Materials for Vacuum Vessel OF Fusion Grade

Machine

Ranjana Gangradey

Institute For Plasma Research

PART -I

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

Shielding blocks & Connecting ducts

Shielding block

Primary shielding SS 304 with 2% boron

Ferromagnetic inserts SS 430

Connecting duct SS 304

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

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.

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

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.

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 250-300 deg C

•Material characterization after manufacturing cycle including the effect of neutron irradiation

•Fracture toughness of the material irradiated in between 250-300 C.

•Welding of the irradiated stainless steel

SS 316 LN-ITER GRADE(IG)

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,NbContent of all the above elements except Co, Ni cannot be changed without affecting steel properties

Required quantity ~ 1600 to 1700 tons for double wall vacuum vessel Ports ~1400 tons

Wt by %Elements Min Max

Fe Balance 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 - 0.01

Ta - 0.01

Ti - 0.15

Cu - 0.3

B - 0.001

Co 0.05

N 0.06 0.08

Cobalt• 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.

SS 316 LN-ITER GRADE(IG)

SS 316 LN-IG

Mechanical & Thermal Properties of SS 316LN-IG

Properties 20° C 250° C 350° C 400° C 500° C

Density (kg/m3) 7966 7867 7824 7803 7760

Young modulus (GPa) 192 174 166 161 153

Poissions ratio 0.3 0.3 0.3 0.3 0.3

Tensile strength (Mpa) 525 461 453 449 433

Yield strength (Mpa) 220 135 121 116 109

Design stress intensity (Mpa) 147 121 109 105 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

SS 304B4 and SS 304B7

For primary Shielding • SS 304B7 with 1.75-2.25 wt. %

of boron for the inboard region • SS 304B4 with 1.00-1.24 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 0.05Nb 0.01

Requirement for a fusion grade machine 1700 tons

Chemical Composition

SS 304B4 SS 304B7

Wt by % Wt by %

Elements Min Max Min Max

Fe Balance Balance Balance Balance

C - 0.08 - 0.08

Mn - 2.0 - 2.0

Si - 0.75 - 0.75

P - 0.045 - 0.045

S - 0.03 - 0.03

Cr 18.0 20.0 18.0 20.0

Ni 12.0 15.0 12.0 15.0

B 1.0 1.24 1.75 2.25

N - 0.10 - 0.10

Co - 0.05 - 0.05

Nb - 0.01 - 0.01

SS 304B4

Mechanical & Thermal Properties of SS 304B4

Properties 20° C 250° C 350° C 400° C 500° C

Density (kg/m3) 7820 7724 7679 7655 7606

Young modulus (GPa) 199 175 164 - -

Poissions ratio 0.3 0.3 0.3 0.3 0.3

Tensile strength (Mpa) 515 445 439 439 -

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 - - -

SS 304B7

Mechanical & Thermal Properties of SS 304B7

Properties 20° C 250° C 350° C 400° C 500° C

Density (kg/m3) 7740 7649 7607 7585 7540

Young modulus (GPa) 199 175 164 - -

Poissions ratio 0.3 0.3 0.3 0.3 0.3

Tensile strength (Mpa) 515 445 439 439 -

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 - - -

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

Elements MinWt by%

MaxWt by %

Fe Balance Balance

C - 0.12

Mn - 1.0

Si - 1.0

P - 0.04

S - 0.03

Cr 16.0 18.0

Ni - 0.75

Co - 0.05

Nb - 0.01

Chemical Composition

Mechanical & Thermal Properties of SS 430

SS 430

Properties 20° C 250° C 350° C 400° C 500° C

Density (kg/m3) 7700 7644 7618 7604 7577

Young modulus (GPa) 202 185 178 172 156

Poissions ratio 0.3 0.3 0.3 0.3 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

SS 304

For Connecting Ducts• Good weldability• Cost consideration• Additional elements for vessel

applicationCo 0.05Nb 0.01

Requirement ~ 300 tonns

Chemical Composition

Elements MinWt by %

MaxWt by %

Fe Balance 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 1.0

Co - 0.05

Nb - 0.01

Properties 20° C 250° C 350° C 400° C

Density (kg/m3) 7930 7837 7793 7770

Young modulus (GPa) 200 180 172 168

Poissions ratio 0.3 0.3 0.3 0.3

Tensile strength (Mpa) 490 385 375 375

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

Mechanical & Thermal Properties of SS 304

SS 304

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 --- 3-7%• Sulphur content ---- 0.005-0.01 to improve weld penetration

Chemical Composition of 16-8-2 filler metal for TIG welding

Elements RangeWt by %

C 0.03-0.045

Mn 1.8-2.5

Si Max 0.5

P Max 0.025

S Max 0.02

Cr 15.5-17.0

Ni 8-9

Mo 1.8-2.2

Cu Max 0.1

PART- II

VV Manufacturability

A Glance at vacuum vessel of Fusion grade

machine

( ITER VACUUM VESSEL)

Size

- Torus OD 19.4 m

- Torus Height 11.3 m

- Double Wall Thickness 0.34-0.75 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 110-8 Pam-

3/sSurface 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

ITER Vacuum Vessel

Main Vessel

Parameter Value in mm

Fabrication tolerance at factory

Sector overall height ± 20

Sector overall width ± 20

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 ± 5

Vessel weld distortion due to field/shop welds at the site

± 5

Surface deviations of the torus from the referencegeometry after assembly at the pit

± 15

Challenge is in achieving the accuracy and tolerances

RIBS

Inboard segment

Equatorial segment

Upper segment segment

Lower segment segment

Segmentation

Intermodular keyInboard housing

Inboard Segment: Design details

Fragment of the inner shell

Fragment of outer shell

Centering key

FABRICATION OF A SECTION

OF A SECTOR

SEGMENTS

UPPER

LOWER

Equatorial

FABRICATION OF A SECTION POLOIDAL SECTOR

Shielding backup slides• Shielding assembly sequence

1. Measure the dimension of the VV

components and location of the support holes

2. Drill holes using position control on the

lower bracket according to measured data obtained at Step 1, welding with the rib

3. Assembly of the shield block

4. Drill holes using position control on the

upper bracket according to measured data obtained at Step 1

5. Fix upper bracket

6. Fix shield block

What is being aimed – SST-2

SST -2 & 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 100 MW 20 MW

Ip(MA) 15 11

Magnetic field at major radius Bo

5.3 T 5.2 T

Integrated full power operation

4700 hours7800 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

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) = 0.525( assessed)

Neutron flux/cm2/sec = 1.8 x1020/680x104

= 2.64 x1013 neutrons/cm2/sec FOR SST -2Total average neutron fluence at the first wall

For 5,000 hours = 0.2 x (4700/24x365) = 0.107 MW a/m2 & for 78000.178 MWa/m2

For 5,000 hours~ 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

1Gwatt=1x109 joules /sec, 17.6 Mev= 2.8x1012 joulesNo neutron3.57x1020 /sec

ITER total burn time 4700 hrs = 1.69 x107 secs ~2x107 sec, 0.63 FPY

Concept of Fusion machine being aimed at

Total Height: 9.55 meterOuter Diameter: 13.8 meterWidth: 5.3 meterInner shell: 40 mm thick plateOuter shell: 40 mm thick platePoloidal Ribs: 30 mm thick plateWall separation: 120 mm at inboard region 320 mm at outboard region

SST-2 Vessel

Material requirement

Vessel ~ 1600 tons of 316LN (IG)

JOIN HANDS TO FACE MATERIAL CHALLANGES

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