Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
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MHI’s Efforts towards ITER
- Challenges to Build Large-size High-performanceSuperconducting Coils KATSUSUKE SHIMIZU*1 NAOKI SAWA*1

AKIO YAMAMOTO*2

TAKASHI MIYAKE*3

TAKERU OKAMOTO*4SHUHO TSUBOTA*5

At present, the ITER Organization undertakes the design, construction, operation,
decommissioning, etc. of equipment/facilities of ITER(International Thermonuclear Experimental
Reactor), while seven participants handle the manufacturing under an adopted “in-kind system”:
in addition, equipment/facility manufacturing is under way. Japan has been assigned to develop apart of the divertor systems, plasma heating systems, plasma diagnostics, and other important
items including Toroidal Field (TF) coils to generate a magnetic field for plasma confinement and
Mitsubishi Heavy Industries, Ltd. (MHI) is involved in nearly all of these. This paper reports on the
results as well as the present status of verification tests, etc. for the manufacturing of actual TF
coils and briefly explains our activities for the divertor systems indispensable for sustaining stable
plasma confinement.

|1. Introduction
The development of nuclear fusion energy has just taken a new step as the construction of
ITER (International Thermonuclear Experimental Reactor) has started. In accordance with the then
US-Soviet Summit (Reagan-Gorbachev) Joint Declaration of 1986, the ITER project started withConceptual Design Activities (CDA) in 1989; the participants in the CDA were the USA, former
Soviet Union (Russia), Europe, and Japan. From 1992, for about nine years in the Engineering
Design Activities (EDA), detailed design, verification tests for manufacturing and new technology
development were carried out.
In 2005, Saint-Paul-lès-Durance of France was chosen as the construction site; this site isadjacent to the Cadarache Research Center of CEA (the Alternative Energies and Atomic Energy
Commission).
On November 21, 2006, a minister-level meeting was held and the ITER Agreement was
signed at the Élysée Palace in Paris. This minister-level meeting was followed by the firstprovisional ITER Council, and approval to bring the ITER international Fusion Energy
Organization (ITER Organization) into being as an international legal entity was obtained under theprovisional application of the ITER Agreement. Inauguration of the ITER Organization was
officially put into effect based on the agreement on the establishment of the ITER Organization on
October 24, 2007 via the ratification of the participants (Japan, the US, Russia, China, Korea, India,
and the European Union (EU)). An in-kind contribution system, where the ITER Organization
undertakes the design, construction, operation, decommissioning, etc. of equipment/facilities while
seven participants deal with the manufacturing of equipment/facilities, was adopted and, at present,
manufacturing of equipment/facilities is under way by participants.
Japan has been given the role of supplying Toroidal Field (TF) coils to generate the*1

Chief Staff Manager, Nuclear Systems Engineering Department. Nuclear Energy Systems Division*2

Chief Staff Manager, Nuclear Plant Component Designing Department, Nuclear Energy Systems Division*3

Chief Staff Manager, Nuclear Plant Manufacturing Department, Nuclear Energy Systems Division*4

Nuclear Plant Manufacturing Department, Nuclear Energy Systems Division*5

Chief Staff Manager, Takasago Research & Development Center, Technology & Innovation Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)16

magnetic field for plasma confinement as well as high-tech equipment such as the divertor system,some plasma heating systems (such as RF heating and NBI systems), and some plasma diagnostics(such as the impurity monitoring system and plasma density measurement system etc.). MHI is
involved in almost all of these objectives. In particular, for TF coils, which are the largest
superconducting coils in the world, prototype tests are almost complete and manufacturing ofactual coils has just started. This paper describes the results and present status of verification andother tests for the manufacture of actual TF coils and gives an outline of our activities for an ITER
divertor indispensable for sustaining stable plasma confinement.

|2. Outline of ITER and TF Coils(1)

Outline of ITER[1]The ITER mission is to verify self-ignition of fusion plasma and sustain long burningwhich is indispensable for tokamak-type nuclear fusion power generation as well as nuclear
fusion reactor engineering technology. ITER plays an important role in acquiring/accumulating
the data that contributes to such areas as the design, manufacture, and construction of ademonstration reactor for the verification of future power generation. The basic parameters of
ITER are shown in Table 1, utilizing deuterium and tritium as fuels, fusion power of 500MW,
plasma burning time of 400 or more seconds, and a fusion energy gain factor (Q)* of 10.
*Fusion energy gain factor (Q): Ratio of fusion output energy to input energy

Table 1 Main Specifications of ITER
Fusion power
Plasma burning time
Fusion energy gain factor
Plasma current
Plasma volume
Machine size dimensions
Total wight of main components(2)

up to 500MW
400 seconds - 3,000 seconds
up to 10up to 15MA
up to 840m3
About 30m in diameter and 30m in height
About 23 thousand tons

Outline of TF coils
ITER has adopted the tokamak-type magnetic field confinement system where themagnetic field generated by the plasma current flowing through the doughnut-shaped vacuum
vessel confines plasmas together with another magnetic field generated by the coils arranged
vertically in the doughnut direction. Here, the doughnut direction is referred to as the toroidal
direction, and coils arranged vertically to the doughnut direction are called toroidal field coils
since these coils generate magnetic fields in the toroidal direction. On the other hand, thecircumferential direction of the doughnut cross-section is referred to as the poloidal direction
and these coils generate a magnetic field in the poloidal direction, so they are called the
Poloidal Field (PF) coils.
The TF coil assembly, generating a magnetic field at a maximum of 11.8T and of 5.3T in
the plasma center, consists of 18 D-shaped coils set at even intervals in the toroidal direction.
The field tolerance for keeping plasma confinement is within the order of a few millimeters andthe required accuracy is as high as ten-thousandths of a millimeter. This also involves thedimensional accuracy of TF coils, which require such accuracy in the order of several
millimeters against a 16m-tall 9m-wide welded structure. TF coils are in an extra thick plate
welded structure, which can endure a tremendous electromagnetic force being exerted on it, andthe structural material is special stainless steel since it is used in the range of a cryogenic
temperature such as -269ºC.
Figure 1 shows the structural concept of ITER TF coils. TF coils comprise a winding
pack (WP) of built-up multi-layered superconductors and a toroidal field coil structure (TFCS)
to encase the WP. The WP consists of seven double pancake (DP) layers and each DP is acomposite of a conductor, groove-worked radial plate (RP) and cover plate (CP) to seal thegroove. The TFCS to encase the WP is divided into subassemblies called AU, AP, BU, and BP,
to each of which several basic segments are joined by welding. Accessories such as support
structures to connect co-adjacent TF coils are attached to AU and BU by welding.

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Figure 1 Concept of toroidal field coil structure(courtesy of: The Japan Atomic Energy Agency)|3. Technical issues on TF coil manufacturing
Since the structural material of TF coils is special stainless steel for cryogenic use, it isnecessary to establish welding, machining, and other base technologies suitable for such material interms of its properties and then manufacturing processes that satisfy the specifications required for(the accuracy of) ITER.
(1) Accuracies required for TF coils
The required accuracies of TF coils are 1 mm for both the profile and flatness of
TFCS/DP, 0.4 mm for the surface flatness of the interface between co-adjacent coils and34,000±5 mm for the perimeter of RP.
Furthermore, as for the conductor material of TF coils, high magnetic field-resistant
Nb3Sn is adopted, requiring the manufacturing processes to be managed so as not to put too
much strain (0.1% or more) on wires heat treated to be superconductive.
(2) Characteristics of structural material for TF coils[2][3]The structural materials of TF coils have been developed, aiming at 1,000MPa of 0.2%
proof stress at -269ºC and a fracture toughness of 200MPam1/2. For the straight portion of the
D-shaped coils on which a considerable force is exerted, a highest-strength material called JJ1
is used. For the other portions, three steel types, SUS316LNL, LNM, and LNH are used, whose
nitrogen contents are different for varying strength requirements. Table 2 shows the chemicalcompositions and mechanical properties of JJ1- and SUS316LN-based material.
(3) Welding technology
It is difficult for the structural material of TF coils to be welded. As shown in Table 2,since larger quantities of nitrogen were included to secure the strength of the material under the
cryogenic circumstance, porosities would easily occur due to gasification of nitrogen during
welding. These materials are a kind of full austenitic steel and hot cracks are prone to occur
during welding.
Based on the characteristics of the materials used for TF coils, electron beam (EB)welding, laser beam (LB) welding and narrow-groove TIG welding were adopted as low-heat
input welding to minimize welding deformation. It is necessary to optimize their process
conditions and establish the process for each method. For this purpose, various kinds of primary
tests, process verification tests with a small-scale mock-up and with a full-scale mock-up werecarried out.

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Table 2
Material

JJ1
316LNL316LNM
316LNH

Chemical composition and mechanical properties of structural materials of TF coil
C

Si

Mn

P

S

Ni

FractureTensile Yield
toughness
C+N strength strength
value(MPa) (MPa)(MPam1/2)> 620 > 300 > 180> 600 > 280 > 180

Cr

Mo

N

11.00
0.035 0.015 11.00
0.030 0.75 9.00
or less or less 11.00 or less or less 13.00 13.00
16.00
0.030 0.75 2.00 0.03 0.020 10.00
or less or less or less or less or less 14.00 18.50

4.00
6.00
2.00
3.00
2.00
3.00
2.00
3.00
2.00
3.00

0.21
0.27
0.05 0.080
0.120 or more
0.100 0.130
0.170 or more

16.00
0.030 0.75 2.00 0.03 0.020 10.00
or less or less or less or less or less 14.00 18.50
16.00
0.030 0.75 2.00 0.03 0.020 10.00
or less or less or less or less or less 14.00 18.50

16.50
SUS316LN 0.030 0.75 2.00 0.045 0.030 10.50(JIS)or less or less or less or less or less 14.50 18.50

> 520

> 210

> 180

> 480

> 205

> 180

> 550

> 245

> 180

> 520

> 230

> 180

0.150 0.180 > 580
0.220 or more > 560
0.12
0.22

> 280

> 180

> 260

> 180

-

-

-

-

Note 1: P+S<0.050 mass%
Note 2: Values of tensile strength and yield strength are shown at room temperature.
Note 3: Fracture toughness is obtained at 4K. (Plate thickness, top: Less than 200 mm, bottom: 200 mm or more)(4)

Machining technology
The structural material of TF coils is also less workable in machining (cutting) thanordinary stainless steel (SUS304) due to its nitrogen content. TFCS and RP have complex
shapes and it is necessary for greater machining efficiency and less deformation due toprocessing to select the suitable tools, set proper machining conditions and establish
appropriate work processes. Thus, as in welding technology, primary tests to understandworkability and process verification tests with the small-scale mock-up and the full-scale
mock-up were performed.

|4. Technology verification
Results of R&D on welding and machining technologies relating to TFCS/RP manufacturing
are roughly as follows:

4.1

Technological verification for TFCS manufacturing

Before actual production of TFCS, the tests were conducted step by step, which produced
successful developments such as the optimization of the welding conditions to prevent porosity andthe decision regarding the welding procedure to minimize the welding deformation.
(1) Primary tests
For welding of TFCS, of which the thickest parts are more than 200mm, a low heat input
welding process such as narrow-gap TIG welding and electron beam (EB) welding was selected
and its applicability was examined.
Narrow-gap TIG welding was tested, targeting higher welding efficiency, and the
optimized welding condition was found to obtain sound weld metal at the deposition rate of
20g/min. in the flat position.
Electron beam (EB) welding was tested to achieve thicker weld metal, and the optimized
welding condition was found to obtain sound weld metal without porosity at a thickness of 40mm.
(2) Process verification tests with small-scale mock-up
Using a 1-meter-long small-scale mock-up which has a cross section equivalent to that of
an AU straight portion (Figure 2), a test was conducted to check welding deformation andestablish a welding procedure that can minimize welding deformation. Primary results aredescribed as follows:- In outer plate/side plate joint welding tests, the deformation was minimized by EB welding
and narrow-gap TIG welding from both sides.
- In joint welding tests between segments, the application of the both-side welding procedure
could minimize angular deformation of the side plate and outer plate.
- No indication was detected by nondestructive inspection or no defects including
micro-cracks were found by cross-sectional observation, proving flawless welding.

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Figure 2 Small-scale mock-up test(3)

Process verification tests with full-scale mock up
To establish the manufacturing process for an actual TFCS, a full-scale mock-up of AU
curved and straight portions was manufactured based on the result of primary and small-scale
tests. In addition, a simulated part of BU was also fabricated and welding between AU and BU
subassemblies was tested.
In testing, deformation was kept balanced by welding from both sides instead of using a
jig to firmly restrain welding deformation. The AU straight portion was manufactured by
combined processes of EB welding and narrow-gap TIG welding and, for the AU curved
portion, the process of narrow-gap TIG welding from both sides was applied due to its shape.
Figure 3 shows the appearance and deformation results of a test-manufactured mock-up.

Figure 3 Full-scale, partial mock-up test (AU curved and straight portion)

4.2

Verification tests of RP manufacturing technology

The RP, into which TF coil conductors are set is formed from forging plate by grooving
process, and about 80% of the forging plate is machined and a huge amount of cutting chips are
generated. The material is high strength, high-nitrogen stainless steel called SUS316LNH (seeTable 2), which has so far never been used by us for welding and is much harder to weld andmachine than ordinary stainless steel. Process verification was, therefore, given after welding andcutting conditions were selected and the machining method and assembling procedure were
established for full-scale manufacturing. Major results are given below.
(1) Primary tests[1] Selection of the welding work conditions for RP
In consideration of the RP weld method, to weld 120mm-thick plate for a large
D-shaped coil (14m×9m), rather than EB welding which requires a vacuum chamber, acombined process of laser beam (LB) welding and narrow-groove TIG welding was adopted.
The applicable range of LB welding using a 30kW fiber laser (rated among the world’s
largest-output power for general purposes) was verified and it was decided to weld a 50-mm
thickness from both sides. For the remaining part, narrow-groove TIG welding was to be
used and element tests indicated good prospects of securing the necessary flatness. Figure 4
shows a macroscopic cross section of the welded part.

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Figure 4 Primary tests for RP welding[2] Selection of machining conditions
With a view to establishing a grooving method, machinability and life verification tests
were conducted using several kinds of tools, and then the tools that achieved good results
were tested for verification of machinability to improve cutting conditions in roughmachining.
- Machinability and life verification tests
Figure 5 shows the processing performed during verification tests. The grooving
process is roughly divided into four phases and Table 3 shows the result of the roughmachining phase that shares much of the processing (about 70%). Among the tools used,
only one type exhibited good machinability.
- Tests for the improvement of cutting conditions in rough machining
Tools that exhibited good results in the above-mentioned tests were further
examined, aiming at the improvement of their machinability. As a result, the cutting
amount per minute could be improved 1.35 times and tool life also increased to 40 minutes
or more, thus virtually assuring the prospect of stable machining (Figure 6).
Table 3 Crude processing tool selection result
Test
No.

Tool maker

1
2
3
4
5

Company A
φ40 shoulder cutter
StableCompany B
φ25 plunge cutter
StableCompany C φ25 high feed cutter-1 Stable
Company C φ30 high feed cutter-2 Stable
Company D φ42 high feed cutter Defective

Tool to be used

Figure 5 Primary tests for RP
grooving(2)

Stability

Amount of cutting
powder discharged(cc/min.)
61.2
36.0
24.7
50.0
81.4

Tool life(min.)Overall rating

20 or more
13
20
25
4

Excellent
Bad
Moderate
Moderate
Bad

Figure 6 Improvement of cutting performance in Test
No. 1 tool (Table 3)

Process verification tests with small-scale mock-up
Results concerning trial manufacturing of RP segments are described as follows:
In these trials, materials prepared by profile forging were machined into the final
cross-sectional shape to verify the machining property as well as to examine the process forMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)21

controlling deformation. Primary results are described as follows:- φ40 plunge machining performed stable machining. Moreover, basic data for further
elongation of tool life could also be obtained.
- The flatness, with a reading of 0.39 mm in Test #1 and 0.14 mm in Test #2, could be kept
within its tolerance of 1 mm. For full-scale manufacturing application, matters to be
improved, etc., could also be extracted, allowing high-accuracy machining process
databases to be thoroughly prepared.
(3) Process verification tests with full-scale mock-up
To verify all manufacturing processes of RP, the full-scale D-shaped sRP (s is anabbreviation of side) was conducted to verify the manufacturing process.
As shown in Figure 7, the AU side consists of four segments and the BU side, six
segments. Each segment was processed while checking the flatness of RP during processing.
One segment is joined to another by TIG as well as LB welding so that four sectors are to beformed before the D-shaped processing. Final machining of the D-shape is carried out, using
specialized machinery as shown in Figure 8 (a), after the conductor length of heat-treatedwinding was checked. The final flatness of the full-scale D-shaped sRP was 0.95mm, which
satisfied the required tolerance of 1mm. The groove length was also no more than the toleranceof ±0.01. The appropriateness of the machining processes adopted here could, therefore, be
verified.

Figure 7 Concept of RP structure (Courtesy of: The Japan Atomic Energy Agency)

Figure 8

Final machining of sRP full-scale mock-up|5. Current status of actual production of TF coils
The first unit of the TF coil was contracted in August 2012, and the contracts for
manufacturing the five units of TF coils in total were finally made. While learning the welding
procedure and verifying the work process with a variety of mock-ups, manufacturing of an actual
radial plate (RP) and TF coil structure (TFCS) started in October 2013 and April 2014,
respectively.
In TF coil production, with winding pack fabrication entrusted to Mitsubishi Electric
Corporation, MHI takes the role of manufacturing the structure encasing a winding pack of TF
coils (TFCS) and RP into which the conductor is placed, heat treatment of conductors, welding of
cover plates (CP), and final assembly. Part of the structure (BU and BP) is manufactured byHyundai Heavy Industries Co., Ltd.

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At the Main and Futami Plants of MHI Kobe Shipyard & Machinery Works, full-scale
fabrication is under way, using the equipment necessary for TF coil production that is
manufactured and installed there. Various specialized facilities for RP fabrication (LB welding
systems, automatic TIG welding systems, and the final machining system of RP) were prepared in
the Main Plant. In the Futami Plant, in addition to existing welding systems and machines, the coilwinding machine, heat treatment furnace for conductors, conductor transferring machine, CPwelding systems electrical insulation machines, and impregnating equipment were prepared andthese machines have operated to manufacture the actual product. For example, typical machines areshown in Figure 9; these machines are the heat treatment furnace manufactured by MHI and thewinding machine manufactured by Mitsubishi Electric Corporation.

Figure 9

Example of special equipment for manufacturing TF coils

As for TFCS, mock-up tests were completed and actual product fabrication is under way
based on test results.
RP fabrication has also made steady progress, with 140 segments or a two-units-equivalentof 350 segments (5 units × 7 RPs × 10 segments) already manufactured and some 10% of 140
sectors (5 units × 7 RPs × 4 sectors) already fabricated or now being machined. Figure 10 shows
TFCS and RP manufactured.

Figure 10

Full-scale manufacturing status of actual product|6. Activities regarding ITER divertor
The divertor infusion reactors, functioning to extract helium ashes from fusion product,
unburnt fuel particles, and impurities, is one of the important components in ITER for securing
stable confinement of plasma. It is necessary to endure a high heat load of tens of megawatts/m2 on
the divertor’s surface facing plasmas.
As shown in Figure 11, divertor development has been taken charge of by Japan, Europe,and Russia, and Japan is to share the Outer Vertical Target (OVT).
MHI participated in the OVT prototype program, and the mock-ups of the Plasma Facing
Unit (PFU) as a component to receive the high heat flux and Steel Support Structure (SSS), which
supports the PFU, were fabricated to check manufacturability and so on. Figure 12(a) shows theexternal appearances of the mock-ups. The PFU was tested for thermal resistance using the high

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heat load test facility at the Efremov Scientific Research Institute of Electrophysical Apparatus(NIIEFA), Russia, and our PFU met the specific requirements of the heat load for the ITER
divertor and was first qualified in the large-sized PFU. Figure 12(b) shows the external appearance
after a thermal resistance test.

Figure 11 Structural concept of an ITER divertor(Courtesy of: The Japan Atomic Energy Agency)

Figure 12 Trial manufacture and tests of an outer vertical target prototype of a divertor(Courtesy of: The Japan Atomic Energy Agency)

Carbon fiber composite (CFC) has so far been used for the high heat load region of PFU, but
CFC was replaced with tungsten (W) due to the recent design change, coming from a reevaluationof the behavior of impurities in plasmas. According to the design change, a new mock-up test of
W-PFU is now being performed.

|7. Conclusion
Manufacturing of ITER TF coils is very challenging because we have no experience of sucha large-size coil, about 16.5m in height and 9m in width with high tolerances of 0.1%, which are
the characteristics of the largest superconducting magnets in the world. We started to manufacture
the actual TF coil based on technologies established through basic tests and verification tests.
We are making efforts to establish manufacturing technologies for the ITER divertor withcollaborative support of the Japan Atomic Energy Agency.

Reference
1. The website of Japan Atomic Energy Agency (http://www.naka.jaea.go.jp/ITER/index.php)
2. A. Nishimura, H. Nakajima, “Development of Structural Materials Standard for Superconducting Magnet
Structures in Fusion Reactor Plant”, TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICALENGINEERS Series A. 2012, 78(790), 804- 807.
3. K. Hamada, “Special Topic Article: Superconducting Magnet Technologies for Magnetic Confinement
Nuclear Fusion Reactor”, 2. Cryogenic Structural Materials, J. Plasma Fusion Res. Vol.83, No.1 (2007),
33-38