FABRICATION DESIGN AND CODE REQUIREMENTS FOR THE ITER VACUUM VESSEL

Transcription

1 Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference PVP2011 July 17-21, 2011, Baltimore, Maryland, USA PVP 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 K. Ioki, B. Giraud, C. H. Choi, & Y. Utin ITER Organization St. Paul lez Durance, France 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 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. 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. 1 Copyright 2011 by ASME

2 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. 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. 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. Parameter Torus outer diameter Torus inner diameter Configuration - Inboard straight region - Inboard top/bottom - Outboard region Operating condition 1 - Normal operation - During baking Design condition - Main vessel - Ports Interior surface area Interior volume Table 1: Main VV Parameters. Description (value) 19.4 m 11.4 m Cylindrical Double curvature Mainly double curvature 100 ºC / 0.8 MPa (Absolute) 200 ºC / 2.1 MPa (Absolute) 200 ºC / 2.6 MPa (Absolute) 250 ºC / 5.0 MPa (Absolute) ~850 m 2 ~1600 m 3 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 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]. Items Table 2: Materials used for VV and IWS. Material Main Vessel SS316L(N)-ITER Grade 1 Ports SS316L(N)-ITER Grade 1 SS 304L (EN grade ) SS 304 (EN grade1.4301) In-wall shielding 304B4 or 304B7 (UNS S30464/7) (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 The VV design shall take into account the various loads combinations for which the VV safety functions are needed including seismic events. 2 Copyright 2011 by ASME

3 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. 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 Category IV Category III Level N2 Level N3 Level N3 custom-machined splice plates which will be thicker than the VV wall to allow misalignment between sectors [2]. 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. 3 Copyright 2011 by ASME

4 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 (width) x 1.16 (height) Equatorial - Regular - Heating Neutral Beam - Heating/Diagnostic Neutral Beam (width) x 2.2 (height) (width) x 1.36 (height) (width) x 1.36 (min. height) (width) x (min. height) Lower - RH/Diagnostics - Cryopumps - Local penetrations (width) x (height) 1.39 (width) x (height) Various sizes (a) (b) (c) 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 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. 4 Copyright 2011 by ASME

5 (a) (b) (c) (d) (a) (b) (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. Cutting and forming of inner and outer shells 2. Welding of inner shells 3. Welding of ribs to the inner shell 4. Machining FSH holes and welding FSH to the inner shell 5. Machining port hole and welding port stub 6. Welding support ribs for IWS 7. Assembly of IWS 8. Welding the outer shells 9. 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. (e) (f) (g) (h) Figure 8: Fabrication Sequence of an Inboard Segment (PS1). (a) Bending shells and machining of holes for key. (b) TIG welding of ribs and E-beam welding of keys. (c) E-beam welding of keys. (d) Hole machining for FSHs. (e) E-beam welding of FSHs. (f) Upper & Lower shells welding and IWS ribs. (g) Installing IWS. (h) 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 5 Copyright 2011 by ASME

6 (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 40 o side ribs for leak tightness of one complete sector. (b) Interface bracket of gusset welding. (c) T-rib welding on the upper part of inner shell. (d) Lower part of inner shell and support pad welding. (e) Rail support welding. (f) T-rib welding on the lower part of inner shell. (g) Welding upper and lower part of inner shell. (h) Waterstopping flange welding and joint area cutting. (i) Outer shell welding. (a) (b) (c) (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. 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. 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. (a) (b) (c) (d) (e) (f) (g) (h) (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 6 Copyright 2011 by ASME

7 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. (a) Figure 15: Inner Shell Shape of Upper Segment. 7 Copyright 2011 by ASME

8 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. 8 Copyright 2011 by ASME

9 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, 2. ITER 2009 Baseline Plant Description (PD), ITER Organization, ITER DDD 1.5 Vacuum Vessel, ITER Organization, 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 Arrete du 12 decembre 2005 relatif aux equipements sous pression nucleaires (ESPN), Order dated 12 th 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, , Bak, J. S. et al., Preparations for the ITER Vacuum Vessel Construction, 23 rd IAEA Fusion Energy Conference, October 2010, Deajeon, Korea. 8. Kim. B. C. et al., Fabrication Design Progress of ITER Vacuum Vessel in Korea, 23 rd IAEA Fusion Energy Conference, October 2010, Deajeon, Korea. 9 Copyright 2011 by ASME