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Construction

The cryostat is a vacuum-tight container that will completely surround the machine and provide an ultra-cool vacuum environment for the vacuum vessel and superconducting magnets. (Click to view larger version...)
The cryostat is a vacuum-tight container that will completely surround the machine and provide an ultra-cool vacuum environment for the vacuum vessel and superconducting magnets.
In a 5,000-square-metre workshop on site, the Indian Domestic Agency is assembling the cryostat—a huge vacuum containment vessel that is also the single largest component of the ITER machine.

Completely surrounding the vacuum vessel and superconducting magnets, the 29 x 29 metre cryostat has two important roles to play—providing a vacuum environment to critical "cold" components (the magnets operating at 4.5 K and thermal shield operating at 80 K), and contributing structural reinforcement by supporting the mass of the machine and transferring horizontal and rotational forces to the radial walls.

The cryostat is a fully welded single wall stainless steel structure with a flat bottom, a rounded lid and wall thicknesses that range from 25 to 200 millimetres. A number of large openings provide access to vacuum vessel ports at three levels; others allow access for coolant pipework, cryo and current feedlines, and remote handling. Advanced welding techniques such as automated, all-position narrow groove gas tungsten arc welding have been specially developed for the fabrication of this challenging component.

Manufacturing is taking place in three stages: the fabrication of 54 segments in India; their subsequent assembly at ITER into four large sections (base, lower cylinder, upper cylinder, top lid); and the final assembly and welding of the large sections in the Tokamak Pit.

From the air, three of the cryostat sections are visible—two cylinders cocooned and stored at right, and the cryostat base as it enters the Cleaning Facility/Assembly Hall. Photo: ITER Organization/EJF Riche (April 2020) (Click to view larger version...)
From the air, three of the cryostat sections are visible—two cylinders cocooned and stored at right, and the cryostat base as it enters the Cleaning Facility/Assembly Hall. Photo: ITER Organization/EJF Riche (April 2020)
Cryostat segments fabricated in India are shipped according to need dates to the ITER site and stored in the Cryostat Workshop.

Beginning with the cryostat base—the first cryostat section needed in the Tokamak assembly sequence—and ending with the cryostat lid, the sections are assembled and welded on large assembly frames. These frames act both as support platforms during the welding activities and as support fixtures that interface with the transport vehicles when the time comes to move the completed components out of the workshop.

Using optical metrology techniques and strict dimensional control, operators carefully align the segments to be welded on the assembly frames. A small team of highly specialized technicians—working singly or in teams (one above, one below)—fill the gaps between each segment with weld material. Given the importance of high vacuum in the cryostat, each weld is verified through a variety of leak detection techniques.

In helium leak detection, one-metre sections of the weld to be verified are "enclosed" within leak-tight boxes positioned on opposite surfaces. Helium injected on one side of the weld can be detected—if it has filtered through a crack—by a mass spectrometer on the other side, thereby signalling a leak that must be repaired by grinding out the faulty weld and replacing it.

Three other quality assurance techniques will be used: radiographic and ultrasonic testing to detect the presence of flaws that could challenge the structural integrity of the welds, and liquid penetrant testing (LPT) for surface checks.

In total, the Indian Domestic Agency estimates that one kilometre of full penetration weld joints will have to be carried out to exacting standards for the sub-assemblies in the site workshop, followed by several hundred metres of weld joints to assemble the cryostat sections in the Tokamak Pit.

It took approximately three years (2016 to 2019) to finalize the on-site assembly and welding operations for the cryostat base—a 1,250-tonne component formed from a tier 1 "disk" and a tier 2 vertical ring and pedestal.

On an adjacent assembly platform, the less-complex lower cylinder (490 tonnes) was assembled in two years (2017 to 2019) and removed to storage on the platform to make room for the assembly of the upper cylinder, which was completed in March 2020. The steel segments required for top lid assembly are expected on site in mid-2020.

In Mayl 2020, the 1,250-tonne cryostat base was transferred to the Assembly Hall, lifted by overhead crane, and inserted into the bottom of the Tokamak assembly pit. This was the first major act of the ITER machine assembly phase. (See more information on assembly here.)

Cryostat Assembly Facts:

  • Number of segments fabricated in India: 54
  • Sections assembled on site: cryostat base, lower cylinder, upper cylinder, top lid
  • Length of weld joints for in-workshop assembly (four sections): ~ 1,015 m
  • Length of weld joints for in-pit assembly: ~ 390 m
  • Diameter of sections: 30 metres (approximate)
  • Weight of each section: base: 1,250 tonnes; lower cylinder: 375 tonnes; upper cylinder: 430 tonnes; lid: 665 tonnes
  • Start of welding activities: 2016
  • First section completed (lower cylinder): March 2019
  • Cryostat base installed in pit: Q2 2020
  • Procurement responsibility: India
  • Contractors: Larsen & Toubro Heavy Engineering Division, India (manufacturing design, fabrication and assembly); MAN Energy Solutions, Germany (Larsen & Toubro sub-contractor for on-site welding);  SPIE Batignolles TPCI, France (Larsen & Toubro sub-contractor for the realization of the Cryostat Workshop)