To develop a practical fusion power system, scientists need to fully understand how the plasma fuel interacts with its surroundings. The plasma is superheated, which means some of the atoms involved can strike the wall of the fusion vessel and become embedded. To keep the system working efficiently, it's important to know how much fuel might be trapped.
"The less fuel is trapped in the wall, the less radioactive material builds up," said Shota Abe , a staff research physicist at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL).
Abe is the lead researcher on a new study published in Nuclear Materials and Energy. The study looks specifically at how much deuterium – thought to be one of the best fuels for fusion – might get stuck in the boron-coated, graphite walls of a doughnut-shaped fusion vessel known as a tokamak . Boron is used in some experimental fusion systems to reduce plasma impurities. However, researchers do not fully understand how a boron coating might impact the amount of fusion fuel that leaves the plasma and becomes embedded in the vessel walls.
"Understanding how boron coatings can interact with deuterium can help us improve materials for future fusion power plants, such as ITER ," said Abe. ITER is the multinational facility under assembly in France, which will study plasma that can heat itself and sustain its own fusion reactions.
In addition to researchers from PPPL, a sizable team of experts from institutions across the country contributed to the new study on fuel retention, including researchers from Princeton University, the University of California-San Diego , General Atomics , the University of Tennessee and Sandia National Laboratories . Their world-leading work is critically important to making fusion a viable source of electricity on a commercial scale.
Deuterium also stands in for tritium in experiments
In a commercial fusion system, the fuel will likely be made of deuterium and tritium, which are both forms of hydrogen. Tritium is radioactive, but deuterium is not. So, the experiments used deuterium as a stand-in for tritium, as they are similar in many respects. But tritium is an element that must be carefully managed in commercial-scale fusion systems.
"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 also 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."
Interestingly, the researchers say the main cause of the trapped fuel isn't the boron coating. It's carbon. Even small amounts of carbon increased the amount of deuterium fuel trapped in the samples during the experiment. These boron film samples were created using a plasma made of a gas containing boron and deuterium (as well as with some impurities) in DIII-D, a tokamak at General Atomics. The carbon and the boron together can bind so tightly to deuterium that it would take temperatures around 1000°F to break the bond, making it very challenging to remove the fuel without damaging the fusion system.
"The carbon is really the troublemaker," said PPPL Staff Research Physicist Florian Effenberg , who is also 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."
In fact, exposure to a plasma with small amounts of carbon contamination increased the amount of deuterium significantly. The researchers found that for every five units of boron trapped in a sample, two units of deuterium were trapped.
Graphite tiles to be swapped out
The DIII-D fusion system was used in the experiments and currently has walls made from graphite, a form of carbon. "We want to get rid of all the carbon and have clean tungsten walls," said Effenberg, to ensure the calculations are even closer to what will be experienced in ITER.
One of the strengths of the research is that some of the samples were exposed to plasma in the DIII-D fusion vessel. The machine is one of several experimental tokamaks that operate using magnetic fields to hold plasma in a doughnut shape. Given that the research suggests that even trace amounts of carbon can drastically increase the amount of tritium stuck in the walls of a tokamak, the results could have important implications for meeting regulatory limits in future fusion power plants.
Other researchers on the project include Michael Simmonds, Igor Bykov, Jun Ren, Dmitry L. Rudakov, Ryan Hood, Alan Hyatt, Zihan Lin and Tyler Abrams. This research was supported by the DOE's Office of Fusion Energy Sciences under awards DE-FC02-04ER54698, DE-AC02-09CH11466, DE-SC0022528, DE-SC0022528 and DE-SC0023378.
