Michael Amdi Madsen
On 21 December 2021, in a facility nine miles south of Oxford’s city centre, the Joint European Torus (JET) fusion device fired up. Using powerful magnets to confine and fuse hydrogen, the tokamak reactor produced a record-shattering 59-megajoule fusion energy pulse over a period of five seconds. The almost forty-year-old JET was able to make the historic breakthrough, thanks in part to a retrofit that replaced its carbon inner walls with metals — a change with important implications for the future of fusion energy development.
A new IAEA coordinated research project is helping better understand this change and investigate the materials used in future fusion reactors. Gathering international experts from 15 countries, the IAEA and its partners are using a neutron source to recreate the conditions faced by a fusion reactor’s inner wall and determine how hydrogen permeates the metallic components within it. The project’s results could hold important answers to questions regarding the cost, efficiency and waste produced from large fusion experiments and reactors, such as ITER and DEMO as well as future fusion power plants.
“One of the most challenging aspects of a fusion reaction is dealing with the damage caused by released neutrons,” said Kalle Heinola, an IAEA Atomic Physicist coordinating the project. “Performing materials research utilizing neutrons typically requires an expensive neutron source and involves special requirements for radiation safety. Therefore, physicists and engineers have traditionally experimented with materials by bombarding them with ions or through computer simulations. However, these alternatives have drawbacks or limitations and may not accurately represent the damage neutrons can cause. Our project addresses that.”
Pickpockets of power
Harnessing fusion energy relies on tightly confining atoms at extremely high temperatures (over 150 million degrees Celsius) so that they fuse together. The process releases a huge amount of energy in the form of heat, as well as neutrons. Fusion machines like JET and the one still under construction, ITER, rely on magnets to confine a fuel of ionized deuterium and tritium — two types of hydrogen isotopes — but cannot constrain neutrons, which have no charge.
“The neutrons eject out of the fusion reaction with high energy. This is really important as engineers hope to utilize them to create more fusion fuel through a ‘tritium breeding blanket’, which is a lithium-based layer in the reactor. But on their way to the blanket, the neutrons can damage, or pockmark, the reactor’s inner walls at the atomic level,” Heinola explained. The pockmarks are troublesome in that they not only affect the integrity of the inner wall and thus the performance of the reactor’s super-hot plasma, but they create microscopic cavities into which the deuterium and tritium fuel can permeate and remain for long periods of time.
This is problematic because tritium is very rare, so valuable fuel is essentially being lost to the walls, and is radioactive, making the wall radioactive too. Reducing hydrogen retention will help reduce radioactive waste, fuel costs, and the need for maintenance.
Neutrons produced from the fusion reactions create atomic cavities in the reactor’s wall materials, in which tritium can get trapped or permeate through if not recycled back to the plasma. (Image: K. Heinola /IAEA)
Testing materials of the future
Dmitry Terentyev is a Fusion R&D Programme Manager at the Belgian Nuclear Research Centre known as SCK CEN. He and his team are receiving materials through the IAEA project and blasting them at their neutron source facility in Mol. In February, they received their first batch.
“We are using the fission research reactor ‘BR2’ as a neutron source to irradiate fusion reactor component materials,” said Terentyev. “The whole process mimics the effects of the ejected neutrons from a fusion reaction, and we can adjust our experiment to replicate the various levels of intensity of neutron bombardment. Though the neutron energy from a fission reaction is lower than from a fusion one, we can still perform invaluable research on the effects induced by fast neutrons.” Over the next 6 months, SCK CEN will test various types of vessel armor materials (tungsten, tungsten-based smart alloys, molybdenum, iron and steels) together with joining and pipe materials (copper and copper-chromium-zirconium).
The core of the BR2 reactor lies in a pool of water 6 metres deep. Material for irradiation is installed directly inside the fuel element, where intense Cherenkov light emission can be seen. (Photo: D. Terentyev / SCK CEN)
The way neutrons collide with a metallic material is akin to an extremely heavy ping pong ball racing down a bowling alley, hitting, shifting and knocking down pins, and sometimes making a full strike – at the atomic level, the neutrons will do the same in the plasma chamber wall: creating spaces by knocking atoms out of their positions in small or very large amounts, and sometimes even transforming atoms into other chemical elements through neutron absorption. These processes cannot be experimented with by any other means than with a neutron source. “The neutron irradiation will give researchers unparalleled insight on how neutrons create intrinsic defects in materials and help them to uncover the driving mechanisms behind hydrogen permeation in different metals,” Terentyev said.
One particularly interesting material the project will test is EUROFER97, a new generation steel that is planned to be used in the structure of future DEMO fusion projects and in ITER. The steel was provided to the project by Fusion for Energy — the European Union’s body involved with ITER — and prepared for neutron irradiation by the United Kingdom Atomic Energy Authority (UKAEA).
“The EUROFER97 we will test is the latest batch developed for extremely demanding fusion energy projects, like ITER, and this batch has never been studied for hydrogen permeation in combination of neutron damage in this way,” Heinola said.
This part of the coordinated research project is just the latest IAEA activity in supporting the development of fusion energy. The IAEA regularly holds technical meetings and coordinates research activities on fusion science and technology development and deployment. The Agency also maintains numerical databases of fundamental data for fusion energy research. The IAEA is exploring synergies in technology development between nuclear fission and fusion for energy production, and on the long term sustainability and legal and institutional issues for fusion facilities.
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