J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Neutron Transport Analysis for In-vessel Diagnostics in ITER
Masao Ishikawa, Takashi Kondoh, Takeo Nishitani, Yasunori Kawano, Yoshinori KusamaFusion Research and Development Directorate, Japan Atomic Energy Agency, Ibaraki, 311-0193, Japan(Received: 12 November 2009 / Accepted: 21 January 2010)

Nuclear transport analysis using the MCNP code and heat analysis using a general purpose Finite Element
Method code ANSYS 11, respectively, have been carried out for in-vessel components of the microfission chamber(MFC) and the poloidal polarimeter. The nuclear heating rates of the MI cable and the exhaust pipe of the MFC are
highest in the gap between two adjacent blanket modules and decrease at greater distances from the gap. Heat
analysis using the nuclear heating rate calculation indicates that the maximum temperature of the exhaust pipe canbe lowered by changing the distance between two cooling clamps. The nuclear heating rates of the first mirrors of
the poloidal polarimeter in the upper port plug are high (> 1 W/cc), and the radiation shield could reduce this rate
only by a factor of 2 ~ 4. This heating likely is caused by the effect of streaming neutrons due to an overlap of thebeam lines. However, it has been found that the nuclear heating rate can be reduced by reducing the diameter of thebeam transmission line.
Keywords: ITER, ITER Diagnostics, MCNP, Neutron Transport Analysis, Nuclear heating, Fusion Power
Diagnostics, Neutron Measurement, Microfission Chamber, Polarimeter, Current Profile.

Neutron transport analysis for the optical mirrors
installed in a port plug is also important to ensure theirappropriate operation. The first mirrors have a direct
sight-line into the plasma. The poloidal polarimeter,
which is also procured by JADA, measures the profile ofthe toroidal current in the core region using an opticalmirror system [6]. The system will be installed in theupper and equatorial ports. It must have a labyrinthine
structure to shield the mirrors from neutron/gamma
radiation and an active mirror cooling system to offset
nuclear heating. In order to design the optimum
diagnostics
system,apreciseestimation
ofneutron/gamma fluxes, nuclear heating and the dose rateis necessary. In this work, neutron transport analysis also
has been carried out on the design of the upper port
diagnostics system.
In this paper, results of neutron transport analysis
applied to the MFC and the poloidal polarimeter arepresented. In section 2, an explanation of the MFC
system and results of nuclear heating analysis and heatanalysis are presented. Nuclear heating analysis for theoptical mirrors of the poloidal polarimeter as well as theexplanation of the measurement system is reported in
section 3. Finally, a summary is presented in section 4.

1. Introduction
Neutron transport analysis is very important for thedesign and optimization of diagnostics in ITER.
Especially, in-vessel diagnostics are exposed to strong
neutron and gamma radiation that could lead to damage
and temperature increases due to the nuclear heating of
the components of those diagnostics. A high dose rate
due to strong radiation also makes maintenance difficult.
Therefore, an evaluation of the neutron/gamma flux,
spectrum and nuclear heating at the location of thediagnostics through neutron transport analysis is essential
for designing a radiation shield and/or a cooling system.
The microfission chamber (MFC), which is adiagnostic for measuring total neutron source strength inITER, is one of many in-vessel diagnostics [1]. The MFC
for ITER is being developed by the Japanese Domestic
Agency (JADA). Heat transport analysis of the MFC
based on neutron transport analysis indicates that thedetector temperature could be maintained at less than theoperational temperature limit without active coolingsystems [2]. One of the reasons an active cooling system
is unnecessary is that the MFCs are installed behind theblanket module and are not directly exposed to theplasma. However, the double coaxial mineral insulated(MI) cable, which is the signal cable for the MFC, and theexhaust pipe pass through the gap between two blanket
modules en route to the upper port [3]. As such, thetemperature of the MI cable and the exhaust pipe may
increase beyond their operational temperature due to highnuclear heating as a result of the strong radiation of
streaming neutrons and gamma rays in the gap. Nuclear
heating analysis and heat analysis using MCNP 5 [4] anda general purpose Finite Element Method code ANSYS 11[5], respectively, have been carried out to design a coolingsystem consistent with neutron transport calculations.

2. Neutron transport analysis for the MI cable andthe exhaust pipe of the MFC system
2.1 The structure of the MFC system
The MFC is a pencil-sized gas counter containing
235
U, which was developed as an in-core monitor forfission reactors. In the MFC, a coating of UO2 covers theouter cylindrical electrode. The active length is 76 mm,
and the MFC contains a total of 10 mg of 235U. The MFC
is filled with 95% Ar and 5% N2 gas at 14.6 atm as anauthor’s e-mail: ishikawa.masao@jaea.go.jp

43©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

M. Ishikawa et al., Neutron Transport Analysis for In-Vessel Diagnostics in ITER

Fig.1

Schematic view of the MFC for preventing Argon
gas leakage into the vacuum vessel [3].

Fig. 3(a) The poloidal section of MCNP calculation
model for nuclear heating calculation of the MI
cable and exhaust cable. (B) correspond to thoseinstallation locations behind the blanket module.

Fig.2

Nuclear Heating (watt/cc)

0.5(a) Location of MFCs, MI cable and exhaust pipe on
the ITER poloidal cross section and (b) schematic
view of those installation positions at the lower
outboard location.

ionization gas. In previous design work, an MFC to beapplied to the ITER environment was designed as shownin Fig.1. The designed MFC is enclosed in a stainlesssteel case. The case prevents gas leakage into the vacuum
vessel even when Ar gas leaks from the MFC due to any
insufficient airtightness of the seal. An exhaust pipe isattached to the stainless steel case in order to exhaust anddetect any leaked Ar gas [3]. A tri-axial mineral insulated(MI) cable is used to transfer signals and to supply power
to the MFC. The cable uses SiO2 as an electrical insulator
with a packing density of 30%. The central conductor(Cu) is also insulated with a SiO2 insulator.
MFCs will be installed behind blanket modules atboth upper and lower outboard positions in order that thelinear combination of output of MFCs at both positions isinsensitive to changes in the shape and position of theplasma as shown in Fig.2 [1, 3]. The MI cables and theexhaust cables also will be installed behind blanketmodules and travel from each MFC position to the upper
port, where the MI cables are connected to a soft cableoutside the vacuum vessel via a feed-through in the upper
port. The MI cables and the exhaust pipes pass throughthe gap between blanket modules en route to the upper
port as shown in Fig.2 (b).
.
2.2 Nuclear heating analysis for the MI cables and theexhaust pipe
The nuclear heating rates of the MI cables and theexhaust pipe were analyzed through neutron transport
calculations using MCNP 5, where FENDL 2.1 [8] wasused as the nuclear library for the calculation. A 40°
toroidal section which includes the first wall, the blanket

SUSSlica
Cu

0.4
0.3
0.20.100102030

4050

Distance from Gap (cm)Fig.4

Calculated nuclear heating rate for various
distances along the gap from the front reference
position (0 cm)

module, the vacuum vessel, poloidal coils and toroidal
coils are modeled in this calculation. The poloidal
cross-section of this calculation model is shown in Fig.3.
The nuclear heating rate of the MI cable and the exhaust
pipe behind the blanket modules was modeled as shown in
Fig.3 (b). The neutron source is set as a toroidally
symmetrical source with 14 MeV, mono-energetic energy,
and the neutron profile is set based on the main scenariosof ITER operation (500 MW fusion power). In thiscalculation, the nuclear heating rate of the MI cable and theexhaust pipe at various distances from the rear of the gap(Fig.3 (b)) in the direction of the arrow has been calculated.
Figure 4 shows the calculated nuclear heating rate for eachmaterial used for the MI cable (SUS, Silica and CU) andthe exhaust pipe (SUS). The horizontal axis in Fig.4
corresponds to the distance from the reference position atthe rear of the gap (0 cm) shown in Fig.3 (b). It was foundthat the nuclear heating rates of SUS and Cu are muchhigher than that of Silica (the electrical insulator). It wasalso found that the highest nuclear heating rate is at therear of the gap, with the nuclear heating rates decreasing
as the distance from the gap increases. It is consideredthat this heating is caused by the ratio of streaming
neutrons to the total neutron flux [8].

44M. Ishikawa et al., Neutron Transport Analysis for In-Vessel Diagnostics in ITER

Calculated temperature distribution of the exhaustpipe.
Maximum Temperature (!)

Fig.6

Fig.5 Calculated modes of the heat analysis for the exhaustpipe; (a) whole model, (b) model of the clamp

400
350
300
250
200
150
100
0

5

10

15

20

2530

Distance between two clamps (cm)Fig.7

2.3 Heat analysis for the exhaust pipe
Heat analysis for the exhaust pipe was carriedout using a general purpose Finite Element Method code
ANSYS 11, using the calculated nuclear heating rate
described in Sec. 2.2. The model for the calculation isshown in Fig.5. In order to offset the nuclear heating of theexhaust pipe, we have designed a clamp as shown in Fig.5(b). In this calculation, we assumed that the exhaust pipe
would be cooled by heat conduction to the vacuum vessel(VV) via only the welding joints on each side of the clamp
attached to the VV and that the temperature of the VV
would be kept at 100°C.
Figure 6 shows the calculated temperature
distribution of the exhaust pipe. The temperature at thecenter between the two clamps is the highest because that
position is farthest from the cooling point of the welding
joint of the clamp. Figure 7 shows the maximum
temperature as a function of the distance between the twoclamps. It was found that the maximum temperature wasreduced as the distance between the two clamps
shortened. This suggests that the maximum temperature
can be controlled by changing the distance between thetwo clamps. In the future, the distance between the twoclamps will be optimized by taking into account thermal
expansion, stress and the electromagnetic force of theexhaust pipe.

Maximum temperature of the exhaust pipe as afunction of the distance between two clamp

vacuum windows. Far-infrared laser beams (a wavelengthof 118m) are launched into the plasma through both
port plugs using the mirror systems. The laser beams passthrough the plasma and are reflected back along the same
path by retro-reflectors installed in the first walls. The
Faraday rotation angle of a polarization plane of areflected beam depends on the toroidal current of theplasma. Then, q(r) can be identified using the Faradayrotation angle [6]. In previous design work, view chordsof the beam in the upper port system were optimized aslisted in table 1. Molybdenum (Mo) was considered oneof suitable material of the optical mirror and the diameterof the beam transmission line was optimized 140 mm.
Since the first mirrors are directly exposed to theplasma, strong neutron and gamma radiation could lead tohigher nuclear heating and thus, high temperature
increases. In order to avoid strong neutron and gamma
radiation, a complex optical mirror alignment, which hasa labyrinthine structure to shield it from neutrons and amirror cooling system to offset nuclear heating, isnecessary. Therefore, nuclear heating analysis isnecessary to design a radiation shield and cooling system
for the optical mirror system.
3.2 Nuclear heating analysis of the mirror systems in
the Upper port
The nuclear heating rates of the Mo mirrors in theupper port were analyzed using MCNP 5. The basic
calculation model is the same as shown in Fig.3.
However, the model of the upper port was modified toyield a precise estimation of the nuclear heating rate of
the mirrors as shown in Fig.8. In order to estimate how

3. Nuclear heating analysis of the optical mirrorsof the poloidal polarimeter
3.1 Measurement system of the poloidal polarimeter
The poloidal polarimeter system consists oftwo systems of the upper port and the equatorial port
diagnostics systems. Each system consists of mirror
systems made up of first mirrors, second mirrors and45

M. Ishikawa et al., Neutron Transport Analysis for In-Vessel Diagnostics in ITER

Fig.8 Schmatic view of the Polodal Polarimeter system.

Tab.1 Optimized view chords of the mirrors in the upperport.
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012/

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3///

02-/

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51/

-,4//

04,/

/

33//

02+/

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041/

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04+.

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043/

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013/

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3+2/

02+/

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042/

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Fig. 9(a) The poloidal section of MCNP calculation modelfor nuclear heating calculation of the mirrors of thepoloidal polarimenter and (b) correspond to thoseinstallation in the upper port. No. 1 ~ 6 corresponds
to the first mirror’s number, respectively.

orders). In order to investigate any variance in the effectof the streaming neutrons on the nuclear heating of thefirst mirrors due to an overlap of the beam lines, the roleof the diameter of the beam transmission lines on thenuclear heating rate was evaluated by changing thediameter of the beam transmission line in the previouscalculation. Figure 10 shows the results of thecalculations. It was found that the nuclear heating rate is
reduced as the diameter of beam transmission line isreduced. This suggests that one of the ways to suppress
nuclear heating is to change the diameter of the beamtransmission lines. However, the diameter of the beamtransmission line affects measurement accuracy.
Optimization of the diameter of the beam transmission
lines is therefore necessary.

the radiation shield could reduce the nuclear heating rate,
the nuclear heating rate of the mirrors was calculated fortwo cases, one in which the port plug is not filled withany materials, and the other in which the port plug isfilled with a neutron shield (70 % stainless steel and 30 %water), absent the mirrors and beam lines (shown in Fig.9(b)). The calculated nuclear heating rates for both cases
are shown in table.2. The nuclear heating rate for the firstmirrors is relatively high. Especially affected are thosenear the plasma (No.1, 2, 3), with rates that exceed 1 w/cc.
The radiation shield is estimated to reduce the heating
only by a factor of 2 ~ 4 times for the first mirrors(No.1~5). Because these mirrors have direct sight to theplasma center, where is the large neutron and gamma-ray
source region, and the radiation shield hardly affect toreduce neutrons and gamma-rays from the plasma center
due to streaming neutrons by overlap of the beamtransmission lines. On the other hand, even though themirror No.6 has direct sight to the plasma, the shield canbe reduced the nuclear heating rate by more than 1 order.
It is considered that the reasons for larger reduction of thenuclear heating than that of No.1 ~3 are that the mirror
No.6 doesn’t have direct sight to the plasma center andthe effect of streaming neutrons much smaller. Thenuclear heating rates of the second mirrors are relatively
low, and the neutron shield is estimated to reduce theheating rates by more than 1 order (up to more than 2

4. Summary
In this work, neutron transport analysis andheat analysis were performed for MFC and the poloidal
polarimeter to assist in the design of a cooling system andradiation shield. The nuclear heating rates of the MI cableand the exhaust pipe of the MFC were calculated using
MCNP. The nuclear heating rate of the MI cable and theexhaust pipe of the MFC is highest (0.34, 0.28 w/cc forCu, SUS) in the gap between blanket modules anddecreases as the distance from the gap increases. Heatanalysis based on the nuclear heating calculations
indicates that temperature increases of the exhaust pipe
can be controlled by changing the distance between
cooling clamps. The nuclear heating rate of the Mo

46

M. Ishikawa et al., Neutron Transport Analysis for In-Vessel Diagnostics in ITER

No.
1
2
34
56

1st mirror heating (w/cc)
w/o shiled
with shiled
1.97E+00
8.39E-01
1.59E+00
8.23E-01
1.18E+00
5.17E-01
8.66E-01
2.64E-01
6.46E-01
1.58E-01
2.25E-01
1.96E-02

Nuclear heating rate (W/cc)

Tab.2 Calculated nuclear heating rate of the mirrors of thepoloidal polarimeter.
2nd mirror heating (w/cc)
w/o shiled
with shiled
9.74E-01
1.50E-01
7.10E-01
6.63E-02
2.94E-01
1.41E-02
2.26E-01
6.48E-03
9.01E-01
1.06E-01
4.51E-01
1.37E-02

mirrors of the poloidal polarimeter were also calculated.
The nuclear heating rates of the first mirrors are high (> 1w/cc for mirrors No.1~3), because those are directly
exposed to neutron and gamma radiation. The radiation
shield (SUS 70% and water 30%) was able to reduce thenuclear heating rate by a factor of only 2 ~ 4 times. It isconsidered that this rate is caused by the effect of
streaming neutrons due to an overlap of the beam lines.
Dependence of the diameter of the beam transmission
lines on the nuclear heating rate subsequently wasevaluated. It was found that the nuclear heating rate could
be lowered by reducing the diameter of beamtransmission lines. This suggests that one of the ways tosuppress nuclear heating is to change the diameter of thebeam transmission line.
In this work, heat analysis for the MI cable was
not carried out because the thermal conductivity of Silica
is unknown. In the future, in order to carry out heatanalysis and to design a cooling system for the MI cable,
an investigation of the thermal conductivity of thepowder and fiber of Silica will be necessary. Detailed
estimates of heat transport of the mirrors of the poloidal
polarimeter also are necessary to design a radiation shieldand cooling system.

References

[6]
[7]
[8]

No.1
No.2
No.3

0.8
0.6
0.4
0.2

Fig.10

The authors would like to acknowledge the ITERJADA team for their help and advice. The authors would
like to thank the FNS group in JAEA for their support andadvice related to the calculations included in this article.

[5]

1080

100120140160180!"#$%&%'()*(&+%(,%#$(-$$.

Acknowledgment

[1]
[2]
[3]
[4]

1.2

M. Yamauchi et al., Rev. Sci. Instrum., 74 (2003), 1730.
T. Nishitani, et al., Fus. Eng. Des., 82 (2007), 1192.
M. Ishikawa, et al., Plas. Fus. Res. SERIES, 8(2009), 334
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Los Alamos National Laboratory (2003).
ANSYS Inc.,
http://www.amsus.com/products/multiphysics.asp
T. Yamaguchi, Plasma Phys. Control. Fusion 50 (2008)
045004.
B. Forrest, A. Trkov, INDC(NDC)-451, IAEA
Headquarters, Vienna, Austria (2004).
M. Ishikawa, et al., Rev. Sci. Instrum., 79 (2008),
10E507.

47

Nuclear heating rate of the first mirrors of thepoloidal polarimeter as a function of the diameterof the beam transmission line.