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

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

Divertor System of Tokamak Divertor system of tokamak is responsible for safe extraction of heat and particles escaping out of plasma core region. Poloidal Cross-Section of ITER tokamak showing Divertor location

Reference: Engineering of Plasma-Facing Components – Mario Merola, PFMC-11 (Oct-2006), Griefswald, Germany Parts of divertor system

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 ( C, 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.

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

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

Cobalt - Niobium - Boron COBALT (Desired weight percentage in SS316L(N) is < 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 < 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 < 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.

Operating conditions for steels in divertors

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) 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 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 Quantity Required : –0.01 Ton by Year 2010; –0.05 Ton by Year 2012; –5.00 Ton by Year 2017;

SS316L(N) Type-1 : Desired Properties Desired Material Properties Yield Strength: –Min 20C; –Min 130 Ultimate Tensile Strength: –Min C; –Min C; Total Elogation: –Min 20C; –Min 300C; Grain Size (ASTM E-112): –Minimum ASTM No. 3 or finer;  -Ferrite Content: –Max 1 %; Impact Energy (ASTM RT) –KCU Min 140 J/cm^2 (Initial state); –KCU Min 100J/cm^2 (after 100 hours at 750C );

SS316L(N) Type-2: Desired Properties Desired Material Properties Yield Strength: –Min 20C; –Min 130 Ultimate Tensile Strength: –Min C; –Min C; Total Elogation: –Min 20C; –Min 300C; Surface Roughness: –Max 9 microns; Grain Size (ASTM E-112): –Minimum ASTM No. 6 or finer;  -Ferrite Content: –Max 1 %;

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

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. ~3m ~2m ~0.8m ~0.4m

Divertor Cassette Body : Water Channels and End Plates

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; ~1m SS316L(N) Support Structure of Divertor Target

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 ( years); –Minimization of decay heat during decommissioning;

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 Challenges for fabrication of steel components

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 (~ °C); –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;

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/m 2 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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