Like a fire-breathing dragon
With the precision of a surgical tool, the plasma jet cuts through the thickness of the steel plate. As sparks fly, the large circular opening at the centre of the cryostat lid is bathed in pinkish-mauve light—the colour of the ultra hot (30 000 °C) hydrogen-nitrogen-argon plasma. Towering above the operator, the computer-controlled 10-tonne robot slowly moves its cutting head up one of the component's ribs, slicing through steel like a thread through butter. One by one, the ribs are machined in order to align perfectly with the edge of the opening.
The plasma machining underway on the central part of the top lid, where a central cylinder will soon be fitted and welded, is the most spectacular of the ongoing works in the Cryostat Workshop. While the dragon-like robot breathes its fire through the steel, a dozen welders from contractor MAN Energy Solutions are busy finalizing the radial joints between the component's segments, and an operator from the Indian manufacturer Larsen & Toubro Ltd, under the constant supervision of a quality inspector, performs ultrasonic tests on the completed welds.
The large circular opening at the centre of the cryostat lid is bathed in the pinkish-mauve light of the plasma-cutting robot. The machine is seen here slicing through the steel of one of the vertical ribs that strenghten the lid's structure.
"At this point, approximately 50 percent of the welding is done and 4 out of 12 radial joints have been checked by ultrasound," explains Anil Bhardwaj, ITER Cryostat Advisor. The operation is long and delicate: every radial joint must be leak-tested on both surfaces (top and underside) of the steel plate and the process takes about two days. After all welds have passed this first round of testing, helium leak tests, scheduled in mid-October, will demonstrate that the top lid, like the three other sections of the 8,500 m³ cryostat, is leak-tight as can be.
Precisely aligned, the nozzle of the robot's cutting head delivers a super heated jet of hydrogen-nitrogen-argon plasma (30,000 °C) that slices through steel like a thread through butter.
At 665 tonnes, the cryostat top lid is the second heaviest single component of tokamak assembly. "With the base, it is also the most critical part of the cryostat," adds Anil. "Its shape is particularly complex, the amount of welding is considerable and access is not always easy."
Every radial joint must be leak tested on both surfaces of the steel plate (top and underside), a process that takes about two days. 50% of the welding is now finalized.
Like a roof on a house, the installation of the cryostat top lid will mark the completion of core machine assembly.
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