ITER – Vacuum vessel welding


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 ITER is one of the most ambitious energy projects in the world today.

In southern France, 35 nations* are collaborating to build the world’s largest tokamak, a magnetic fusion device that has been designed to prove the feasibility of fusion as a large-scale and carbon-free source of energy based on the same principle that powers our Sun and stars.

The experimental campaign that will be carried out at ITER is crucial to advancing fusion science and preparing the way for the fusion power plants of tomorrow.

Thousands of engineers and scientists have contributed to the design of ITER since the idea for an international joint experiment in fusion was first launched in 1985. The ITER Members—China, the European Union, India, Japan, Korea, Russia and the United States—are now engaged in a 35-year collaboration to build and operate the ITER experimental device, and together bring fusion to the point where a demonstration fusion reactor can be designed.


The ITER experiments will take place inside the vacuum vessel, a hermetically sealed steel container that houses the fusion reactions and acts as a first safety containment barrier. In its doughnut-shaped chamber, or torus, the plasma particles spiral around continuously without touching the walls.

The vacuum vessel provides a high-vacuum environment for the plasma, improves radiation shielding and plasma stability, acts as the primary confinement barrier for radioactivity, and provides support for in-vessel components such as the blanket and the divertor. Cooling water circulating through the vessel’s double steel walls will remove the heat generated during operation.

Forty-four openings, or ports, in the vacuum vessel provide access for remote handling operations, diagnosticsheating, and vacuum systems. (Neutral beam injection will take place at equatorial level, for example, while on the lower level, five ports will be used for divertor cassette replacement and four for vacuum pumping.)

The blanket modules lining the inner surfaces of the vessel will provide shielding from the high-energy neutrons produced by the fusion reactions. (Some blanket modules will also be used at later stages to test materials for tritium breeding concepts.) Along with the magnet systems, the ITER vacuum vessel is entirely enclosed in a large vacuum chamber called the cryostat.

In a tokamak device, the larger the vacuum chamber volume, the easier it is to confine the plasma and achieve the type of high energy regime that will produce significant fusion power.

The ITER vacuum vessel, with an interior volume of 1,400 m³, will provide an absolutely unique experimental arena for fusion physicists: the volume of the plasma contained in the centre of the vessel (840 m³) is fully ten times larger than that of the largest operating tokamak in the world today. The ITER vacuum vessel will measure 19.4 metres across (outer diameter), 11.4 metres high, and weigh approximately 5,200 tonnes. (With the installation of the blanket and the divertor, the vacuum vessel will weigh 8,500 tonnes.)

Diagnostics & Instrumentation : First Welding on the Vacuum Vessel

 Beginning in 2035, ITER will open a window into “burning plasmas”—a state of matter that exists in the core of stars only. Observing, assessing and monitoring this yet unexplored territory of physics will require a vast array of measuring systems and diagnostics. In the Assembly Hall, welding specialists are taking the first steps toward their physical installation on the vacuum vessel.

”Bosses” that will support instrumentation and cable trays are being welded to the outer surface of the vacuum vessel by specialists from Italian contractor Ansaldo Nucleare. (The larger round, red devices in white squares are metrology targets temporarily attached to the vacuum vessel.)

Like an elephant at rest, the first of the nine vacuum vessel segments lies on its side at the far end of the vast and crowded hall. Its grey skin, which was perfectly smooth when the component was delivered in late July, is now pockmarked with dozens of small cylindrical devices, some perfectly aligned, others distributed in a seemingly haphazard fashion. Alex Martin, who heads the ITER Vacuum Vessel Section, uses another image—what he sees in this scene is “a stranded whale, its skin colonized by barnacles…”

The number of cylindrical devices (or barnacles if you prefer) is growing by the day: since work began last Wednesday, close to 150 of these objects, called “bosses,” have been welded to the component’s outer surface and the total should reach over 700 within a few weeks.

Bosses are like studs onto which other components can be bolted or welded to the vacuum vessel. They are designed to support the instrumentation that monitors the main electromagnetic parametres of the plasma—the shape of its boundary along with other data. Bosses are also used to support the cable trays that guide the sensors and instrumentation cables away from the vacuum vessel, along the vacuum vessel wall, through the cryostat, the bioshield, and eventually to the control cubicles and electronics located in the Diagnostic Building.

Implemented in ITER for the first time, ”laser templating” techniques are used to project an outline of the element to be welded at its precise pre-defined position on the component’s surface, providing the welder with a perfect template.

Welding 700 studs to a steel surface may appear to be a simple, repetitive operation. It is not. “We are dealing with the first confinement barrier of a nuclear device,” explains Alex, “and in order to ensure its integrity we have to observe a whole set of procedures that are controlled by nuclear safety, quality and third party nuclear inspectors.”

The precise positioning of the bosses on the curved surface of the vacuum vessel sector is achieved using an innovative technique called “laser templating.” Drawing from 3D models of the vacuum vessel and diagnostics, a laser beamer projects the outline of the element to be welded at its precise pre-defined position on the component’s surface, providing the welder with a perfect template. It is the first time that laser templating has been implemented on ITER for such an application.

Identical operations will be repeated on the exterior walls of the eight other vacuum vessel sectors prior to pre-assembly. As for the equipment planned on the inner walls, it will be installed once the vacuum vessel pre-assemblies are inside the Tokamak pit. In total, 30,000 welded attachments, fitted to within a few millimetres of tolerance, need to be welded to the vacuum vessel.

First Sector Safely Docked

Delivered to ITER on 7 August 2020, equipped and tested during the eight months that followed, vacuum vessel sector #6 is now safely docked on one of the sector sub-assembly tools. It is the first element of the Tokamak’s core to be ready for pre-assembly.

Over the past 11 months, there have been both heavier and larger loads. The cryostat baselower cylinder and lower cryostat thermal shield are without contest spectacular components, but none of them is part of the torus-shaped core of the machine; rather, they are large, sophisticated steel structures designed to envelop and insulate the machine. Vacuum vessel sector #6 is something else: it is the first element of the tokamak itself, a 40-degree segment of the vast toroidal chamber hosting the burning plasma— a state of matter that only exists at the core of the stars.

On 20 March 2021,  sector #6 was transferred from the laydown area to upending tool. On 26 March, bolted and strapped in the upending tool, the component was lifted and tilted from horizontal to vertical. Ten days later, it was time to perform the third stage of the operation and position it inside the sector sub-assembly tool.

This was the culmination of an eight-month preparatory campaign since the arrival of sector #6 at ITER. Following site acceptances tests (including a leak test in September), months were spent fully instrumenting the component, first by welding bosses and other attachments onto its outer shell, then by welding magnetic sensors and other instrumentation onto the bosses.

On 6 April, attached to a complex rigging system (see box below) and delicately pulled out of its berth, vacuum vessel sector #6 travelled a few dozen metres to face sub-assembly tool #2 and made a right angle turn.

The last phase of the operation was the most spectacular: lifted by the double overhead crane, the component rose above the 20-metre high sub-assembly tool, before slowly descending through the tool’s V-shaped opening to eventually settle, hanging from the radial beam that finds a home on supporting blocks at the top of the tool.

The central and most massive element of the first pre-assembly is now in place. In the coming months, it will be encased in thermal shield panels and associated to two toroidal field coils. Before the end of the year, the 1,200-tonne pre-assembly will be lowered into the pit, marking the first step in creating the tokamak’s toroidal chamber.

Steps to transfer a 440-tonne vacuum vessel sector from its horizontal orientation on the shop floor to the V-shaped sector sub-assembly tool:

  • The upending tool is used to raise the sector from horizontal to vertical.
  • Because the overhead travelling cranes cannot “grasp” these major vertical loads directly, several layers of attachment are installed between crane and component. These adaptive devices are brought together before the lift operation. (Next bullet.)
  • The 90-tonne dual crane heavy lifting beam (attaches to the four hooks of the double crane to allow the cranes to work in tandem) is connected to the sector lifting tool (equipped with a balance control system that can be activated to perform horizontal-plane load adjustments along the X and Y axis), and a radial beam (which will remain attached to the vacuum vessel sector through to its final position in the assembly pit).
  • The assembled lift attachments are brought above the vacuum vessel sector in its vertical position.
  • The support and alignment units of the sector lifting tool are connected to two lifting hooks on the vacuum vessel sector. This is a tricky step due to the position and angle of the lifting hooks, which have been welded to the sector.
  • Through controllers and actuators, the sector lift tool adjusts its balancing cross beam to match the calculated centre of gravity of the vacuum vessel sector. The sector is lifted to verify that the pre-set centre of gravity is satisfactorily aligned, and that the pins in the upending tool line up with the “guiding pins” on the vacuum vessel sector.
  • If alignment is confirmed, the pins slowly disengage and the vacuum vessel sector is fully lifted and transported in front of the sector sub-assembly tool.
  • The dual crane heavy lifting beam rotates the load 90° for mounting on the V-shaped tool.
  • The load is raised more than 20 metres above the shop floor and brought above the sector sub-assembly tool.
  • The load is lowered into the tool. From the top, the sector is supported by the radial beam, which is braced on the tool. At the bottom, a temporary pillar supports the lower port extension. Two hydraulic jacks in the pillar help to adjust the vertical angle of the sector through a “push and pull” movement until the required position is achieved.
  • The sector lifting tool performs a final balancing adjustment operation before disengaging from the radial beam.
  • The sector sub-assembly operation can proceed: panels of thermal shielding and two toroidal field coils are rotated in to the sector and assembled.

ITER concludes contract with ENSA

The contract covers the welding of ITER’s 9 vacuum vessel sector sub-assemblies and 54 ports. The work will take place inside of the Tokamak pit, as sector sub-assemblies are lowered one by one and welded according to a plan than aims to minimize deformation.

The ITER Organisation has concluded a contract with the Spanish company ENSA (Equipos Nucleares SA) for the welding of the ITER vacuum vessel.

ENSA is no stranger to the vacuum vessel welding project; since 2012 the company has been working under a phased contract with the ITER Organization to first develop specific welding processes and tools and then to procure the equipment. Phase three of the project—the execution of the welding works—was awarded to the firm in October 2020 following an international competition based on a negotiated procedure.

This is the third major machine assembly contract after the TAC1 and TAC2 Tokamak Assembly Contracts were signed in 2019.

The scope of the new contract—called VVW2P, for Vacuum Vessel Welding Production Phase—covers the welding of 9 vacuum vessel sector sub-assemblies and 54 ports. (A sub-assembly is the unit formed by one 40-degree vacuum vessel sector, two toroidal field coils, and a 40-degree thermal shield sector.) The estimated duration of the contract is 57 months, with a targeted completion date of March 2025. ENSA estimates peak personnel needs on site at 150 people.

Work will take place inside of the restricted space of the Tokamak pit, as the sub-assemblies are successively lowered into place by the TAC-2 ITER Organization assembly contractor and attached to their supports. ENSA contractors will join and weld the vacuum vessel sectors according to a plan that aims to minimize deformation. The first sectors to be welded together in 2022 are sector #6 (already on sub-assembly tooling) and sector #7 (expected from Korea in August 2021).

ENSA will be supported by the Indian firm Larsen & Toubro as the sub-contractor for the welding of the major part of the ports.

Mockups built by ENSA during the development of welding processes and tools (like this one at ENSA’s Special Projects workshop in the outskirts of Santander, Spain) will be used at ITER to train operators. Welding activities will begin as soon as two sectors are side by side in the ITER Tokamak pit (2022).

The ENSA team will face a challenging task, evolving in a crowded Tokamak pit environment with co-activity on every side. The weld robots will have access from inside the vessel only, as the thermal shield panels and the toroidal field coils block access to the vessel’s outer shell. Weld thickness is consequential—up to 60 mm—and weld shrinkage will have to be managed at every step of the process to ensure the correct shape of the final vessel assembly. Finally, stringent volumetric examination protocols will be required to comply with French nuclear regulations.

“As a nuclear pressure vessel, the ITER vacuum vessel is subject to conformity assessments performed by a body authorized by the French Nuclear Regulator,” explains Frantz de la Burgade, Group Leader for the Sector Modules Delivery & Assembly Division. “The technical documentation will have to be edited sufficiently in advance to allow enough time for this Agreed Notified Body to review.”

During the initial phase of the contract (P0), which has been underway since the kick-off meeting in November 2020, the ENSA team is focusing on a selection of preparatory tasks, including documents, engineering tests, trials, qualification, and training.

“To perform a good weld is one thing; to be in a position to justify it has been perfectly done is an additional challenge,” says De la Burgade. “The contractor is asked to predict shrinkage behaviour at each step of the assembly process, so that the expected shrinkage can be compensated by the relative pre-positioning of two consecutive sectors in the pit before the welding begins. Another important challenge of the initial phase of the contract is to mobilize experienced operators for both welding and non-destructive examination.”

The early team, mobilizing to organize the preparatory activities on site, will be followed by increasing numbers of workers as the tasks begin in the ITER Tokamak pit. A core team of experienced engineers from ENSA is already collaborating with the ITER engineering teams, as they familiarize themselves with the overall construction work packages and the engineering input required for the optimization of work execution.

A machining workshop is planned on site for the customization of splice plates—material used to ease the assembly of two consecutive sectors. The company will also dispose of space to install mockups from its facility in Santander, Spain, that had been used during the tool qualification process and that will now serve for the training of operators.