Prototype Divertor System: Steels and Fabrication Technologies

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Prototype Divertor System Steels and
Fabrication Technologies

• Sameer Khirwadkar
• (Prototype Divertor Development Division)
• 21-July-2008
• Institute for Plasma Research (IPR)
• Bhat, Gandhinagar, Gujarat State, India

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Outline

• Divertor System of Tokamak
• Iron Nickel based alloys for Divertors
• Neutron induced transmutations
• Desired properties of SS316L(N)
• Divertor fabrication process
• Challenges for manufacturing steel materials
• Challenges for fabrication of steel components
• Summary

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Divertor System of Tokamak
Divertor system of tokamak is responsible for
safe extraction of heat and particles escaping
out of plasma core region.
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Parts of divertor system
Reference Engineering of Plasma-Facing
Components Mario Merola, PFMC-11 (Oct-2006),
Griefswald, Germany
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• Divertor Targets Dome
• Used to intercept high energy plasma particles
  and safe extraction of heat energy and particles
• Experience Neutronic Load, Thermal Loads
  Electromagnetic Loads due to close interaction/
  vicinity with plasma core
• Divertor Cassette Body
• Used as support structure for mounting Divertor
  Targets
• Used for supply of water (100-150C, 4MPa) to
  Divertor Targets for heat removal
• Used as neutron shield for vacuum vessel near
  divertor region
• Support Structure for Divertor Cassette
• Provide support to Divertor Cassette against
  forces due to thermo-mechanical loads,
  electromagnetic loads, earth movement, etc.

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Iron Nickel based alloys for divertor system

• A Iron based alloys
• SS316L(N) Austenitic Stainless Steel (UNS S31653)
  Divertor Cassete Body, Invessel support
  structures, Water supply pipes and manifolds
• XM-19 Austenitic Stainless Steel (UNS S20910)
  Divertor Attachment Links
• Steel 660 Austenitic Stainless Steel (UNS
  K66286) Fastners
• B Nickel based alloys
• Alloy 718 Super Alloy (UNS N07718) Bolts and
  cooling manifold support
• Inconel 625 Super Alloy (UNS N06625) Welding
  transition from CuCrZr to SS316L(N)
• Nimonic 80A Super Alloy (UNS N07080) Divertor
  keys for fixing divertor target in cassette body

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Neutron induced transmutations
Fe Ni Mn Cr Co Nb
Mn54, Mn56, Fe55, Co57, Co58,
Co60, Ni57, Cr51
Transmutation
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Cobalt - Niobium - Boron

• COBALT (Desired weight percentage in SS316L(N) is
  lt 0.05)
• Cobalt is an impurity.
• Cobalt reduction from 0.25 to 0.05 decreases
  the total decay heat by 20 and helps to reduce
  the activation of waste.
• Cobalt is one of the main components of activated
  corrosion products in the water cooling system.
  It has implications on occupational dose.
• NIOBIUM (Desired wt in SS316L(N) for In-Vessel
  Components is lt 0.1)
• Niobium is present as a trace element picked up
  during the melting process from ferroalloy
  addition.
• Nb produces long-lived radioisotopes that could
  become important for the decommissioning and
  waste disposal of in-vessel components.
• Exception Niobium is also added to some
  austenitic steels as an alloying element to
  stabilize the austenitic structure, to prevent
  susceptibility to inter-granular corrosion
  (associated with chromium depletion of grain
  boundaries) and to decrease the grain size.
• BORON (Desired wt in SS316L(N) is lt 10ppm)
• Boron produces Helium under neutron irradiation.
  Helium formation at weld joints can initiate
  crack formation.
• The effect of Boron on the Helium generation is
  most significant for the steel close to the water
  cooling channels due to thermalizing neutrons by
  water.
• Exception Boron is added to some steels used
  for neutron shielding purpose e.g. SS304B7 (UNS
  S30467). Such steels are not used for structural
  applications.

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Operating conditions for steels in divertors
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SS316L(N) Type-1 2

• SS316L(N) Type-1
• Usage
• Divertor Cassette Body
• Support Structures
• Production Routes
• Powder HIP
• Cast HIP
• Flat Rolled Plate (Thickness 5-200mm)
• Forging
• Quantity Required
• 10 Ton by Year 2010
• 50 Ton by Year 2012
• 500 Ton by Year 2017

• SS316L(N) Type-2
• Usage
• Water supply pipes
• Water supply manifolds
• Shape Size Required
• Tube diameter 10 20 mm
• Tube thickness 1 2 mm
• ASTM Standard
• ASTM A 771-88
• Quantity Required
• 0.01 Ton by Year 2010
• 0.05 Ton by Year 2012
• 5.00 Ton by Year 2017

SS316L(N) material specifications are based on
design and construction rules for mechanical
components of the FBR Nuclear Islands, RCC-MR,
Edition 2002, Section II, Materials, product
specifications RM 3321, RM 3324, RM 3331, RM 3342.
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SS316L(N) Type-1 Desired Properties

• Desired Material Properties
• Yield Strength
• Min 220MPa _at_ 20C
• Min 130 MPa _at_300C
• Ultimate Tensile Strength
• Min 525 MPa _at_ 20C
• Min 410 MPa _at_ 300C
• Total Elogation
• Min 45 _at_ 20C
• Min 33 _at_ 300C
• Grain Size (ASTM E-112)
• Minimum ASTM No. 3 or finer
• ?-Ferrite Content
• Max 1
• Impact Energy (ASTM E23-92 _at_ RT)
• KCU Min 140 J/cm2 (Initial state)
• KCU Min 100J/cm2 (after 100 hours at 750C )

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SS316L(N) Type-2 Desired Properties

• Desired Material Properties
• Yield Strength
• Min 220MPa _at_ 20C
• Min 130 MPa _at_300C
• Ultimate Tensile Strength
• Min 525 MPa _at_ 20C
• Min 410 MPa _at_ 300C
• Total Elogation
• Min 45 _at_ 20C
• Min 33 _at_ 300C
• Surface Roughness
• Max 9 microns
• Grain Size (ASTM E-112)
• Minimum ASTM No. 6 or finer
• ?-Ferrite Content
• Max 1

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Divertor Fabrication Processes

• Processes for divertor cassette body fabrication
• Powder HIP (Hot Isostatic Press)
• Casting (/- Solid-HIP)
• HIPing of Solid Plates
• Welding of Flat Rolled Plates
• Processes for tube-to-tube tube-to-plate
  joining
• Electron Beam Welding
• Laser Beam Welding
• TIG Welding

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• Approximate size of a divertor cassette 3m x 2m
  x (0.4m to 0.8m)
• Approximate weight of a divertor cassette 10
  Tons
• Total number of cassettes 54 cassettes (total
  weight of 540 tons approx.)
• SS316L(N) constitutes about 70 of total cassette
  material used for fabrication of cassette body,
  support structures, coolant tubes, etc.

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Divertor Cassette Body Water Channels and End
Plates
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• Large SS316L(N) steel support structure with
  complex shapes are used in fabricating Divertor
  Targets
• Hundreds of SS316L(N) coolant pipes are used for
  supplying water from Cassette Body to Divertor
  Targets and back

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Challenges for manufacturing steel materials

• Control on microstructure of material for
• Minimizing material damage due to neutrons and
  gamma radiations
• Minimizing corrosion of cooling tubes welds due
  to water
• Control on constituent elements of materials and
  impurities for
• Minimization of Hydrogen and/or Helium formation
  that generally degrades material strength
• Minimize production of long lived radioactive
  elements
• Radiation dose level of irradiated components
  should reduce to safe level within short time
  (50-100 years)
• Minimization of decay heat during decommissioning

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Challenges for fabrication of steel components

• Fabrication of steel structure of divertor
  cassette (cassette body) weighing 7 Ton in parts
  using one or more alternatives
• Powder HIP
• Casting (/- solid HIP)
• HIPing of solid plates
• Welding of Flat Rolled Plates
• Joining various parts together to fabricate
  complete cassette structure
• Fabrication of large size structures with tight
  dimensional tolerances
• Attaching SS316L(N) pipes with various components
  such as water manifold, cassette body, copper
  alloy heat sink tube/plate, etc.
• Minimization of fabrication costs without
  compromising on quality of components

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• Assessment of effect of fabrication / repair
  process and operating conditions on material
  performance
• Material characterization after the manufacturing
  cycle
• Fracture toughness of material irradiated at high
  temperatures (250-300C)
• Welding of irradiated stainless steel
• Stress corrosion cracking, corrosion fatigue of
  the HIPed material
• Irradiation Assisted Stress Corrosion Cracking
  (IASCC) of steel after components manufacturing
  cycle

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Summary

• SS316L(N) material selected for divertor cassette
  has stringent requirements on chemical
  composition of alloying elements as well as
  impurities. Production of such a material on
  industrial scale is a challenge.
• Divertor cassette body is a large, complex and
  heavy object that need to be fabricated with
  tight dimensional tolerances using multiple
  fabrication processes. Fabrication of divertor
  cassette body to required size and shape without
  affecting properties of material and joints is a
  challenge.
• Neutron irradiation damage is not severe for
  assumed neutron wall load of 0.5MWa/m2 on
  divertor. However, neutron irradiation effects on
  materials and joints at operating temperatures
  need to be understood to estimate lifetime of the
  components.

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Thank You All