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New U.S. regulations and a wave of startup interest are breathing new life into TRISO-fueled reactors, which have struggled to take off due to high fuel costs.
As the U.S. looks to revive its stagnant nuclear industry, a group of companies is racing to realize the promise of a “meltdown-proof” fuel that for decades has struggled to progress beyond federal lab experiments.
Tri-structural isotropic fuel, known as TRISO, is safer and more stable than the fuel rods used by the large-scale water-cooled reactors that make up the vast majority of the world’s nuclear power plants. Both fuel sources use enriched uranium, but in TRISO, the element is balled into poppyseed-sized spheres with ceramic coating that can absorb dangerous radioactive materials.
The hitch is the cost: TRISO is orders of magnitude more expensive than conventional assemblies of low-enriched uranium. Given that hefty price tag, only a few TRISO-fueled reactors have ever been built worldwide, even though the technology has existed for years and the world is hungry for nuclear projects that promise to avoid the worst accidents of the past.
But renewed interest is growing as TRISO producers claim they can cut the cost of the fuel by as much as 50% in the coming years, and as a growing number of companies make progress building reactors that can demand that supply.
In mid-May, the U.S. Nuclear Regulatory Commission made history when it advanced a new nuclear plant with a mere monthslong environmental assessment rather than the yearslong environmental impact statement required of every other atomic power station in the country. Regulators said they moved faster in part because the 80-megawatt design from the next-generation developer X-energy relied on TRISO.
But X-energy is not the only developer seeking to use TRISO.
One of the two new nuclear plants currently under construction in the U.S. is a TRISO-based design from Kairos Power. Of the nearly two dozen U.S. startups hawking next-generation reactor designs, at least seven companies are aiming to build fission plants that run on TRISO.
New licensing pathways at the NRC could also give TRISO a boost, as the fuel’s well-documented safety qualities make permits easier to come by for the startups using it.
The momentum comes as public support for nuclear power reaches its highest level in years in the U.S. Some advocates see nuclear energy as a necessary tool to meet surging power demand, while others — particularly blue-state lawmakers and environmental groups that have abandoned anti-nuclearism — view the most reliable carbon-free energy source as a bedrock of a future electrical grid free of fossil fuels.
The 94 operating nuclear reactors in the U.S. are all large-scale models that are cooled by water; most of them were designed and built more than a half century ago. Firms looking to develop reactors that run on TRISO are part of a broader wave of startups that think the best way to build new atomic energy is by pursuing lower-powered small modular reactors, whose designs can be refined and made cheaper through assembly-line repetition.
Companies looking to go the route of microreactors and small modular reactors, however, face not only the challenges that plague large-scale reactors, such as pushback over radioactive waste and costly fuel sources, but new ones, too. For TRISO, those challenges are cost and an immature supply chain — plus the fact that the fuel’s performance remains largely untested at any commercial scale.
Research on TRISO began in the late 1950s in the United Kingdom. At the time, British nuclear regulators had started work on the so-called Dragon reactor at a U.K. Atomic Energy Authority site in Dorset, England. Cooled with pure helium, which can absorb three to five times as much heat as water, the high-temperature gas-cooled reactor needed a fuel that could match its heat tolerance. Thus, TRISO was born.
The first attempts to run a reactor on TRISO worked. The British Dragon ran on the fuel, as did similar experimental high-temperature gas-cooled reactors completed in the 1960s, such as Peach Bottom Unit 1 in Pennsylvania, Germany’s Arbeitsgemeinschaft Versuchsreaktor, and Japan’s government-owned high-temperature test reactor.
But the market remained tiny. Of those four midcentury test reactors, just the Japanese unit remains in operation. In Colorado, the Fort St. Vrain nuclear plant, the only American attempt at commercializing a high-temperature gas-cooled reactor running on TRISO, shut down in 1989 after a decade of high maintenance costs and frequent technical troubles.
Right now, the only commercial TRISO-fueled reactor in the world is in China. It came online in 2023 and uses a form of the fuel based on low-enriched uranium, rather than the more potent high-assay low-enriched uranium, or HALEU, that many U.S. firms are eyeing.
The small number of TRISO-fueled reactors has made it hard for fuel companies to dedicate any space or capacity to producing the fuel, and has kept prices high as a result. TRISO costs an estimated $30,000 per kilogram — more than nine times the roughly $3,300 per kilogram for conventional fuel.
These fuel costs can be offset in part by the fact that TRISO designs don’t need to build expensive concrete containment domes. A reactor’s containment dome alone, after all, made up on average about 4% of the total upfront costs for reactors built between 1976 and 1987, according to a 2020 study in the journal Joule. And that doesn’t include the cost of the steel containment vessel that encloses the reactor under the dome. Nor does it account for the fact that, as the study concluded, productivity at recent U.S. plants — likely meaning the infamously budget-busting pair of gigawatt-sized Westinghouse reactors at Southern Company’s Plant Vogtle in Georgia — was 13 times lower than industry expectations.
While there’s no obvious magic number on which the economics hinge, producers agree that the TRISO market will not take off until the fuel is produced at a large enough scale to significantly bring down costs.
X-energy, for its part, owns a TRISO-manufacturing subsidiary to ensure its own fuel supply. It plans to get the facility online by as soon as next year and said it’s using a standard fuel design honed as part of a decades-long DOE program to support research into the fuel.
BWX Technologies, the nuclear fuel giant that generates fuel for the U.S. government, is currently working on a dedicated $500 million plant to manufacture the fuel in Gillette, Wyoming.
If operations begin as expected by 2031, the plant alone would slash the price of TRISO in half, said Erik Nygaard, BWXT’s director of product development. Recent supply contracts from firms including Antares and Kairos have buttressed BWXT’s investment plans for the facility.
“If you’re making hundreds of kilograms a year in a factory that has all this other cost and overhead, and then you go to a dedicated factory that’s completely designed around how you want your process flow, you get to spread your cost over a much, much greater volume,” Nygaard said. “It doesn’t take a lot to make the economics work. The problem has just been needing enough demand booked to put the shovel in the ground and build this really big facility.”
TRISO-based designs may be at a disadvantage when it comes to fuel cost and supply — but in the U.S., a series of recent regulatory moves have given this class of reactors a leg up when it comes to permitting.
In March, the NRC made the first major update to its licensing standards since 1956 — the culmination of an effort to speed up nuclear licensing that started with a bipartisan law signed by former President Joe Biden and that has been accelerated under President Donald Trump.
Known as Part 53, the licensing pathway allows developers of next-generation technologies to assess the safety of reactors through a risk-informed analysis. It’s a less prescriptive approach than the traditional pathways, which were written to account for the specific safety concerns that come with building a conventional, large light-water reactor.
That new flexibility could benefit TRISO-based designs. Part 53 offers an open-ended option to prove, for example, that the inherent qualities of a TRISO-based design may make it unnecessary to build an expensive concrete containment dome for a reactor.
“Absolutely, there are substantial safety benefits to using TRISO,” said Jeremy Bowen, the head of the NRC’s newly created Office of Advanced Reactors.
In fact, when the NRC greenlit construction permits for the first two demonstration units Kairos Power is building at its debut plant in Oak Ridge, Tennessee, the agency cited the unique safety of the Google-backed developer’s molten salt-cooled, TRISO-fueled reactors. Those reactors, under construction with a target commissioning date of 2030, are the first TRISO-based reactors underway in the U.S since Fort St. Vrain, which closed 37 years ago.
“We were able to credit the safety of TRISO,” Bowen said of assessing Kairos’ applications.
The combination of the molten salt coolant, which can reach much higher temperatures than water, and the TRISO fuel amount to what the NRC called “functional containment design,” meaning a meltdown was so impossible to fathom that a concrete dome over the reactor wasn’t needed.
“Those aspects gave us confidence there was minimal risk of a radiation release and public safety concerns for impacts on the environment,” Bowen said.
The newly proposed Part 57 — another licensing pathway the NRC designed for smaller reactors — also peels back some of the initial safety layers that apply to traditional atomic stations, including both concrete containment domes and miles-wide emergency planning zones.
The NRC created both Part 53 and the proposed Part 57 in large part to speed up permitting processes for smaller nuclear reactors. But these pathways may be viable only for the crop of next-gen nuclear firms that use TRISO in their designs.
To use a licensing pathway such as Part 57, for example, a developer would need to prove that an accident would risk the release of less than 1 rem of radiation, the same amount the human body gets from a CT scan, throughout the duration of the incident.
“TRISO, according to the NRC, is its own containment vessel,” said Adam Stein, the director of nuclear energy innovation at the Breakthrough Institute. “It would be difficult for a different fuel source to meet that 1 rem criteria.”
Other firms may be forced to rely on the traditional, slower-moving Part 50. It’s not a death sentence — X-energy opted for that approach for both of its debut power stations — but other developers have complained that Part 50 puts them at a disadvantage in terms of timing, and requires pushing back against measures in the process that were designed for large water-cooled reactors.
The safety features of TRISO are all the more appealing given that many nuclear startups are staffed by people with no prior experience working with reactors, said Paul Dickman, an expert at the American Nuclear Society and the former chief of staff to the NRC chair from 2006 to 2010, the last time the U.S. had planned to embark on a large buildout of new reactors.
Under the new regulatory structures, “you’re putting a lot of reliance on the fuel — that’s your primary defense mechanism,” Dickman said.
“This is a bunch of guys who don’t have pilot’s licenses designing airplanes. That isn’t to say their airplane isn’t going to fly, but there’s a lot of things you learn from experience,” he added. “To them, it’s all about reinventing everything. That’s a problem.”
But a solution, he said, is turning to a fuel that has inherently safe properties — even if it comes with different challenges in terms of cost and availability.
For his part, Dickman expects the market for TRISO to expand in response to the new regulatory options.
Kurt Terrani, the chief executive of the TRISO manufacturing startup Standard Nuclear, also expects the same.
“They’re not saying you have to use TRISO, but it’s pretty, pretty easy to arrive at the fact that if you really want to meet the limits, it behooves you to use TRISO,” he said.
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