetorus hall on the mid plane during nine years of the LHD
deuterium experiment. Here polyethylene, (CH2 )n, is assumed as a typical organic material. It is very clear that theneutron contribution is about 10 times larger than that of
gamma-rays.
The radiation damage on the electronic devices withhigh integrated circuits, such as a PLC (ProgrammableLogic Controller) for diagnostics, and control systems is
one of the most urgent issues for the LHD deuterium experiment. Figure 7 shows the profiles of the dose on Silicon,
which is a major composition of the electronics devices
during nine years of the LHD deuterium experiment. Contrasting with Fig. 6, the gamma-ray contribution is dominant. The gamma-ray dose is almost insensitive to the2405057-3

Plasma and Fusion Research: Regular Articles

Volume 11, 2405057 (2016)
tests on the electronic devices with high integrated circuits such as a PLC at the 60 Co irradiation facility ofNagoya University. There, some devices broke downaround 100 Gy [5]. If we consider the dose only, suchelectronic devices may survive nine years. However, theneutron irradiation provides not only energy deposition but
also atomic replacements on the material and single event
errors in the electronic devices. Thus, the soundness ofsuch electronic devices should be considered more carefully taking account of the neutron irradiation eﬀect, which
will be investigated in the near future.

4. SummaryFig. 6 Equivalent dose rate profiles for polyethylene, (CH2 )n, in
the torus hall on the mid plane.

Fig. 7 Dose profiles for Si in the torus hall on the mid plane
during nine years of the LHD deuterium experiment.

target material, however, the neutron dose depends on thetarget material. If we consider the energy deposition byelastic collisions, the neutron dose decreases with the mass
number of the target material. The energy transfer to Si ismuch smaller than polyethylene which contains much hydrogen. Thus, the diﬀerences between Figs. 6 and 7 arereasonable.
Recently we performed the gamma-ray irradiation

Monte Carlo neutron calculations are carried out withthree-dimensional modeling of LHD using MCNP-6 codeby dividing six degrees toroidal angle pitch for the LHD
deuterium operation. Three-dimensional map of the total neutron flux, neutron and gamma-ray spectra, and thedose rate profile were obtained. Neutron and gamma fluxes
by MCNP-6 code are about 2/3 of those by DORT code,
which is probably due to the two-dimensional modeling of
the DORT calculation. Outside the cryostat, scattered neutrons are dominant. The total equivalent dose rate reaches
to about 1 Sv/s near the cryostat, where the neutron contribution is about 100 times larger than that of gammarays. The gamma-ray absorbed dose for Si is 20 - 70 Gy
during nine years of operation, indicating that high integrated electric devices such as the PLC may survive if weconsider the gamma-ray eﬀect only.
[1] M.B. Emmett et al., the DORT section of A User’s Manualfor MASH 1.0 - A Monte Carlo Adjoint Shielding Code System, ORNL/TM-11778 (Oak Ridge National Laboratory,
Oak Ridge, 1992).
[2] X-5 Monte Carlo Team, MCNP User’s Guide - Code Version 6.1.1beta, LA-CP-14-00745 (Los Alamos National
Laboratory, Los Alamos, 2014).
[3] H.D. Lemmel, et al., ENDF/B-VI Release 8 (Last release ofENDF/B-VI) The U.S. Evaluated Nuclear Data Library for
Neutron Reaction Data, IAEA-NDS-100, Rev. 11 (IAEA,Vienna, 2001).
[4] Safety management plan of the deuterium experiment at theLarge Helical Device (National Institute for Fusion Science, Toki, 2012) in Japanese.
[5] K. Ogawa et al., Eﬀects of gamma-ray irradiation on electronic and non-electronic equipment of the Large HelicalDevice, presented at the 25th Int. Toki Conf., 3-6 November, 2015, Toki-city, Japan.

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