TVA: From high pressure (vacuum) to nuclear fusion

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Fusion energy is created when two atomic nuclei fuse together. According to Albert Einstein’s famous formula E = mc2, the difference in mass between the two original nuclei and the resulting fusion nucleus is released as energy. This involves unimaginable amounts of energy: converting one gram of deuterium-tritium mixture into helium would produce thermal energy of around 100 megawatt hours. In comparison, to generate this amount of energy from coal, it would be necessary to burn more than 12 tons of coal! Even better, the amount of hydrogen required is practically inexhaustible here on Earth, and the residues from nuclear fusion are chemically completely harmless, unlike the environmentally harmful greenhouse gases produced when burning fossil fuels. It’s no wonder, then, that researchers have dreamed of tapping into this miraculous source of energy for over a century.

In Cadarache, a small village in the south of France, this dream seems within reach. It is home to the ITER project, a research collaboration of 35 industrialized countries with the ambitious goal of producing energy through controlled nuclear fusion. To this end, an experimental tokamak fusion reactor is currently being built to test the possibility of using fusion energy permanently from 2025. In the reactor, nuclear fusion is generated using a deuterium-tritium plasma focused by extremely powerful then merged magnetic fields. It sounds simple enough, but in practice it’s incredibly complicated. For example, a temperature of 150 million degrees Celsius must prevail in the plasma (10 times hotter than the core of the sun) for nuclear fusion to begin. The reactor must also be hermetically sealed under high vacuum conditions.

This is where the Swiss company VAT comes in, valve specialist, world leader in high-performance vacuum valves and exclusive development partner of ITER for several years. Highly specialized valves have been developed for the ITER project to withstand the extreme temperature and radiation conditions around the tokamak reactor. This allows ITER developers and their external partners to easily select all the valves required for the development of a specific module from the catalog specified by ITER – safe in the knowledge that there is full compatibility and the most possible high level of security.

Most ITER valves are all-metal valves that use special metal-to-metal (VATRING) seals instead of elastomers. In their pneumatic valve actuators, special O-rings, which are more resistant to radiation than conventional elastomer seals, are also used. VAT looks forward to seeing how the valves will prove themselves in the long term under such extreme conditions. “For the VAT team, the ITER developments are of particular importance because we have to test the limits of what is technically feasible here”, explains Phil Schneider, product manager in charge of all-metal valves at VAT. Already, the ITER cooperation has provided Swiss VAT valve professionals with countless valuable insights into valve development.

In the ITER reactor, two very powerful neutral beam injectors are used to heat the plasma to fusion temperatures. A third neutral beam injector is used to diagnose the plasma. Each injector forms a vacuum chamber which must be able to be ventilated independently of the tokamak in the event of a breakdown (eg fire, earthquake, pressure build-up or coolant leak). In the search for a valve solution suitable for these purposes, attention quickly turned to the innovative VATRING technology – a sealing technology developed by VAT for all-metal valves for repeated hermetic closing under UHV conditions. . Initially, there were doubts about the possibility of adapting this technology to the requirements of ITER, but this naturally raised the ambition of the TVA developers: as part of a feasibility study, they developed the Absolute Valve DN1600, which not only meets all the requirements of ITER specifications but, with an opening diameter of 1.6 meters, it is also the largest all-metal valve ever developed!

Stainless steel seals with silver coatings are installed in the valve, ensuring high vacuum tightness up to a pressure gradient of 0.2 MPa / 2 bar on the valve disc, with a simultaneous leak rate less than 10-7 mbar litre/s. The test prototype of this magnificent valve seal is now in the spotlight on a hill near the ITER construction site and is a popular point of interest for all visitors to the site.

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