More than a kilometre of gas injection lines will be required for the boronisation system being developed for the International Thermonuclear Experimental Reactor (ITER), under construction in France. When the ITER Organisation decided in 2023 to change the armour tiles of the plasma chamber from beryllium to tungsten, it meant adding a new wall conditioning system to buffer the plasma from the associated increase in impurities.
The new system, called boronisation, will apply a thin layer (~10-100 nanometers) of boron over all plasma-facing surfaces to capture, or “getter,” oxygen that could otherwise increase radiative losses of the plasma, particularly during the sensitive discharge-initiation phase.
ITER engineers and scientists are working on various aspects of the project – including designing a gas injection system to deliver the carrier gas to different points in the vessel. Plans for the system are advancing quickly, with initial modelling and preliminary design reviews near completion and a long-term strategy defined.
“It’s been a good challenge to integrate boronisation into the existing wall conditioning system,” said Tom Keenan, the ITER wall conditioning engineer who is the technical responsible officer for the system. “We’re working with a proven technology, but it’s never been done on this scale or in a tritiated environment, so it’s interesting territory.”
When the design for the new wall conditioning system began, one of the first decisions made was to use diborane, a compound of hydrogen and boron. There will be a 5% concentration of diborane in a carrier gas, with helium being the preferred option. Because diborane is both toxic and explosive, a highly secure storage site called a “gas cabin” must be built outside of the Diagnostics Building.
In addition, 21 gas injection points will be added to the inside of the vacuum vessel for the boronisation system. The design team has developed an optimised model for the placement and size of the injection points given the available space.
Another requirement is a gas injection system that will bring the diborane into the tokamak. The preliminary design calls for more than one kilometre of injection lines inside the Tokamak Building, another 400 metres of lines in the vessel, and 21 gas injection points to be integrated into the vacuum vessel. An optimised distribution of in-vessel gas supply locations has been achieved that is consistent with the performance benefits identified by the modelling activities and which respects the constraints of the existing available vacuum vessel gas feedthroughs. Gabor Kiss, a fuelling process integration engineer at ITER who is working on the integration study, said the adaptations are not expected to impact plant installation sequences.
Once injected into the machine, diborane decomposes and deposits on the plasma-facing walls. ITER already plans to use glow discharge cleaning for wall conditioning during maintenance periods, but adapting it for more frequent use and for boronisation raises challenges. The first issue is whether ITER’s anode design,which delivers about ten times more energy than those in current tokamaks,is compatible with frequent boronisation cycles. Upcoming tests at the EAST tokamak in China are expected to answer this.
The second question involves determining the optimal placement of anodes to ensure even boron coverage on ITER’s plasma-facing surfaces. Through modelling and collaborative testing with the ASDEX Upgrade (Germany) and WEST (France) tokamaks, the team decided to add four additional anodes to the vacuum vessel to obtain the most effective boron distribution.
“This has been a major joint effort with experts from the International Tokamak Physics Activity,” said Tom Wauters, who specialises in plasma-wall interactions at ITER. “We’ve had very good support from the international science and engineering community that has helped us move forward with ITER’s boronisation system.”
Determining how often boronisation should be performed in ITER depends on how much oxygen the boron layer can absorb and how quickly the plasma erodes these layers. Recent studies suggest that a single boron application could be effective for anywhere between 2.5 and 12.5 weeks of campaign time, leading to a planned maximum boronisation interval of every two weeks. Upcoming dedicated tests on operational tokamaks will also accurately measure how much oxygen can be captured by the boronised surfaces.
In a glow discharge boronisation procedure, a low percentage of diborane will remain non-decomposed. The diborane that is pumped out of the tokamak must be safely neutralised due to its high toxicity. Two destruction methods are currently being evaluated: one involves heating the diborane to 700 °C for thermal breakdown, while the other uses a proprietary chemical trap, used in the semiconductor industry, to absorb and neutralise the gas.
“We are very confident in both systems,” noted Peter Speller, the process engineer overseeing diborane treatment. “Thermal destruction has already worked at WEST and DIII-D (USA) tokamaks, while the chemical trapping system has been successful at ASDEX Upgrade, so we just need to determine which is best suited to ITER.”
Whatever method is chosen, space is being made in the Tritium Building for the diborane removal system, which will treat the exhaust gases from the tokamak during boronisation. Installation of the boronisation system is expected to begin in 2028.
