New research reveals a hidden carbon culprit that could complicate tritium management in tomorrow’s clean energy plants
The gleaming, iridescent walls inside the DIII-D fusion vessel might look beautiful, but they’re concealing a potential roadblock on the path to practical fusion energy. Scientists have discovered that these walls are quietly stockpiling precious fusion fuel during experiments, with unexpected consequences for future power plants.
In a study published this month in Nuclear Materials and Energy, researchers from the Princeton Plasma Physics Laboratory (PPPL) and partner institutions identified a surprising mechanism that traps deuterium—a crucial fusion fuel—within the protective coatings used in today’s experimental fusion devices.
“The less fuel is trapped in the wall, the less radioactive material builds up,” explained Shota Abe, a staff research physicist at PPPL and lead author of the study. This seemingly simple issue could have profound implications for tomorrow’s commercial fusion plants.
A High-Stakes Accounting Problem
Future commercial fusion plants will likely run on a combination of deuterium and tritium. While deuterium is stable and abundant, tritium is radioactive and subject to strict regulatory limits. During operation, some of this fuel inevitably escapes the plasma and embeds itself in the vessel walls—creating what amounts to a radioactive accounting challenge.
“There are very strict limitations on how much tritium can be in a device at any given time. If you go above that, then everything stops, and the license is removed,” said Alessandro Bortolon, a managing principal research physicist at PPPL who contributed to the work. “So, if you want to have a functioning reactor, you need to make sure that your accounting of tritium is accurate. If you go over the limit, that’s a showstopper.”
The researchers examined samples with boron coatings—materials frequently used in fusion experiments to reduce plasma impurities. These samples were created at the DIII-D tokamak operated by General Atomics, then exposed to different plasma conditions to measure how much fuel they retained.
The Carbon Connection
The results pointed to an unexpected culprit: carbon. Even small amounts of carbon significantly increased fuel retention, forming bonds so strong that temperatures around 1000°F would be needed to break them.
“The carbon is really the troublemaker,” said PPPL Staff Research Physicist Florian Effenberg, a co-author of the paper. “Carbon must be minimized. While we cannot get it to zero, we use all the means we have to reduce the amount of carbon as much as possible.”
The team found that for every five boron atoms in the samples, two deuterium atoms became trapped—a ratio with potentially significant implications for tritium management in future facilities. This retention increased when samples were exposed to plasma containing even trace amounts of carbon contamination.
From Graphite to Tungsten
The DIII-D fusion system, which was used in these experiments, currently has walls made from graphite—a form of carbon. The findings suggest that future fusion power plants might need different materials to minimize fuel trapping.
“We want to get rid of all the carbon and have clean tungsten walls,” noted Effenberg, referring to changes that would bring experimental conditions closer to what will be used in ITER—the international fusion project under construction in France.
The study represents a collaboration between researchers from multiple institutions, including PPPL, Princeton University, the University of California-San Diego, General Atomics, the University of Tennessee, and Sandia National Laboratories.
By measuring precisely how much fuel gets trapped in fusion vessel walls under various conditions, the research helps establish crucial safety parameters for future commercial fusion power. Understanding these interactions will be essential for developing systems that can efficiently manage the tritium fuel cycle while maintaining regulatory compliance.
For fusion to succeed as a practical energy source, scientists need to account for every atom of tritium—and this research shows that the materials lining fusion vessels may be holding more than previously thought.
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