Masataka Nakahira

Manufacturing and maintenance technologies developed for a thick-wall structure of the ITER vacuum vessel

Development of welding, cutting and non-destructive testing (NDT) techniques, and development of remotized systems have been carried out for on-site manufacturing and maintenance of the thick-wall structure of the International Thermonuclear Experimental Reactor (ITER) vacuum vessel (VV). Conventional techniques, including tungsten inert gas welding, plasma cutting, and ultrasonic inspection, have been improved and optimized for the application to thick austenitic stainless steel plates. In addition, advanced methods have been investigated, including reduced-pressure electron-beam and multi-pass neodymium-doped yttrium aluminum garnet (NdYAG) laser welding, NdYAG laser cutting, and electro-magnetic acoustic transducer inspection, to improve cost and technical performance. Two types of remotized systems with different payloads have been investigated and one of them has been fabricated and demonstrated in field joint welding, cutting, and NDT tests on test mockups and full-scale ITER VV sector models. The progress and results of this development to date provide a high level of confidence that the manufacturing and maintenance of the ITER VV is feasible.

The thermal shields for the ITER magnet system: thermal, structural and assembly aspects

Abstract The thermal shield system is a continuous barrier between the magnet system operating at 4.5 K and the warm tokamak components. It provides a substantial reduction of both total and local thermal loads to the cold structures to achieve the limits required for normal operation of the superconducting magnet system and maximum heat load of the cryogenic plant. This paper describes details of the design of the different types of thermal shield and presents some results of thermal–hydraulic and structural analyses and some aspects of the assembly procedure for the vacuum vessel thermal shield, which is a challenging task considering the rather small dimensional tolerances that have to be obtained on the fully assembled shield.

Design and Development of the ITER Vacuum Vessel

Abstract In ITER, the vacuum vessel (VV) is designed to be a water cooled, double-walled toroidal structure made of 316LN stainless steel with a D-shaped cross section approximately 9 m wide and 15 m high. The design work which began at the beginning of the ITER-EDA is nearing completion by resolving the technical issues. In parallel with the design activities, the R&D program, Full-scale VV Sector Model Project, was initiated in 1995 to resolve the design and fabrication issues. The full-scale sector model corresponds to an 18° sector (9° sub-sector×2) and is being fabricated on schedule. To date, 60% of the fabrication had been completed. The fabrication of full-scale model including sector-to-sector connection will be completed by the end of 1997 and performance tests are scheduled until the end of ITER-EDA. This paper describes the latest status of the ITER VV design and the Full-scale Sector Model Project.

Mechanical characteristics and position control of vehicle/manipulator for ITER blanket remote maintenance

Abstract In International Thermonuclear Experimental Reactor (ITER), blanket maintenance requires the 4-tonne module handling with high positioning accuracy of ±2 mm. In order to meet this requirement, it is essential to suppress the dynamic deflection and vibration of the remote handling equipment due to sudden transfer of the module weight from/to the back-plate supports to/from the equipment itself during installation and removal. A new control scheme was proposed and tested so as to suppress the dynamic behaviors. As a result, the dynamic deflection of the rail and the acceleration of the manipulator were sucesessfully decreased to nearly zero. Based on the test results, the proposed control scheme was concluded to be effective so as to suppress this kind of dynamic effect during heavy component handling.

Eddy current experiments with closely placed solid boxes simulating a next step fusion device

Eddy current experiments with closely placed solid boxes driven by a coil alternative current were carried out. The experiment was to designed to identify approaches to the problems about: (1) computing accuracy of the eddy current calculation with thin plate approximation; and (2) modeling baselines for a computational study of eddy currents in a super conducting (SC) fusion device. The following conclusions were drawn from experimental data and their comparison with eddy current analyses: (1) the skin effect is a major cause of computational error; (2) if the error due to the skin effect cannot be ignored, the double shell model calculates accurate eddy currents; (3) the mesh size with a narrow gap should be small enough (for example smaller than five times the gap width) so as not to cause additional error. The Finite Element Method (FEM) model for the eddy current computation is well modeled using these conclusions.

Technical Code Issues of ITER Vacuum Vessel and Their Resolutions

The ITER vacuum vessel is a double-walled torus with large-sized quadrilateral ports needed to provide quite a high degree of vacuum for deutrium-tritium fusion reaction. Access to the outside of vacuum vessel is limited during assembly welding, because it is installed after assembled with surrounding toroidal field coils. From the radiological safety aspect, the vacuum vessel functions as a physical barrier to enclose radioactive materials. Therefore, construction of the vacuum vessel needs application of newly developed technologies on design, fabrication and examination. The technologies include design approach by finite element analysis, and partial penetration T welded joints to join ribs to outer shell. Several issues have to be resolved for applying those technologies to the vacuum vessel. This paper describes several newly developed technologies and key issues for such applications.

Design and Structural Analysis of Support Structure for ITER Vacuum Vessel

The International Thermonuclear Experimental Reactor (ITER) vacuum vessel (VV) is a safety component confining radioactive materials such as tritium and activated dust. An independent VV support structure with multiple flexible plates located at the bottom of VV lower port is proposed as a new concept, which is deferent from the current design, i.e., the VV support is directly connected to the toroidal coils (TF coils). This independent concept has two advantages comparing to the current one: (1) thermal load due to the temperature deference between VV and TF coils becomes lower and (2) the TF coils are categorized as non-safety components because of its independence from VV. Stress Analyses have been performed to assess the integrity of the VV support structure using a precisely modeled VV structure. As a result, (1) the maximum displacement of the VV corresponding to the relative displacement between VV and TF coils is found to be 15 mm, much less than the current design clearance of 100 mm, and (2) the stresses of the whole VV system including VV support are estimated to be less than the allowable ones defined by ASME Section III Subsection NF, respectively. Based on these assessments, the feasibility of the proposed independent VV support has been verified as a VV support.

DIVERTOR PLATE SUPPORTING SYSTEM FOR FUSION EXPERIMENTAL REACTOR

The divertor plates of the fusion experimental reactor (ITER/FER) are operated under severe heat/particle fluxes and are categorized as the scheduled maintenance components. Thus, reliable supporting system to be compatible with remote maintenance is inevitably required for locking/lifting the divertor plate. For this purpose, the hydraulic supporting system composed of movable cotter by hydraulic driving mechanism is being developed. The basic feasibility of the hydraulic driving system has been successfully demonstrated and full scale mockup test is under way. This paper gives the design concepts and latest R&D achievements of the hydraulic supporting system for divertor plate.

Numerical evaluation of experimental models to investigate the dynamic behavior of the ITER tokamak assembly

The most recent assessment method to evaluate the dynamic behavior of the International Thermonuclear Experimental Reactor (ITER) tokamak assembly is outlined. Three experimental models, including a 1/5.8-scale tokamak model, have been considered to validate the numerical analysis methods for dynamic events, particularly seismic ones. The experimental model has been evaluated by numerical calculations and the results are presented. In the calculations, equivalent linearization has been applied for the non-linear characteristics of the support flange connection, caused by the effects of the bolt-fastening and the friction between the flanges. The detailed connecting conditions for the support flanges have been developed and validated for the analysis. Using the conditions, the eigen-mode analysis has shown that the first and second eigen-mode are horizontal vibration modes with the natural frequency of 39 Hz, while the vertical vibration mode is the fourth mode with the natural frequency of 86 Hz. Dynamic analysis for seismic events has shown the maximum acceleration of approximately twofold larger than that of the applied acceleration, and the maximum stress of 104 MPa found in the flange connecting bolt. These values will be examined comparing with experimental results in order to validate the analysis methods.

Progress and achievements of R&D activities for the ITER vacuum vessel

The ITER vacuum vessel (VV) is designed to be large double-walled structure with a D-shaped crosssection. The achievable fabrication tolerance of this structure was unknown due to the size and complexity of shape. The Full-scale Sector Model of ITER Vacuum Vessel, which was 15m in height, was fabricated and tested to obtain the fabrication and assembly tolerances. The model was fabricated within the target tolerance of 5mm and welding deformation during assembly operation was obtained. The port structure was also connected using remotized welding tools to demonstrate the basic maintenance activity. In parallel, the tests of advanced welding, cutting and inspection system were performed to improve the efficiency of fabrication and maintenance of the Vacuum Vessel. These activities show the feasibility of ITER Vacuum Vessel as feasible in a realistic way. This paper describes the major progress, achievement and latest status of the R&D activities on the ITER vacuum vessel.