Vacuum technology
Record-breaking sealing performance
9 Sep 2019
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Robert Pearce, Vacuum Section Leader
The ITER vacuum vessel, its ports and port extensions, and port plugs all provide the vacuum boundary and first safety confinement barrier of the ITER machine. In this context, the leak tightness of the large metallic vacuum seals around the ports is critical. In order to leave nothing to chance, the ITER Organization is testing vacuum sealing on a real-size test rig. Recent test results were excellent.
Celebrating the world-record-breaking performance were ITER team members David Laugier, Robert Pearce and Eamonn Quinn, and Adrien Bommenel, Gines Murcia, and Frédéric Sauvinet from Technetics.
The high-performance vacuum sealing of the ITER vacuum vessel's 50+ large ports is a "first of a kind" challenge due to the rectangular shape of the ports, their large size, and the need to remove and replace port plugs (large, stainless-steel components that "plug" the openings and also play a role as structural host to systems such as diagnostics).
Eamonn Quinn is responsible officer for the Large Seal Test Rig (LSTR) program. "To leave nothing to chance, a full-size replica of the largest equatorial ports was designed, manufactured and installed on site with the purpose of testing the vacuum sealing of what will be the largest ports ever built on a tokamak."
The equipment was installed in the European Poloidal Field Coils Winding Facility on site, where the "clean" conditions were ideal for the tests, and space was made available courtesy of the European Domestic Agency.
The test rig is of an impressive size (nearly 5 metres in height and as many in length) and it weighs around 19 tonnes. It gives a preview of how the ITER machine will look when seen through the equatorial ports cells, as they are starting now to take shape on the construction site. It was built and installed by Indian high-tech company Vacuum Techniques.
There was much preparation, along with repeated inspections and some anxiety as the first large, double all-metallic seal was positioned on the flange. The flanges were brought together with the repeated tightening of 100 bolts, and after a number of hours of muscle flexing the seals were compressed.
The port flange is inspected prior to installing the seal.
Then came the leak tests: first a quick one—no leak found. Then the thorough one, during which the leak rate across each seal was <10-11 Pa.m3.s-1 —a world-record-breaking sealing performance for the largest non-circular all metal vacuum seal! (In lay terms, that means that no more than the volume of air contained in a glass would leak over a period of ... 100 million years.)
The performance on this first test was well beyond the requirements, beyond all expectation. In short, a world record breaker!
The metal seals were made by Technetics (US). Company Vice President Bob Panza had time to comment: "I heard the wonderful news yesterday. This is a result of good collaboration between our companies and we should all be proud of the results."
The seal test rig not only allows the largest demountable rectangular seals to be tested, but also enables us to prepare installation techniques which will be critical to achieving the required vacuum quality.
Since the first test, the validation program has successfully continued with heating the flanges to 100 °C and then on to 240 °C and still the sealing performance has been maintained.