How will engineers maintain the components of the ITER Divertor?

One could try describing the ITER Divertor as a massive ashtray, consisting of 54 heavy metallic components known as “cassettes”, where the impurities of the fusion reaction will be driven to. These cassettes will take most of the heat from the super-hot plasma which will reach 150 million °C and during the lifetime of the ITER machine will need to be maintained and replaced. Europe will supply 104 of them because it is envisaged to replace the entire set with another one during ITER’s operation. How can one fix these bulky components without being physically present in the chamber of the machine? The answer lies in remote handling— the technical area that combines sophisticated human-in-the-loop robotics and tooling that will operate with extreme dexterity and milimetric accuracy.

F4E has signed a contract with Assystem UK for the remote handling system of the ITER Divertor, which among other parties includes RACE (Remote Applications in Challenging Environments), the new centre of the UKAEA for robotics development. The laboratory builds on the know-how it has acquired in the field of remote handling used in JET (Joint European Torus) and is currently extending its expertise through its contribution to ITER.

For more than a year, a team of experts has been working to identify key technologies to perform the cutting and welding operations required for the pipes of the cassettes. For experts working in the field of remote handling for the ITER Divertor, this is one of the most complex tasks to be performed. Indeed, in order to cope with the high temperatures that the cassettes will be exposed to, each of them will contain pipes which will be connected to ITER’s piping system where cooling water will be circulating through them. The piping system of the ITER Divertor will go through the vacuum vessel. Cutting the pipes, when we will have to replace the cassettes, will be no easy task because no lubricants will be applied due to the fact that no liquids can be used in the vacuum vessel. Therefore, dry cutting techniques will be deployed to perform the delicate procedures.

Welding together 3 mm thick stainless steel pipes, will also prove to be a challenging task because they will need to be carefully aligned, bearing in mind that they will have to operate in vacuum. Before operation, the resulting welds will need to be tested remotely through nondestructive examination (NDE) which is a complex task to perform in remote conditions. To make matters a bit more complicated, a number of factors will need to be taken into account such as space, which is limited in ITER forcing engineers to find the most compact solution; the levels of radiation in the vessel and the suitability of tooling for remote deployment.

How did RACE tackle this mission impossible? First, a technology survey was launched identifying the technics available to carry out these operations and were subsequently narrowed down to those complying with the ITER environment. After having analysed possible alternatives, and conducting risk assessments, RACE identified the best options and successfully performed various trials of welding and cutting. We visited the laboratory when these tests took place and we were able to observe step-by-step how they were carried out. Although trials have been performed with the intervention of technicians, so as to identify the best technologies, in ITER this type of repair work will be performed remotely. Therefore, the quest for the right tooling is now on. When the design work is completed the teams will move ahead with the fabrication of tooling expected to start in 2020.