Nuclear fusion reactors offer the hope of vast, clean energy from the same process that powers stars. But despite decades of research, a reactor that can supply practical amounts of power has proven elusive. Now startup Commonwealth Fusion Systems has revealed in depth what it says is the most complex aspect of the reactor it is constructing—the way the reactor controls the plasma responsible for generating power.
The company says their findings support their vision—a reactor that can generate 1.1 gigawatts of fusion power and deliver 400 megawatts of net electricity to the grid. “That can power about 280,000 average American homes for a year, all using an amount of fuel you could deliver in a pickup truck,” says Brandon Sorbom, co-founder and chief science officer of Commonwealth Fusion Systems (CFS) in Devens, Massachusetts.
The ARC (affordable, robust, compact) fusion reactor that CFS is developing is a tokamak. This is essentially a doughnut-shaped bottle that magnetically traps plasma at pressures and temperatures high enough to force atomic nuclei to fuse together. A fraction of the mass of these atoms gets converted into energy. “We’re basically creating a miniature star,” Sorbom says.
High-Temperature Superconductor Magnets
The key innovation of the ARC reactor is the use of high-temperature superconductor (HTSC) magnets instead of typical superconducting magnets, which require frigid temperatures near absolute zero to work. Although HTSCs still require temperatures in the range of about 20 to 77 K (–200 to –250 °C), the relative warmth in which they operate means they require dramatically less cooling equipment. This makes ARC significantly more compact and simple than previous fusion reactor designs, such as the International Thermonuclear Experimental Reactor (ITER).
The fusion reactions generate neutrons, the energy from which heats a continuously flowing loop of molten salt around the reactor’s magnetic bottle. This blanket of molten salt then heats a fluid to drive a turbine that generates electricity.
CFS researchers collaborated with scientists at MIT, Columbia University, the Max Planck Institute for Plasma Physics and other institutions around the world to describe the scientific underpinnings of the ARC reactor. They detailed their research in five peer-reviewed studies published today in the Journal of Plasma Physics.
“We demonstrate that the ARC power plant has a solid foundation in physics,” Sorbom says. “The papers confirm that when we build the ARC fusion power plant, it will work.”
Roughly two-thirds of the 58 authors of the studies come from outside CFS. “These papers are not just the stamp of our validation, but that of the global fusion science community,” Sorbom says. “And then they underwent peer review from more institutions for independent checks to make sure all our calculations were correct.”
Managing Plasma Disruptions in Tokamaks
The new studies detail how ARC will deal with a major challenge all fusion reactors face. Plasma disruptions occur when instabilities within the plasma flow lead it to spiral out of control and make contact with the reactor wall. These can not only inflict a great deal of damage—the plasma is 150 million degrees Celsius and carries 12 million amps of electrical current—but also extinguishes the plasma.
“Plasma physics is really hard,” Sorbom says. “It’s the most complicated part of the machine.”
In the new studies, the researchers describe methods for limiting the impacts of such disruptions, such as rapidly injecting massive amounts of gas into ARC as a cushion to keep the plasma from damaging the reactor. But they also have designed ARC to withstand one disruption per day and to restart the plasma within a minute without interrupting power output, Sorbom says.
“We designed ARC considering that even on the wrong side of all the uncertainties we still face, ARC will still work.” —Brandon Sorbom, Commonwealth Fusion Systems
“Even if the plasma is off, the molten salt doesn’t decrease dramatically in temperature immediately,” Sorbom says. The salt can therefore continue to supply heat for electricity generation until fusion restarts.
ARC will use deuterium and tritium, two hydrogen isotopes, as its fuel. Ultimately, ARC will breed more tritium for future use, as neutrons from the plasma striking the molten salt will transmute some of the lithium within the salt to the rare hydrogen isotope. The tritium can then serve as fuel for the reactor, or help seed other power plants, “enabling the rapid scaling of this technology,” Sorbom says.
ARC Fusion Reactor Lifetime and Maintenance
The projected lifetime of ARC is 25 to 30 years. Its longevity depends on how long the superconducting magnets can survive damage from neutrons escaping the salt blanket. If the researchers want a fusion plant with a longer life, “we can make it slightly larger to put in more shielding between the blanket and the magnets,” Sorbom says.
The new studies explain that the reactor’s plasma fuel is held within a vacuum vessel that erodes over time. “It lasts somewhere between one to two years before it has to be replaced,” Sorbom says.
CFS has designed the vacuum vessel to be swapped out as quickly as possible. The reactor can be opened up and the salt blanket drained away so the company can cut up an old vacuum vessel and place in a new one.
ARC will have to shut down during such times, but Sorbom notes other kinds of power plants often experience outages every few years for routine maintenance as well. The startup hopes ARC will have short maintenance cycles, “a couple of months at most,” he says. The company is now collaborating with a grid operator to plan around such maintenance.
Sorbom adds that between replacements, research and development could design better vacuum vessels. “Every time we replace it, we can upgrade it,” he says. “The first may last one year. The next year, two years. Then after that, 2.5 years.”
All in all, these new studies suggest ARC is going to work, Sorbom says. “We designed ARC considering that even on the wrong side of all the uncertainties we still face, ARC will still work.”
Currently the startup is building a smaller prototype of ARC called Sparc. “Sparcis now more than 75 percent complete,” Sorbom says. The company aims for Sparc to generate its first plasma in 2027, and aims to build ARC at a site in Virginia by the early 2030s.
As thorough as the new studies are, the ARC reactor is still evolving, Sorbom adds. “We will be able to use what we learn from Sparc to make final design tweaks on ARC.”
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