A research group from the National Institute for Fusion Science and Toho Kinzoku Co., Ltd. has confirmed no damage in test pieces of in‐reactor components used in fusion reactors, called divertors, which are exposed to high heat loads, even after more than 8,000 high‐temperature plasma exposures over a period of about three months. The technology for bonding dissimilar metals that has been developed so far has also shown high reliability, and further research into practical applications is expected to progress. The group presented their findings at the Annual Meeting of the Atomic Energy Society of Japan held in spring 2023.
In the not‐too‐distant future, fusion reactors are anticipated to become a source of clean energy in a sustainable society. The research group is jointly promoting the development of a divertor, one of the key components of a fusion reactor. Divertors must have both high thermal resistance and heat removal performance. It is essential to establish a technology to bond tungsten, a metal with a high melting point, and copper alloys with high thermal conductivity properties. Powder solid bonding has been developed as a new dissimilar metal bonding technology to replace conventional methods such as brazing, with the aim of applying it to divertors and industrial equipment.
The divertor under development applies a heat load by irradiating the tungsten surface with an electron beam, and while its soundness has already been confirmed under a high heat load environment of 20 MW/m², it was not possible to confirm its soundness in harsh environments close to nuclear fusion reactor conditions, such as repeated exposure to high‐temperature plasma that contains ions as well as electrons.
The research group has now installed the divertor test piece under development in the Large Helical Device (LHD) at the National Institute for Fusion Science and subjected it to repeated exposure to the high‐temperature plasma produced more than 8,000 times during an about‐three‐month plasma experiment that began at the end of September last year.
During the experiment, the researchers measured various values, such as the temperature of the divertor, the type and amount of impurities emitted from the surface, the temperature and density of the plasma and the vacuum, but observed no anomalies in the equipment itself or adverse effects on the plasma.
Furthermore, at the end of February, after the experiment was completed, the divertor prototype was examined in the LHD vacuum vessel. Although tungsten surfaces exposed to high thermal loads often show cracks and abnormalities on bonded surfaces, visual observation of the test piece did not reveal any cracks or abnormalities, confirming the soundness of the entire device. Moving forward, the research group plans to conduct microscopic analysis with electron microscopes and other equipment.
The fact that a divertor was manufactured using the newly developed dissimilar metal bonding technology and successfully tested in a high‐temperature plasma irradiation environment confirms the high reliability of this technology. In the future, the group will conduct experiments in large plasma experimental units in Japan and abroad, with the aim of implementing the technology in future fusion reactors. This spin‐off technology from nuclear fusion research on high‐temperature plasma and high heat flux is also highly promising for applications and expansion in industrial equipment where high heat is handled.
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