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For the advancement of a functional fusion power system, it is crucial for scientists to comprehend the interactions between plasma fuel and its environment. The fuel, in the form of superheated plasma, can lead to certain atoms striking the walls of the fusion vessel and potentially becoming trapped. Understanding the extent of this fuel entrapment is essential to maintain system efficiency.
“Minimizing the amount of fuel trapped in the vessel’s walls reduces the accumulation of radioactive materials,” stated Shota Abe, a research physicist at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL).
Abe leads a recently published study in Nuclear Materials and Energy that examines how deuterium, regarded as an optimal fusion fuel, can become trapped within the boron-coated graphite walls of a tokamak, a doughnut-shaped fusion reactor. Boron is incorporated in some experimental fusion setups to mitigate plasma impurities, yet the full impact of boron coatings on the retention of fusion fuel remains unclear.
“Gaining insight into the interaction between boron coatings and deuterium could enhance our material choices for prospective fusion power plants like ITER,” Abe explained. ITER is a collaborative project currently under construction in France, aimed at producing self-heating plasma that can sustain its own fusion reactions.
The study also benefited from contributions by a diverse team of researchers from various U.S. institutions, including Princeton University and the University of California-San Diego, which underlines the collaborative efforts needed to make fusion a reliable source of commercial electricity.
Deuterium serves as a proxy for tritium in research
In potential fusion energy applications, the combination of deuterium and tritium is anticipated as the fuel source. While tritium is radioactive, deuterium is stable, allowing researchers to use it as a substitute in experimental scenarios due to their similar properties. However, the management of tritium is critical in large-scale fusion projects.
“There are stringent regulations governing the maximum amount of tritium permitted in any device,” cautioned Alessandro Bortolon, a principal research physicist at PPPL and co-contributor to the study. “Exceeding this limit can halt operations and result in license revocation. Accurate tracking of tritium levels is vital for the reactor’s functionality.”
Interestingly, the research revealed that the primary contributor to fuel entrapment is not the boron coating, but rather carbon. Even minimal amounts of carbon can significantly increase the quantity of deuterium trapped within the samples. The boron-coated samples were created using a plasma consisting of a mix including boron and deuterium, produced at the DIII-D tokamak operated by General Atomics. The chemical bond formed between carbon and boron with deuterium is so robust that temperatures reaching about 1000°F are required to release it, complicating the removal process without causing harm to the reactor.
“Carbon is the main issue we face,” remarked PPPL Staff Research Physicist Florian Effenberg, who co-authored the study. “Although we cannot eliminate carbon completely, we strive to reduce its presence as much as possible.”
In fact, the presence of carbon in the plasma substantially increased deuterium retention. Data indicated that for every five units of boron trapped, two units of deuterium were also captured.
Replacing graphite tiles
The DIII-D fusion system used in the study features walls made of graphite, which contains carbon. “Our goal is to eliminate carbon and replace it with clean tungsten walls,” Effenberg stated, emphasizing the need for more accurate predictive calculations relevant to ITER’s future operations.
This research’s strength lies in its real-world application, as some samples were tested directly within the DIII-D’s plasma environment. As this research underscores the significant impact that even trace levels of carbon can have on tritium retention, the findings hold substantial implications for adhering to regulatory standards in future fusion power plants.
Other contributors to this project include Michael Simmonds, Igor Bykov, Jun Ren, Dmitry L. Rudakov, Ryan Hood, Alan Hyatt, Zihan Lin, and Tyler Abrams. The research received support from the DOE’s Office of Fusion Energy Sciences through several awards.
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