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In magnetic-confinement fusion reactors, the plasma is heated to temperatures of more than 100 million degrees Celsius (°C) to enable fusion of the deuterium and tritium fuel. The main challenges for fusion power science are finding materials that can resist extreme heat radiating from the plasma as well as the effects of irradiation from the neutron bombardment. Other important challenges include identifying those materials that are able to provide safe, reliable and predictable performance, and a long service life at elevated temperatures, with minimum radioactivity in end-of-life components for simplified recycling or disposal. Studying the effects of irradiation in fusion devices presents a particularly difficult challenge, as fully suitable test beds are not available. Materials research is a strategic priority in many IEA member countries. Given the potential of fusion power and the applications to other high-potential energy options, continued efforts in this area will be needed.
The focus of co-ordinated research under the Implementing Agreement for a Programme of Research and Development on Radiation Damage in Fusion Materials (FM IA) is to develop materials for the first wall and blanket of a power plant that will operate under high temperatures and survive the high flux of neutrons and charged particles produced in the plasma chamber. This work also includes developing protocols for measuring material production processes, joining methods and design properties. Research underway includes materials irradiation in fission reactors, ion beams, and computational simulation. The theories are paired with experiments and modelling efforts to represent true fusion conditions and lifetimes. There are nine Contracting Parties, including China, India and Russia.
A unique aspect of the fusion reaction is the substantial production of gases that affect the mechanical and physical properties of the materials.
One such gas, helium, is produced in significant quantities. As helium gas is not soluble, it forms ‘bubbles’ which accumulate to form voids in the materials, affecting its integrity and structure. It is essential to quantify these effects to develop safe and reliable fusion systems. As a result, helium bubbles are a topic of much research worldwide, and a focus of the FM IA. Currently it is difficult to explore the effects of helium under prototypical conditions due to a lack of appropriate neutron irradiation sources.
The large number of small bubbles and the few, large voids are significant. Growth in the number and size of the voids could cause premature failure of the steel.
Novel experimental techniques combined with multi-scale modelling are being used in the activities of the FM IA to reveal the micro-level effects of helium on typical structural materials such as the reduced-activation ferritic or martensitic (RAFM) and advanced oxide dispersion strengthened (ODS) steels.
One strategy for managing high levels of helium is to provide many places for bubbles to form so that they do not transform into voids. This is one reason for the resurgence of interest in ODS steels. The high number of small oxide particles in ODS steels provides many places to ‘trap’ the helium, increasing resistance to unexpected structural failure.
* Photo courtesy of Battelle Memorial Institute.
For more information: www.frascati.enea.it/ifmif
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