Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference
PVP2011
July 17-21, 2011, Baltimore, Maryland, USA
PVP2011-57143
FABRICATION DESIGN AND CODE REQUIREMENTS
FOR THE ITER VACUUM VESSEL
H. J. Ahn, B. C. Kim, J. W. Sa, Y. J. Lee, K. H. Hong, H. S. Kim, J. S. Bak, K. J. Jung
National Fusion Research Institute
Daejeon, Republic of Korea
K. H. Park, T. S. Kim, J. S. Lee, Y. K. Kim, H. J. Sung
Hyundai Heavy Industries
Ulsan, Republic of Korea
ABSTRACT
The ITER vacuum vessel (VV) is a double walled torus
structure and one of the most critical components in the fusion
reactor. The design and fabrication of the VV as nuclear
equipment shall be consisted with the RCC-MR code based on
French fast breeder reactor. The VV is a heavy welded structure
with 60 mm thick shells, 40 mm ribs and flexible housing of
275 mm diameter. The welding distortion should be controlled
since the total welding length is over 1500 m. To satisfy the
design requirement, the electron beam welding (EBW) and
narrow gap gas tungsten arc welding (GTAW) techniques are to
be applied and developed through the fabrication of mock-ups.
The fabrication design has been developed to manufacture the
main vessel and port structures in accordance with the RCCMR code. All fabrication sequences including welding methods
are also established to meet the demanding tolerance and
inspection requirement by HHI as a supplier.
K. Ioki, B. Giraud, C. H. Choi, & Y. Utin
ITER Organization
St. Paul lez Durance, France
The ITER Organization (IO) was conceived as long ago
as 1985 but was only formally established on 24 October 2007,
following an agreement between the People’s Republic of
China, the European Union (EU), the Republic of India (IN),
Japan, the Republic of Korea (KO), the Russian Federation
(RF) and the United States of America. The ITER VV
procurement sharing includes the production of 7 sectors by the
EU, 2 sectors by KO, the upper ports by RF, the VV supports
and the lower and equatorial ports by KO, the in-wall shielding
by IN and the assembly by IO. As one of the providers, Korea
Domestic Agency (KODA) and Hyundai Heavy Industries
(HHI) have performed the fabrication design of the main vessel
and port structures based on IO’s Build-to-Print design in
accordance with the RCC-MR code. This paper presents details
of the fabrication design and mock-ups developed by KO to
manufacture two sector and port structures.
INTRODUCTION
The international thermonuclear experimental reactor
(ITER), currently under construction in the south of France,
aims to demonstrate that fusion is an energy source of the
future. ITER is based on the 'tokamak' concept of magnetic
confinement so that the plasma is contained in a doughnutshaped vacuum vessel as shown in Figure 1. The fuel, a mixture
of Deuterium and Tritium (two isotopes of Hydrogen) is heated
to temperatures over 150 million °C forming hot plasma.
Strong magnetic fields are used to keep the plasma away from
the walls; these are produced by superconducting coils
surrounding the vessel, and by an electrical current driven
through the plasma [1].
Figure 1: The configuration of the ITER Tokamak.
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DESCRIPTION OF VACUUM VESSEL
The ITER vacuum vessel (VV) is a double walled torus
structure and one of the most critical components in the fusion
reactor. Its primary function is to provide a high quality vacuum
for the stable plasma and it becomes a major safety barrier for
ITER. It consists of nine 40 degree vessel sectors with many
port structures like long nozzles of a pressure vessel as shown
in Figure 2.
Figure 2: ITER Vacuum Vessel.
Main parameters of the VV are summarized in Table 1 [2].
The vessel is a heavy welded structure with 60 mm thick shells,
40 mm ribs and flexible housings of 275 mm diameter. Its
weight is about 5250 tons and its torus outer diameter and
height are 19.4 m and 11.4 m, respectively.
Table 1: Main VV Parameters.
Parameter
Description (value)
Torus outer diameter
Torus inner diameter
19.4 m
11.4 m
Configuration
- Inboard straight region
- Inboard top/bottom
- Outboard region
Cylindrical
Double curvature
Mainly double curvature
Operating condition1
- Normal operation
- During baking
100 ºC / 0.8 MPa (Absolute)
200 ºC / 2.1 MPa (Absolute)
Design condition
- Main vessel
- Ports
200 ºC / 2.6 MPa (Absolute)
250 ºC / 5.0 MPa (Absolute)
Interior surface area
~850 m2
Interior volume
~1600 m3
Note 1) VV operating condition is based on the temperature and
pressure of the cooling water inlet.
The interspace between the vacuum vessel double walls is
filled with in-wall shielding (IWS) and cooling water. The
shielding structures, which occupy about 60% of the in-wall
space, provide efficient neutron shielding. The heat deposition
in the VV due to neutron heating is removed by high pressure
cooling water during plasma operation. The water is supplied
and flows through the lower port and is routed in an internal
supply structure to the bottom of each sector [3].
The VV is supporting blankets, divertors and ELM/VS
coils on the interior surface of the vessel. The blanket modules
are attached to the flexible support housing (FSH) mounted on
the inner shell by flexible supports and keys. The hinge type
gravity supports are located below lower ports and sustain the
VV weight and its electro-magnetic loads. The supports are
radially free to move for thermal expansion but restrained
vertically and toroidally.
MATERIALS OF VACUUM VESSEL
The main material is austenitic stainless steel with
controlled nitrogen contents and tight limitation of impurities
such as cobalt, niobium and boron. The choice of materials for
the VV has significant influence on cost, performance,
maintainability, licensing, detailed design parameters, and
waste disposal. The primary reason for the choice of materials
shown in Table 2 [2] is their high mechanical strength at
operating temperatures, water resistive properties, excellent
fabrication characteristics, and low cost relative to other
candidates. This is achieved as a result of an optimal
combination of the main alloying elements, carbon, nitrogen,
nickel, chromium, manganese and molybdenum, with a tight
specification of their allowable composition range. This steel is
qualified by RCC-MR Code 2007 [4].
Table 2: Materials used for VV and IWS.
Items
Material
Main Vessel
SS316L(N)-ITER Grade1
Ports
SS316L(N)-ITER Grade1
SS 304L (EN grade 1.4307)
SS 304 (EN grade1.4301)
In-wall shielding
304B4 or 304B7 (UNS S30464/7)2
430 (UNS 43000)
Bolts
Alloy 718 or Steel 660 (EN grades)
XM-19 (B8R)
Note 1) Special requirements for 316L(N)-IG are as follows;
• Nitrogen control (0.06~0.08%) to keep consistent strength
• Limitation of impurities:
- Co(0.05%): reduction of contact dose and gamma heating
- Nb(0.01%): reduction of activated waist
- Boron (0.0001%): limit He production
2) Borated steels are containing 1 or 2 weight % boron.
CLASSIFICATION AND APPLIED CODES
The VV provides high quality vacuum for plasma and
primary radioactivity confinement boundary. The VV is
classified into a safety important class (SIC) component based
on the French safety and quality order 1984. The VV design
shall take into account the various loads combinations for
which the VV safety functions are needed including seismic
events.
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The VV consists of an assembly of a number of individual
nuclear pressure equipments (NPE) as per definition of the NPE
Order 2005 [5]. The assembly will appear only after welding of
the components supplied to the ITER site. The VV and some
port sections are multi-chamber equipments. According to
regulatory requirement, NPE Order, the VV are classified into
category and nuclear level as shown in Table 3. An Agreed
Notified Body (ANB) contracted by the IO is in charge of the
conformity assessment in order to demonstrate that the
necessary safety requirements are satisfied [6].
The RCC-MR Code, Edition 2007, is selected as the design
and construction code for mechanical components of nuclear
installations. For items which are not covered by the Code,
ITER Organization’s technical specifications are used. The VV
and ports are classified as Class 2 box structure components
and applicable design rules are provided in the RCC-MR RC
3800 chapter and complemented by Appendix 19. The design
and construction code for IWS blocks is ASME III – NC.
custom-machined splice plates which will be thicker than the
VV wall to allow misalignment between sectors [2].
Table 3: Category and nuclear level for Vacuum Vessel.
Equipment
All cooled by VV primary heat transfer
system (PHTS) (3.0 MPa)
- Main vessel (9 sectors)
- All port stub extensions
- All port extension of lower ports
- Port extension of equatorial NB ports
Upper ports
- Port extensions cooled by First Wall/
Blanket PHTS (5.0 MPa)
Regular equatorial ports
- Port extensions cooled by First Wall/
Blanket PHTS (5.0 MPa)
No.
NPE Order
Classification
Category
IV
Level
N2
18
Category
IV
Level
N3
14
Category
III
Level
N3
1
Figure 3: Composition of a VV Sector.
The segment has inner and outer shells, T-shape poloidal
ribs, flexible support housings, in-vessel coil supports and port
stub like Figure 4. The lower segment has special attachments
such as gussets, pipe penetrations, triangular supports and
divertor rails. These are to be made using forging blocks.
SECTOR DESIGN FOR THE VACUUM VESSEL
The VV is to be fabricated in the factory as nine sectors
each spanning 40 degree. The weight of each sector is about
200 tons and its height and width are 13 m and 6 m,
respectively. 60 mm-thick stainless steel plates forms a doublewall that contains additional 250 tons of IWS. A 40 degree
sector consists of four major segments which are inboard
segment (PS1), upper segment (PS2), equatorial segment (PS3)
and lower segment (PS4) as shown in Figure 3. The baseline
fabrication scheme of a VV sector is the welding of four
poloidal segments with segment splices [3].
According to the baseline assembly scheme for the VV
sector developed by IO, each sector will be sub-assembled with
a pair of Toroidal Field (TF) coils and thermal shield segments
in the ITER Assembly Hall. The sub-assembled sectors are then
transferred into ITER pit, in sequence, to complete sector
assembly. VV sectors are welded together to form three sets of
VV triples – these triplets are then aligned and welded together.
Welding between the sectors to be joined is conducted by fitting
Figure 4: Configuration of Lower Segment.
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DESIGN OF THE VV PORTS
The vacuum vessel has 18 ports at the upper level of the
machine, 14 regular and 3 NB ports at the equatorial level, and
9 port and 18 local penetrations at the lower level. A typical
port structure is attached to the port stub (integral to the main
vessel) and includes a stub extension and a port extension. The
end portion of the port extension is normally equipped with a
closure plate that provides the primary vacuum boundary. The
port extension is connected to the cryostat with a connecting
duct that is a part of the secondary vacuum boundary (except
the NB ports). The basic port arrangement is shown in Figure 5
and a summary of port usage and inside dimensions are
summarized in Table 4 [3].
Figure 5: The Basic Port Arrangement (Typical Sector).
used for the RF plasma heating and current drive, plasma
diagnostics, for positioning the plasma limiters and test
modules, etc. An access to the blanket modules for maintenance
will be also through the regular equatorial ports. The NB ports
will provide access for the neutral beams for the plasma heating
and current drive, and the diagnostics. The lower
RH/Diagnostics ports will be used for the divertor cassette
maintenance and diagnostics and the lower cryopump ports will
be used for the vacuum pumping, pellet/gas injection and
piping. The local penetrations are used for the divertor water
piping, IVV, GDC, location of the VS coil feeders, and so on.
WELDING METHODS
The shapes of the VV and ports are very complicated
double structure and require severe dimension control. Based
on these considerations, narrow gap gas tungsten arc welding
(GTAW) and electron beam welding (EBW) procedures were
considered as the main welding process. GTAW processes are
divided into a manual type and a machine type in terms of their
accessibility and productivity. HHI has developed three
different welding equipments which are to be applied in main
shell butt welding, rib to shell welding and shell to FSH with
narrow gap joint and hot wire system.
T-shape adapters are introduced to the welding joints
between outer shell and rib, which satisfies the code
requirements such as the full penetration weld and the
minimum distance between the welds due to lots weld
components and complexity of assembly. The details of major
welding joint for inboard and outboard segments are
demonstrated in Figure 6 and 7. For EBW between inner shell
and FSHs, tight fit-up with gap less than 0.1 mm is required to
achieve required welding quality. For outer shell welding,
narrow U-type groove as shown in Figure 6(a) shall be required
because of the single side accessibility. For both sides accessing
region, K-type groove is adopted like (b).
Table 4: Summary of Port Usage and Inside Dimensions.
Port
Port Type
Inside Dimensions (m)
No.
Upper
18
1.154 (width) x 1.16 (height)
Equatorial
- Regular
- Heating Neutral Beam
- Heating/Diagnostic
Neutral Beam
14
2
1
1.748 (width) x 2.2 (height)
0.582 (width) x 1.36 (height)
0.582 (width) x 1.36 (min. height)
0.528 (width) x 0.435 (min. height)
4
5
18
1.39 (width) x 2.175 (height)
1.39 (width) x 2.175 (height)
Various sizes
Lower
- RH/Diagnostics
- Cryopumps
- Local penetrations
(a)
(b)
(c)
Figure 6: Welding Joint Details for Inboard Segment.
(a) FSHs and center rib to outer shell.
(b) Ribs on inner shell.
(c) Side ribs on outer shell.
The port structures have many functions. The upper ports
will be used for diagnostics, EC plasma stabilization, and
blanket/VV water piping. The regular equatorial ports will be
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(a)
(b)
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(c)
Figure 7: Welding Joint Details for Outboard Segment.
(a) FSHs and port stub to inner shell.
(b) Ribs on inner shell.
(c) FSHs and port stub to outer shell.
In case of welds to be examined by ultrasonic test (UT),
the minimum distance between the welds is the larger 1.5 times
the thickness of the thickest part to be assembled or 40 mm.
These welding designs comply with the code requirements such
as the full penetration weld and the minimum distance between
the welds as well as 100% volumetric NDE condition.
FABRICATION SEQUENCE OF A SEGMENT
The manufacturing sequence of segments is developed to
satisfy the design requirement. EBW and narrow gap TIG
welding techniques are adopted and developed through the
manufacturing mock-ups. Each segment is to be made
according to the following common fabrication sequence;
1.
2.
3.
4.
5.
6.
7.
8.
9.
Cutting and forming of inner and outer shells
Welding of inner shells
Welding of ribs to the inner shell
Machining FSH holes and welding FSH to the inner shell
Machining port hole and welding port stub
Welding support ribs for IWS
Assembly of IWS
Welding the outer shells
Final machining of a segment
An inboard segment has a lot of attachments welded which
are 48 FSH, 12 intermodular keys, 6 centering keys and so on.
These FSH and keys are welded on the inner shell by EBW
process under full vacuum in order to minimize welding
distortion and to increase productivity. Due to the limitation of
the vacuum chamber in available facility, inner shell of the
inboard segment divides into two pieces for EBW. Figure 8
shows the fabrication sequence of an inboard segment briefly.
Several strong jigs will be used to minimize welding distortion
as sketched in red.
Figure 8: Fabrication Sequence of an Inboard Segment (PS1).
(a) Bending shells and machining of holes for key.
(b)
(c)
(d)
(e)
(f)
(g)
(h)
TIG welding of ribs and E-beam welding of keys.
E-beam welding of keys.
Hole machining for FSHs.
E-beam welding of FSHs.
Upper & Lower shells welding and IWS ribs.
Installing IWS.
Cover welding of outer shell.
SEGEMTNS ASSEMBLY SCHEME
The baseline fabrication scheme of a VV sector is welding
of four poloidal segments with segment splices. For minimizing
the welding deformation during final joint of segments, HHI
has been developed the design of segment joints without
poloidal rib splices to reduce the butt welding work of poloidal
ribs as shown in Figure 9. Reduced weld joints will release the
risk of sector tolerance mismatch.
Figure 9: Segment Joint Configuration of PS2 and PS3.
Figure 10 shows the joint details for each segment joint.
Prior to machining on final welding of segment, 3-dimensional
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(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(3D) measurement of each segment should be conducted to
collect their locations and adjust machining quantities. And the
margin will be machined to provide proper welding condition.
Scallops are needed in the welding line intersecting areas and
can provide the space to take radiographic test (RT) on inner
shell weld. The scallop will not be applied on both 40o side ribs
for leak tightness of one complete sector.
(a)
(c)
(b)
(d)
Figure 10: Joint Details for Final Segment Assembly.
(a) PS1 and PS2.
(c) PS2 and PS3.
(b) PS1 and PS4.
(d) PS3 and PS4.
Interface bracket of gusset welding.
T-rib welding on the upper part of inner shell.
Lower part of inner shell and support pad welding.
Rail support welding.
T-rib welding on the lower part of inner shell.
Welding upper and lower part of inner shell.
Waterstopping flange welding and joint area cutting.
Outer shell welding.
MOCK-UP FABRICATION
HHI decided to make the partial full scale mock-up to
develop their fabrication procedures as shown in Figure 12 [7]
[8]. The first mock-up, VV inboard segment mock-up (VISM)
is to develop and stabilize EBW techniques. The lower parts of
the inboard segment are made for the optimization of EBW
techniques including repair and NDE method development. The
second mock-up is 20 degree VV upper segment mock-up
(VUSM). This is for the verification of forming, machining
technique, welding sequence optimization and distortion
control. By this mock-up, HHI will also ensure that NG-GTAW
welding method, NDE procedures, and dimension inspection
method are applicable to the final production. In addition, 10
degree partial mock-up for the triangular support of lower
segment (VLTM) is planned to develop the copper cladding
fabrication method. Full scale lower port mock-up (VLPM) is
also under the fabrication to assess fabrication feasibility for the
VV ports.
FABRICATION SCHEME OF THE LOWER PORT
The manufacturing sequence of lower port stub extension
is developed to satisfy the design requirement. Narrow gap
GTAW welding techniques are adopted and developed through
the manufacturing mock-ups for port stub extension. Figure 11
shows the sequence of the lower port stub extension briefly.
(a)
(b)
(d)
(g)
(c)
(e)
(h)
(f)
(i)
Figure 11: Fabrication Sequence of a Port Stub Extension.
(a) Upper part of Inner shell welding and machining.
Figure 12: R&D Mock-up for the Fabrication Feasibility Study.
(a) VISM: VV inboard segment mock-up.
(b) VUSM: VV upper segment mock-up.
(c) VLTM: VV lower-segment triangular support mock-up.
(d) VLPM: VV lower port mock-up.
Stainless steel 316L was used for the mock-up fabrication
because its properties and material contents are close to the
ITER graded stainless steel 316L(N)-IG except nitrogen and
radioactive impurity contents control for the ITER grade one.
As a first step of mock-up fabrication, the EBW test with the
specimen was performed to find the optimum welding
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parameters. After the survey of welding parameters by bead on
plate welding, the liner butt joint and circular welding test were
performed. Figure 13 shows the radiographic test of result for
the liner butt joint welding specimen [8]. The both side EBW
shows the optimum results and is selected as a basic welding
scheme.
(b)
Figure 13: RT Result of Linear Butt Joint EBW.
The components of each mock-up are under fabrication
except VLPM for design finalization. Except outer shell, all the
VISM components were assembled. Electron beam welding
method was applied to the assembly of flexible support
housings, center keys, and intermodular keys to minimize weld
distortion as shown in Figure 14. The other components such as
ribs and inner shells were assembled by narrow gap GTAW. To
check assembly feasibility of IWS, one set of IWS was
fabricated. After then, assembly of outer shells will be done
including final dimensional inspection. Major issues for the
VISM fabrication were the distortion of inner shell due to lots
of weld components and feasibility of NDE. Application of
EBW reduced risk of weld distortion considerably and there
were no problems in RT and UT for the inner shell weld
components. During fabrication, design of jigs and fixtures was
finalized.
Figure 14: Fabrication of VV Inboard Segment Mock-up (VISM).
(a) Inner shell side view.
(b) Outer shell side view.
For VUSM, the region of the VV inner shell has 3D shapes
and it consists of four kinds of 3D zones as shown in Figure 15.
Since both zone 1 and zone 2 are divided by two shells, inner
shell of VV upper segment is to be divided by 6 shells. All the
inner shells were 3D formed by the cold forming as shown in
Figure 16 through 19. Each formed shell was measured using
3D measuring system like Figure 20 and machined for weld
preparation as Figure 21. The inner shell is under the stage of
fit-up to assembly jig (Figure 22) for the welding like Figure
23. During the VUSM R&D works, major issues are the weld
distortion and thickness reduction due to 3D forming. The jigs
and fixtures were designed to minimize the weld distortion for
the VV sector fabrication. HHI is considering maximum 9% of
inner shell thickness reduction due to cold forming so that the
structural analysis was done for the load conditions given from
ITER organization. The results of stress analysis including limit
analysis are complied with the RCC-MR code requirements.
Figure 15: Inner Shell Shape of Upper Segment.
(a)
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Figure 16: Formed Inner Shell for Zone 1.
Figure 20: 3-Dimensional Measuring of Zone 1.
Figure17: Formed Inner Shell for Zone 2.
Figure 21: Machining for Weld Preparation.
Figure 18: Formed Inner Shell for Zone 3.
Figure 22: Jig Setting.
Figure 19: Formed Inner Shell for Zone 4.
Figure 23: Fit-up on the Jig and Measuring.
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SUMMARY
The fabrication design has been developed to manufacture
the main vessel and port structures based on IO’s Build-to-Print
design in accordance with the RCC-MR code. All fabrication
sequences including welding methods are also established to
meet the demanding tolerance and inspection requirement by
HHI as a supplier.
Welding designs comply with the code requirements such
as the full penetration weld and the minimum distance between
the welds as well as 100% volumetric NDE condition. Through
the fabrication of several mock-ups, the fabrication design of
VV sectors and ports will be verified and refined in the near
future.
ACKNOWLEDGMENTS
This work has been supported partially by the Ministry of
Education, Science and Technology of the Republic of Korea
under the Korean ITER project contract.
REFERENCES
1. ITER: the world’s largest Tokamak, ITER website,
http://www.iter.org/mach.
2. ITER 2009 Baseline Plant Description (PD), ITER
Organization, 2009.
3. ITER DDD 1.5 Vacuum Vessel, ITER Organization, 2010.
4. Design and Construction Rules for Mechanical Components
of Nuclear Installation, RCC-MR, French Association for
the Design, Construction and Operating Supervision of the
Equipment for Electro-Nuclear boilers (AFCEN), Edition
2007.
5. Arrete du 12 decembre 2005 relatif aux equipements sous
pression nucleaires (ESPN), Order dated 12th December
2005 concerning nuclear pressure equipment (NPE Order
2005).
6. Ioki, K. et al., ITER vacuum vessel design and construction,
Fusion Engineering and Design, vol. 85, 1307-1313, 2010.
7. Bak, J. S. et al., Preparations for the ITER Vacuum Vessel
Construction, 23rd IAEA Fusion Energy Conference,
October 2010, Deajeon, Korea.
8. Kim. B. C. et al., Fabrication Design Progress of ITER
Vacuum Vessel in Korea, 23rd IAEA Fusion Energy
Conference, October 2010, Deajeon, Korea.
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