The
first welding activities began in September 2016 on the cryostat base (tier 1). Today, on the perfectly polished circular surface of the lower base section, welders have given way to specialists in leak detection.
Their role is crucial: once assembled, the 8,500 m³ vacuum chamber must be absolutely leak tight. The challenge is considerable and begins with the quality of the welds. Every millimetre of vacuum-facing weld joints must be leak-tested before commissioning.
In tier 1 of the cryostat base alone, over 100 metres of weld must be tested. The process—although simple in its principle—is meticulous to implement.
If there's a crack in a weld crossing the vacuum boundary, no matter how small, it's a problem. And to detect it, something equally as small must be employed. "We use helium to detect cracks, which is as safe as it is inert¹ and whose atoms are among the smallest² of the periodic table," explains Liam Worth of the ITER Vacuum Section. "Helium atoms also have a have low viscosity—they are 'slippery.'"
Procedures for leak testing of the cryostat welds were prepared by Larsen & Toubro Ltd, the Indian company that manufactures the component; they were submitted to and validated by ITER-India and the ITER Organization and are being implemented by Larsen & Toubro's German contractor MAN.
Acting as "witnesses," representatives of ITER India and Larsen & Toubro are present at every stage of the process.