1. Materials for Vacuum Vessel OF Fusion Grade Machine
Ranjana Gangradey
Institute For Plasma Research

2. PART -I

3. Structure of Vacuum Vessel+ + + =
Inner shell- 40 to 60 mm thick
Rib structure 30 to 40 mm thick
Outer shell- 40 to 60 mm thick
Port structure 40 mm thick
40° sector
Main structural Material 316 Special grade

4. Shielding blocks & Connecting ducts
Connecting duct SS 304
Primary shielding SS 304 with 2% boron
Ferromagnetic inserts SS 430

5. The Stainless Steels
Austenitic stainless steels (SSs) of 304 and 316 type are the main structural materials of the basic machine.
Reasons
They are qualified in many national design codes.
Have adequate mechanical properties
Good Resistance to corrosion
Weldability
Forging and casting potential
Industrially available in various forms
Can be manufactured by well established techniques
Widely used in high technology area
There is extended data base in un-irradiated condition from cryogenic to elevated temperature

6. Stainless steel as structural material of Fusion reactor
Requirement
Degradation of properties during irradiations, mechanical and thermal loads and environmental effects should not result in loss of structural integrity of the components.
Both base metal and welded joints should with stand the irradiation doses within the range of operating temperature.
SS components are exposed to vacuum , to liquid helium, to deareated, demineralised water in cooling channels. Hence material must be compatible to these requirements.
Good weldability of the material in a wide range of thickness is required.
Vacuum vessel being the first safety barrier and for safety of machine, its structural integrity must be guaranteed.
Good strength and fatigue resistance and fracture toughness after neutron irradiation are essential requirements.

7. On the basis of the service experience of Fission Reactors and R&D results obtained in Fast Breeder reactor and Fusion programs 316LN( Low Carbon and Controlled Nitrogen) steel is thought to be the most suitable material to resist high dose of irradiation, relatively high loads and direct contact with water

8. Selection 316L(N) for Fast Breeder Reactors (316LN-FBR)
Background For Selection of Type SS required for Fusion machine
Selection 316L(N) for Fast Breeder Reactors (316LN-FBR)
Reasons
The proposed grade has an optimal combination of main alloying elements carbon, nitrogen, nickel, chromium, manganese and molybdenum with tight specification for their allowable range. The narrow specification provides an optimal microstructure and a good control of the heat to heat variation of mechanical properties.
The tight control of the carbon and nitrogen content provide a the satisfactory resistance to stress corrosion cracking of the base metal and welds, and adequate level of material strength
316LN-FBR has better strength and ductility and design allowable strength is higher than in other SS grades.
less prone to delayed reheat cracking than Ti or Nb stabilized steels.
less sensitive to irradiation embrittlement than 304 steel.
SS316LN-FBR has comprehensive data base including heat to heat variation and product size.

9. SS FOR FUSION MACHINE
With the data available for SS316LN for fast breeder reactors for a fusion machine minor modifications required are to cope for radiological safety limits and with rewelding requirements.
In ITER R&D material development programe the following points were considered
Irradiation embrittlement in the temperature range deg C
Material characterization after manufacturing cycle including the effect of neutron irradiation
Fracture toughness of the material irradiated in between C.
Welding of the irradiated stainless steel

10. SS 316 LN-ITER GRADE(IG) Wt by % Chemical Composition
Elements
Min
Max Fe
Balance C -
0.03 Mn
1.6
2.0 Si
0.5 P
0.025 S
0.01 Cr
17.0
18.0 Ni
12.0
12.5 Mo
2.3
2.7 Nb Ta Ti
0.15 Cu
0.3 B
0.001 Co
0.05 N
0.06
0.08
Chemical Composition
Main allowing elements;-Ni,Cr,Mo,Mn,C,N Any change results in different kind of steel. Produces Significant Change
P,S,Si  Inherently present in the steel as a consequence of metallurgical process. Changes produce change in material properties and quality of steel. The amount is controlled to produce required quality of steel.
Ti, Ta,Nb,Cu,Co,B Impurities in the Ore Scrap No significant effects on material properties. Lowest level defined by industrial process.
the activation is dominated by isotopes of Mn54,Mn56,Fe55,Co57,Co58,Co60, Ni57, Cr51 produced by transmutation of elements produced in steel Fe,Ni,Cr,Co,Mn,Nb
Content of all the above elements except Co, Ni cannot be changed without affecting steel properties
Required quantity ~ 1600 to tons for double wall vacuum vessel
Ports  ~1400 tons

11. SS 316 LN-ITER GRADE(IG) Cobalt Niobium Boron
Reducing the Co content from 0.25 % to 0.05% decreases the total decay heat in vacuum vessel by ~20%.
Cobalt is one of the main components of activated corrosion products in water cooling systems cooling systems.
Niobium
Niobium produces long lived isotopes which become important for the decommissioning and waste disposal of in vessel components. For vacuum vessel the content has been kept as 0.01%.
Boron
SS 316LN-FBR grade boron is less than 20 wppm. Neutronic calculations show that decreasing the boron content to 10 wppm will reduce 31% helium generated. Welding can be successfully carried out if He content is less than 0.5 –1.0 appm.

12. SS 316 LN-IG Mechanical & Thermal Properties of SS 316LN-IG Properties
Density (kg/m3)
7966
7867
7824
7803
7760
Young modulus (GPa)
192
174
166
161
153
Poissions ratio
0.3
Tensile strength (Mpa)
525
461
453
449
433
Yield strength (Mpa)
220
135
121
116
109
Design stress intensity (Mpa)
147
105 97
Thermal conductivity (W/m-K)
13.94
17.24
18.67
19.39
20.82
Specific heat (J/kg-K)
470
518
539
550
571

13. SS 304B4 and SS 304B7 Wt by % Chemical Composition
Elements
Min
Max Fe
Balance C -
0.08 Mn
2.0 Si
0.75 P
0.045 S
0.03 Cr
18.0
20.0 Ni
12.0
15.0 B
1.0
1.24
1.75
2.25 N
0.10 Co
0.05 Nb
0.01
For primary Shielding
SS 304B7 with wt. % of boron for the inboard region
SS 304B4 with wt. % of boron for the outboard region
Addition of Boron for neutron shielding
The steel has low ductility & low fracture toughness
Additional elements for vessel application
Co 0.05
Nb 0.01
Requirement for a fusion grade machine  1700 tons

14. SS 304B4 Mechanical & Thermal Properties of SS 304B4 Properties 20° C
Density (kg/m3)
7820
7724
7679
7655
7606
Young modulus (GPa)
199
175
164 -
Poissions ratio
0.3
Tensile strength (Mpa)
515
445
439
Yield strength (Mpa)
205
177
167
163
Design stress intensity (Mpa)
137
118
112
109
Thermal conductivity (W/m-K)
14.79
18.18
Specific heat (J/kg-K)
492
542

15. SS 304B7 Mechanical & Thermal Properties of SS 304B7 Properties 20° C
Density (kg/m3)
7740
7649
7607
7585
7540
Young modulus (GPa)
199
175
164 -
Poissions ratio
0.3
Tensile strength (Mpa)
515
445
439
Yield strength (Mpa)
205
177
167
163
Design stress intensity (Mpa)
137
118
112
109
Thermal conductivity (W/m-K)
14.38
17.6
Specific heat (J/kg-K)
498
551

16. Ferromagnetic Materials For Vacuum Vessel inserts
An insert of Ferromagnetic material is used in the outboard area inside the double vacuum vessel to reduce the toroidal field ripple.
SS 430 is a suitable material for the ferromagnetic inserts in terms of magnetic , technological, corrosion properties availability and acceptable cost.
SS430 has curie temperature of 660 deg C and saturation magnetic flux density 1.35 T(13500 gauss)
strengths are comparable to SS316 but have lower ductility
lower thermal expansion co-efficient
generally easier to machine
Chemical Composition
Elements
Min
Wt by%
Max
Wt by % Fe
Balance C -
0.12 Mn
1.0 Si P
0.04 S
0.03 Cr
16.0
18.0 Ni
0.75 Co
0.05 Nb
0.01

17. SS 430 Mechanical & Thermal Properties of SS 430 Properties 20° C
Density (kg/m3)
7700
7644
7618
7604
7577
Young modulus (GPa)
202
185
178
172
156
Poissions ratio
0.3
Tensile strength (Mpa)
450
387
366
347
284
Yield strength (Mpa)
205
177
170
162
134
Design stress intensity (Mpa)
138
118
113
108 89
Thermal conductivity (W/m-K)
24.57
25.15
25.3
25.36
25.45
Specific heat (J/kg-K)
449
547
597
627
700

18. SS 304 For Connecting Ducts Chemical Composition Good weldability
Cost consideration
Additional elements for vessel application Co Nb
Requirement  ~ 300 tonns
Elements
Min
Wt by %
Max Fe
Balance C -
0.07 Mn
2.0 Si
1.0 P
0.03 S
0.015 Cr
17.0
19.5 Ni
8.0
10.5 N
0.11 Cu Co
0.05 Nb
0.01

19. SS 304 Mechanical & Thermal Properties of SS 304 Properties 20° C
Density (kg/m3)
7930
7837
7793
7770
Young modulus (GPa)
200
180
172
168
Poissions ratio
0.3
Tensile strength (Mpa)
490
385
375
Yield strength (Mpa)
190
117
103 98
Design stress intensity (Mpa)
127
105 93 88
Thermal conductivity (W/m-K)
14.28
17.74
19.24
19.99
Specific heat (J/kg-K)
472
530
546
556

20. Filler material for SS/SS welding
Weld metal composition ---- to form duplex structure (austenitic + delta ferrite) to reduce the risk of hot cracking
Specified range of delta ferrite %
Sulphur content to improve weld penetration
Elements
Range
Wt by % C Mn Si
Max 0.5 P
Max 0.025 S
Max 0.02 Cr Ni
8-9 Mo Cu
Max 0.1
Chemical Composition of
filler metal for TIG welding

21. PART- II

22. vacuum vessel of Fusion grade machine
VV Manufacturability
A Glance at
vacuum vessel of Fusion grade machine
( ITER VACUUM VESSEL)

23. ITER Vacuum Vessel Size Structure Resistance Required Leak Rate
- Torus OD
19.4 m
- Torus Height
11.3 m
- Double Wall Thickness m
- Toroidal Extent of Sector
40°
- Number of Sectors 9
- Shell Thickness
60 mm
- Rib Thickness
40-60 mm
Structure
double wall
Resistance
- Toroidal
7.9 µΩ
- Poloidal
4.1 µΩ
Required Leak Rate
 110-8 Pam-3/s
Surface Area / Volume (Main vessel)
- Interior Surface Area
850 m2
- Interior Free Volume
1090 m3
- Interior Total Volume
1600 m3
Mass (without water)
- Main Vessel (without shielding)
1611 t
- Shielding
1733 t
- Port Structures
1487 t
- Connecting Ducts
294 t
- Total (not including water)
5124 t

24. Main Vessel

25. Challenge is in achieving the accuracy and tolerances
Parameter
Value in mm
Fabrication tolerance at factory
Sector overall height
± 20
Sector overall width
Sector wall thickness
± 5
Surface deviations of a 20-degree sector from the reference geometry after fabrication at factory
± 10
Assembly/positioning tolerances at site
Mismatch of the sector surfaces at field joints
Vessel weld distortion due to field/shop welds at the site
Surface deviations of the torus from the reference
geometry after assembly at the pit
± 15

26. RIBS

27. Segmentation Inboard segment Upper segment segment
Lower segment segment
Equatorial segment

28. Inboard Segment: Design details
Fragment of outer shell
Fragment of the inner shell
Inboard housing
Intermodular key
Centering key

29. FABRICATION OF A SECTION
OF A SECTOR

30. SEGMENTS
UPPER
Equatorial
LOWER

31. FABRICATION OF A SECTION POLOIDAL SECTOR

32. Shielding backup slides
Shielding assembly sequence

33. What is being aimed – SST-2

34. SST -2 & ITER (0.38 ~0.4 times of ITER Ip(MA) FOR ITER
Total average neutron fluence at the first wall
= 0.59 x (4700 hrs/24x365 ) = 0.31 MW a/m2
= 0.59 x 7800/(24x365) = ( assessed)
Neutron flux/cm2/sec = 1.8 x1020/680x104
= 2.64 x1013 neutrons/cm2/sec FOR SST -2
For 5,000 hours
= 0.2 x (4700/24x365) = MW a/m2
& for 78000.178 MWa/m2
~ 0.11 MW a/m2
Neutron flux/cm2/sec = 0.357x10 20/391x10 4
= 0.91x1013 = 1 x1013 neutrons/cm2/sec
(0.38 ~0.4 times of ITER
Machine
ITER- FEAT
SST-2
Power
500 MW
100 MW
Density ( /m3 )
1x1020
1.47x1020
Plasma Major Radius
6.2 m
4.4 m
Plasma Minor Radius
2.0 m
1.5
Plasma Surface area
680 m2
391 m2
Plasma Volume( m3)
840
322
No of neutrons (per sec)
1.8 x1020
~0.357x1020
Neutron heat
400 MW
80 MW
Neutron wall loading
0.59 MW/m2
0.2 MW/m2
Helium carries heat
20 MW
Ip(MA) 15 11
Magnetic field at major radius Bo
5.3 T
5.2 T
Integrated full power operation
4700 hours
7800 hours (assessed)
~5000 hours
Total average neutron fluence at the first wall
0.3 MWa/m2
0.11 MWa/m2
Neutron wall load at outboard FW at midplane
0.78 MW/m2
~0.2x1.4
~0.28 MW/m2
ITER total burn time 4700 hrs = 1.69 x107 secs ~2x107 sec, 0.63 FPY
1Gwatt=1x109 joules /sec,
17.6 Mev= 2.8x1012 joules
No neutron3.57x1020 /sec

35. Concept of Fusion machine being aimed at
SST-2 Vessel
Material requirement
Vessel ~ 1600 tons of 316LN (IG)
Total Height: 9.55 meter
Outer Diameter: 13.8 meter
Width: ­­ 5.3 meter
Inner shell: 40 mm thick plate
Outer shell: 40 mm thick plate
Poloidal Ribs: 30 mm thick plate
Wall separation: 120 mm at inboard region
320 mm at outboard region

36. JOIN HANDS TO FACE MATERIAL CHALLANGES
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