ITER Vacuum Vessel Research Articles

Prototyping studies for the Blanket Shield Module of the ITER ECH Upper Port Plug

A team of European associations is planning to procure ECH launcher turnkey systems for MHD control in the ITER plasma. ECH launchers will be installed to four ports on the upper level of the ITER vacuum vessel (VV). The structural system of the launchers accommodates the mm-wave components, cooling devices and elements for nuclear shielding. Its main components are the Blanket Shield Module (BSM), including the plasma facing First Wall Panel (FWP) and the port plug mainframe. A removable flange connection between the BSM and the main frame provides access to the internals. Appropriate remote handling capability is also taken as a design requirement. The BSM with the flange connection will be exposed to substantial nuclear heat loads. The manufacturing of machined components requires complex shaping with small tolerances and good quality of the surfaces due to operation under vacuum conditions. For the BSM and the front segment of the main frame a rigid double wall structure with meandering rectangular cooling channels was designed and analysed to meet these requirements. To investigate industrial manufacturing routes, a typical single-piece sample was machined and the manufacturing process was evaluated. Further two prototypes of a characteristic section of the BSM were manufactured, using two different fabrication techniques. These are (a) Hot Isostatic Pressing (HIP), which combines the sintering of metal powder inside of welded capsules and diffusion welding of solid parts and (b) brazing of bent and machined individual parts. The prototypes are under study at the Launcher Handling Test facility (LHT) at FZK, which offers a water circuit to provide coolant with adjustable parameters, simulating different ITER operating conditions. Extensive test series were performed to validate underlying analysis related to homogenous temperature distribution, tolerable pressure drop within the cooling paths and removal of applied heat loads.

A study on the thermal analyses of the ITER vacuum vessel thermal shield

The thermal shield system of ITER plays the role of reducing heat loads transferred by thermal radiation and conduction from warm components to the components and structures that operate at 4.5 K. In the design of thermal shields, it is of importance to ensure that they maintain thermal loads within the specified design criteria. In the present work, a study is conducted on thermal analyses in order to confirm that the thermal shield design can accommodate the thermal loads during the plasma operation state and the baking state of the ITER vacuum vessel. Design of the cooling tube layout of a vacuum vessel thermal shield is followed based on the results of thermal analyses. It is demonstrated that the present results can be applied to the detailed design of the ITER thermal shield system.

Recent progress of ITER vacuum vessel related design activities in Korea

Based on the design done by the ITER international organization (ITER-O) and other participant teams, the more detail engineering design of the ITER vacuum vessel port and support is progressed by the Korea ITER project team. In this work, the recent design elaboration and manufacturing feasibility study from 2006 mainly focused on the equatorial and lower port are reported. The fabrication status and test plan of vacuum vessel support mock-up are also introduced.

A proposal of ITER vacuum vessel fabrication specification and results of the full-scale partial mock-up test

The structure and fabrication methods of the ITER vacuum vessel (VV) have been investigated and defined by the ITER International Team (IT). However, some of the current technical specifications are difficult to be achieved from the manufacturing point of view. To solve such an issue, this paper proposes an alternative specification of the VV to the IT's design. A series of the fabrication and assembly procedures for the mock-up are presented, together with candidates of welding configurations. Finally, the paper summarizes the results of mock-up fabrication, such as non-destructive examination of weld lines, obtained welding deformation and issues revealed from the fabrication experience. Based on the results, it is suggested that several issues such as clarification of conditions of repair welding, demonstration of welding distortion control and detectability/localization of internal defects should be solved before manufacturing the ITER VV.

D-T neutron streaming experiment simulating narrow gaps in ITER equatorial port

Under the ITER/ITA task, we have conducted the neutron streaming experiment simulating narrow and deep gaps at boundaries between ITER vacuum vessel and equatorial port plugs. Micro-fission chambers and some activation foils were used to measure fission rates and reaction rates to evaluate the relative fast and slow neutron fluences along the gap in the experimental assembly. The MCNP4C, TORT and Attila codes were used for the experimental analysis. From comparing our measurements and calculations, the following facts were found: (1) in case of a such narrow and deep gap structure, the calculation with MCNP, TORT and Attila codes and FENDL-2.1 is sufficient to predict fast neutron field inside the gap; (2) by scattering neutrons in the experimental room, experimental error considerably increased at the deeper region than 100 cm; (3) angular quadrature set of upward biased U315 and last collided source calculation on TORT and Attila were very important technique for accurate estimation of neutron transport.

Design progress of the vacuum vessel sectors and ports towards the ITER construction

Recent progress of the ITER vacuum vessel (VV) design is presented. As the ITER construction phase approaches, the VV design has been improved and developed in more detail with the focus on better performance, improved manufacture and reduced cost. Based on the achievements of manufacturing studies being performed in cooperation with the ITER participant teams (PTs), design improvement of the typical VV sector (#1, see the legend to figure 1 in this article) has been nearly finalized. Design improvement of other sectors is in progress—in particular, of the VV sectors #2 and #3 which interface with tangential ports for the neutral beam (NB) injection. For all sectors, the concept for the in-wall shielding has progressed and developed in more detail. The design progress of the VV sectors has been accompanied by the progress of the port structures. Structural analyses have been performed to validate all design improvements.

ITER vacuum vessel, in-vessel components and plasma facing materials

The vacuum vessel (VV) design is being developed in more detail considering manufacturing and assembly methods, and cost. Incorporating manufacturing studies being performed in cooperation with ITER Parties, the regular VV sector design has been nearly finalized. Design of the neutral beam (NB) ports including duct liners has been developed. Design of the in-wall shielding has been developed in more detail considering the supporting structure and the assembly method. Additional ferromagnetic inserts to be installed in the outboard midplane region will minimize the maximum ripple and the toroidal field flux line fluctuation. Detailed studies were carried out on the ITER vacuum vessel to define appropriate codes and standards in the context of ITER licensing in France. The blanket module design has progressed in cooperation with participant teams. Fabrication of mock-ups for qualification testing is under way and the tests will be performed in 2007–2008. The divertor activities have progressed with the aim of launching the procurement according to the ITER project schedule.

Forming residual stresses effects on the electron beam welding distortions of thick components

The purpose of this study is to evaluate the effects of forming residual stresses on the electron beam welding distortions of thick components building up the ITER vacuum vessel. The deformed shape of a thick plate is obtained by a three point rolling process. The numerical simulation of this process is then performed to evaluate the induced residual stresses. Forming residual stresses are taken into account as initial conditions for the welding simulation of two deformed thick plates. A simulation is finally performed to evaluate the influence of the forming residual stresses on the welding distortions.

Design Progress of the ITER In-Wall Shielding

The space between the double walls of ITER Vacuum Vessel is filled with pre-assembled or modular shield blocks called “in-wall shielding”. It has two main functions; one is to provide an effective neutron shielding and the other to reduce the toroidal field ripple in the outboard region. The in-wall shielding is designed to minimize electromagnetic forces acting on the shield blocks. The electromagnetic forces have been calculated with the latest disruption scenarios. Magnetization forces have also been calculated for ferromagnetic inserts. Structural integrity has been validated by structural analyses.

Structural and fracture mechanics analysis of ITER vacuum vessel

The eddy current is induced on ITER vacuum vessel (VV) by plasma centered disruption (CD). The electromagnetic (EM) force is loaded on VV by an interaction of the eddy current and magnetic field. Since it is difficult to perform in-service inspection due to inaccessibility of VV, it is important to estimate the allowable initial defect from the view point of fracture mechanics in order to assure the structural integrity of VV. The purpose of this study is evaluation of acceptable initial crack size of VV by a linear elastic fracture mechanics (LEFM). The fatigue crack growth is evaluated using the cracks modeled by the local three-dimensional (3D) finite element model to calculate a stress intensity factor as a function of crack size. As the results of these analyses, no significant unstable fracture and penetrating VV shell are foreseen by the CD.

Design progress of the ITER vacuum vessel sectors and port structures

Recent progress of the ITER vacuum vessel (VV) design is presented. As the ITER construction phase approaches, the VV design has been improved and developed in more detail with the focus on better performance, improved manufacture and reduced cost. Based on achievements of manufacturing studies, design improvement of the typical VV Sector (#1) has been nearly finalized. Design improvement of other sectors is in progress—in particular, of the VV Sectors #2 and #3 which interface with tangential ports for the neutral beam (NB) injection. For all sectors, the concept for the in-wall shielding has progressed and developed in more detail. The design progress of the VV sectors has been accompanied by the progress of the port structures. In particular, design of the NB ports was advanced with the focus on the beam-facing components to handle the heat input of the neutral beams. Structural analyses have been performed to validate all design improvements.

Results from ITER vacuum vessel sector manufacturing development in Europe

This paper describes the results from several R&D tasks carried out in Europe to improve the prospects for manufacturing the ITER vacuum vessel within the required tight tolerances and to the required high quality. The experience from the manufacture of a part of the sector has highlighted several distortion issues, which can be compensated for. A local machining tool demonstrated the possibility of vibration-free machining without lubrication, as required in the ITER clean conditions scenario. Although a project to establish the feasibility of cold 3-D forming of the walls showed it was with the capacity of available equipment, a problem of buckling instability may make this method unusable. An alternative method of sector manufacture, using e-beam welding, and avoiding the use of one-sided welds, is described.

Detailed design and fabrication method of the upper and lower ports for the ITER vacuum vessel

The detailed design of the vacuum vessel port structures has been developed based on the conceptual design of the ITER IT (international team) and PT (participant team). The special emphasis was laid on the flange joint between the port extension and the in-port plug to develop the design of the upper port. The structural analyses of the lower RH port have been also performed to verify the capacity for supporting the VV. The fabrication method and the sequence of the detailed fabrication for the ports are developed focusing on the cost reduction as well as the structural simplification. Tolerance study has been performed to avoid the mismatch of each fabricated component and to obtain the suitable tolerances in the assembly at the shop and site. From the several structural analyses and the development of the fabrication method, the upper and lower port designs have been improved and it also complies with the design specification.

Interaction of beryllium containing plasma with ITER materials

Beryllium-seeded deuterium plasma is used in PISCES-B to investigate mixed-material erosion and redeposition properties of ITER relevant divertor materials. The beryllium containing plasma simulates the erosion of first wall material into the ITER sol plasma and its subsequent flow towards the carbon divertor plates. The experiments are designed to quantify the behaviour of plasma created mixed Be/C and Be/W surfaces. Developing an understanding of the mixed material surface behaviour is crucial to accurately predict the tritium accumulation rate within the ITER vacuum vessel. The temporal evolution of the plasma interactions with the various mixed surfaces are examined to better understand the fundamental mechanisms in play at the surface and to allow scaling of these results to the conditions expected in the ITER divertor. A new periodic heat pulse deposition system is also installed on PISCES-B to simulate the transient temperature excursions of surfaces expected to occur in the ITER divertor during edge localized modes (ELMs) and other off-normal events. These periodically applied heat pulses allow us to study the effects of transient power loading on the formation, stability and tritium content of mixed-material surfaces that are created during the experiments.

Preliminary Design and Analysis of ITER In-Wall Shielding

ITER in-wall shielding (IIS) is situated between the doubled shells of the ITER Vacuum Vessel (IVV). Its main functions are applied in shielding neutron, gamma-ray and toroidal field ripple reduction. The structure of IIS has been modelled according to the IVV design criteria which has been updated by the ITER team (IT). Static analysis and thermal expansion analysis were performed for the structure. Thermal-hydraulic analysis verified the heat removal capability and resulting temperature, pressure, and velocity changes in the coolant flow. Consequently, our design work is possibly suitable as a reference for IT's updated or final design in its next step.

Lees' Loss Prevention in the Process Industries, 3rd ed., San Mannan (Ed.). Elsevier/Butterworth-Heinemann, Burlington, MA/Oxford, UK (2005), (3680pp., US$476.00)

Explosion indices and explosion behaviour of Al dust/H2/air mixtures were studied using standard 20 l sphere. The study was motivated by an explosion hazard occurring at some accidental scenarios considered now in ITER design (International Thermonuclear Experimental Reactor). During Loss-of-Vacuum or Loss-of-Coolant Accidents (LOCA/LOVA) it is possible to form inside the ITER vacuum vessel an explosible atmosphere containing fine Be or W dusts and hydrogen. To approach the Be/H2 explosion problem, Be dust is substituted in this study by aluminium, because of high toxicity of Be dusts. The tested dust concentrations were 100, 200, 400, 800, and 1200 g/m3; hydrogen concentrations varied from 8 to 20 vol. % with 2% step. The mixtures were ignited by a weak electric spark. Pressure evolutions were recorded during the mixture explosions. In addition, the gaseous compositions of the combustion products were measured by a quadruple mass-spectrometer. The dust was involved in the explosion process at all hydrogen and dust concentrations even at the combination ‘8%/100 g/m3’. In all the other tests the explosion overpressures and the pressure rise rates were noticeably higher than those relevant to pure H2/air mixtures and pure Al dust/air mixtures. At lower hybrid fuel concentrations the mixture exploded in two steps: first hydrogen explosion followed by a clearly separated Al dust explosion. With rising concentrations, the two-phase explosion regime transits to a single-phase regime where the two fuel components exploded together as a single fuel. In this regime both the hybrid explosion pressures and pressure rise rates are higher than either H2 or Al ones. The two fuels compete for the oxygen; the higher the dust concentration, the more part of O2 it consumes (and the more H2 remains in the combustion products). The test results are used to support DUST3D CFD code developed at KIT to model LOCA or LOVA scenarios in ITER.

Prevention of hydrogen and dust explosion in ITER

It is to be expected that, over time, an interaction of plasma with walls and the divertor will create in ITER hundreds of kilograms of beryllium, carbon or tungsten dust – loose particles with characteristic diameter <100 μm. Interaction of the hot Be dust with water in the case of a water leak will produce hydrogen. In such conditions air leaks can lead to hydrogen or dust explosions. The explosions could damage the machine and would present a danger to the public if the release of radioactivity from the vacuum vessel were not otherwise mitigated. Current ITER strategy to cope with the danger of dust/hydrogen explosions is based on administrative limits on accumulation of dust inside the ITER vacuum vessel. We are investigating a different approach based on active prevention measures – injections of an inert gas (CO 2) in the vacuum vessel in case of detection of air leaks. Analysis done with the code MELCOR shows that injection of CO 2 in the vacuum vessel may prevent the explosions. However, a detailed analysis of O 2/CO 2/H 2 space distributions and some experiments will be needed before a decision is made to base ITER safety on this approach.

ITER vacuum vessel sector manufacturing development in Europe

The ITER vacuum vessel, to be constructed on site from nine toroidal sectors, places as-welded manufacturing tolerances on the surfaces several times smaller than usual, in relation to its large size. Framatome ANP has proposed a manufacturing route for the sector construction, accepted as the reference by the ITER International Team. To facilitate the achievement of the required tight tolerances, EFDA has implemented a development programme, including the manufacturing part of the sector and the analysis of its distortion using SYSWELD computer modelling. This paper describes the manufacturing route and the results of the development programme so far.

Development and control towards a parallel water hydraulic weld/cut robot for machining processes in ITER vacuum vessel

This paper presents a special robot, able to carry out welding and machining processes from inside the ITER vacuum vessel (VV), consisting of a five degree-of-freedom parallel mechanism, mounted on a carriage driven by two electric motors on a rack. The kinematic design of the robot has been optimised for ITER access and a hydraulically actuated pre-prototype built. A hybrid controller is designed for the robot, including position, speed and pressure feedback loops to achieve high accuracy and high dynamic performances. Finally, the experimental tests are given and discussed.

Dynamic identification of the hydraulic Maestro manipulator—Relevance for monitoring

Maintenance tasks at ITER divertor level require the use of powerful force-reflective remote handling devices. CEA in collaboration with CYBERNETIX and IFREMER has developed the Maestro hydraulic manipulator (Modular Arm and Efficient System for TeleRObotics). For teleoperated tasks inside the ITER vacuum vessel, model-based monitoring of the robot can be used to warn the operator of possible failures, without additional sensors. In this paper, we present the experimentally identified dynamic model of the Maestro arm. Then, the use of this model to detect abnormal situations has been investigated. Even if close monitoring is limited to the case of moving axes, due to friction predominance at low velocities, the results help to draw up a detection strategy for unpredicted behaviour, such as collision of the robot with its environment.