22P136-P
Progress in the development of Microfission chamber for ITER
ITERマイクロフィッションチェンバーの開発の進展	 
Takashi Kondoh, Masao Ishikawa, Yoshinori Kusama
近藤	  貴, 石川正男，草間義紀
Japan Atomic Energy Agency
801-1, Mukoyama, Naka, Ibaraki 311-0193, Japan
原子力機構 〒311-0193	  茨城県那珂市向山801-1

A microfission chamber to be installed in the vacuum vessel has been developed formeasurement of total neutron emission rate and fusion power in ITER. In-vessel component
including detector, mineral insulated (MI) cable and feedthrough have been designed andprototypes have been made to test feasibility. Conceptual design review has been completed inOctober 2010 and it was concluded that the present design of MFC was applicable to ITER.
1. Introduction
The microfission chambers (MFCs) measureneutron flux at the vacuum vessel and provide
total neutron emission rate and fusion power in
ITER [1-4]. MFC units will be mounted on theinner shell of the vacuum vessel behind blanket
module at two poloidal and two toroidal locations(total 4 locations) as shown in Fig. 1. One MFC
unit consists of two chambers with 235U and adummy chamber to measure γ-ray backgroundnoise.
2. In-vessel components
2.1 Detector
MFC is a pencil-size gas counter that wasdeveloped as an in-core monitor for fission
reactors. A total amount of 10 mg of 235U iscoated on the outer cylindrical electrode. Figure1(A) shows a schematic view of a typical MFC

developed for fission reactor. Figure 1(B) shows
a drawing of the modified MFC designed to fulfill
the requirements of the vacuum vessel
environment of ITER. This modified MFC isenclosed in a stainless steel case and connected toguard vacuum pipe to detect gas leak and vent thegas outside the ITER vacuum vessel.
2.2. MI cable
Tri-axial mineral insulated (MI) cables will be
used to apply bias voltage between the electrodes ofthe MFC and to carry signals. Particular challenge
for implementation on ITER was the developmentof the in-vessel connector for the tri-axial MI cable.
In order to install the MFC and the MI cable atdifferent times, a joint for connecting them in thevacuum vessel is necessary. Vacuum tight tri-axial
connector of the MI cable has been designed andprototype has been made [4].

Fig. 1. Schematic layout of microfission chambers in ITER

shown in Fig. 4 (a). To prevent functional errors ofdigital circuits, digital processing should beinstalled in diagnostic building as shown in Fig. 4(b). Pre-amplifier and power supplies should beshielded by lead of 10 - 20 cm thickness.

Fig.2. (a) Schematic view of the MFC, (b) theMFC designed for preventing Argon gas leakage

2.3. FeedthroughFeedthrough, which carries the signal from inside
to outside of the vacuum vessel, has been designed.
Space for the feedthrough is limited as shown in Fig.
3. To meet the ITER vacuum requirement, guardvacuum is needed. A prototype of the feedthroughwas fabricated to assess its feasibility.	  Helium
leak test was conducted and it was confirmed thatthe leak late was less than 10-12 #-G93/s, whichmeets ITER requirement (10-10 #-G93/s).

Fig. 3. Design of feedthrough of tri-axial MI cable.

3. Effect of high γ -ray dose on electronics
Gamma-ray dose rate in the port cell, in where
pre-amplifier and signal processing unit will beinstalled, is estimated to 1 - 2 Gy/h due to activationof cooling water during plasma operation at fusion
power of 500 MW. The activity from water arises
because of the production of 16N due to 16O(n,p)16N
reactions. Energy of γ-ray is 6.13 and 7.12 MeV.
Expected lifetime limit of many commercial
analog electronics is around 50 Gy of γ-rayexposure. Functional errors are also expected fordigital circuits. Integrated dose in 4600 hours fullplasma operation is 4600 - 9200 Gy at the port cell.
Then reduction of γ-ray by 2 - 3 orders ofmagnitude and radiation-hardened electronics areneeded.
In the previous design, a pre-amplifier and digitalprocessing unit will be installed in the port cell asFig. 4. Schematic diagram of signal processing. (a)Previous design (b) Revised design

4. Conclusion
Conceptual design review has been completedin October 2010 and it was concluded that thepresent design of MFC was applicable to ITER.
Procurement arrangement of MFC between ITER
organization and Japan Atomic Energy Agency
will be signed early in 2012 and detail design also
will start in 2012.
Acknowledgments
The authors would like to acknowledge Power
Systems Company of Toshiba Corporation andToshiba Electron Tubes & Devices Co., LTD forthe development of MFC.
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74 (2003) 1730.
[2] M. Ishikawa, T. Kondoh, T. Nishitani and Y.
Kusama: Rev. Sci. Instrum. 79 (2008) 10E507.
[3] M. Ishikawa, T. Kondoh, T. Nishitani and Y.
Kusama: J. Plasma Fusion Res. SERIES 8 (2009)334.
[4] M. Ishikawa, T. Kondoh, K. Ookawa, K. Fujita, M.
Yamauchi, A. Hayakawa, T. Nishitani and Y.
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The views and opinions expressed herein do notnecessarily reflect those of the ITER Organization.