The Nuclear Regulatory Commission (NRC) has taken a historic step by voting to issue construction permits for Kairos Power’s 70-MWth Hermes 2, a “low power” advanced test facility comprising two 35-MWth molten salt reactors. “Following the Commission’s vote, Hermes 2 is now the first electricity-producing Gen IV plant to be approved for construction in the United States,” said Kairos Power.
Kairos plans to build Hermes 2 at the East Tennessee Technology Park Heritage Center (ETTP) in Oak Ridge, Tennessee. The site will also host the proposed single 35-MWth Hermes 1 test reactor, a non-nuclear engineering test unit, and potentially a Kairos Power fuel fabrication facility.
Construction on Hermes 1, a non-power demonstration reactor, began in July 2024 and is expected to be completed by 2027, making it Kairos Power’s first nuclear project. If construction of Hermes 2 proceeds on schedule, the facility’s two reactors—capable of a combined electrical output of 20 MWe—could begin operation by December 2027.
The NRC’s issuance of the Hermes 2 construction permit represents a key milestone in the two-step CFR Part 50 licensing process. Kairos Power will need to apply for and secure an operating license before the plant can commence operations. Once operational, Hermes 2’s two reactors are anticipated to have a licensed lifetime of 11 years.
How Hermes 2 Fits Into Kairos’ Overall Iterative Strategy
The licensing milestone is another notch for the Alameda, California–headquartered engineering company, which has been developing and marketing nuclear power plant designs based on its fluoride salt-cooled, high-temperature reactor (KP-FHR) technology since its founding in 2016. The company is pursuing a notable “rapid iterative development approach,” which involves using iterative hardware demonstrations and in-house manufacturing “to achieve disruptive cost reduction and provide true cost certainty for commercialization.” Under the stepwise approach, Kairos is also looking to de-risk its technology and supply chain logistics and clear regulatory hurdles before scaling to full commercial deployment.
As POWER has reported, Kairos’s KP-FHR comprises a graphite-moderated, “randomly packed” pebble‐bed reactor with molten fluoride salt coolant (a chemically stable molten fluoride salt mixture of 2LiF:BeF2 [Flibe] enriched in Li‐7), operating at high temperature and near‐atmospheric pressure.
The company’s targeted commercial offerings will include the KP-X Commercial Demo Plant, a single-unit, 50-MWe fluoride salt-cooled high-temperature reactor designed to operate at 650C and near-atmospheric pressure, and KP-FHR Commercial Plant, a dual-unit, 150-MWe reactor configuration (2 x 75 MWe) designed for high efficiency with a 650C reactor outlet temperature. Kairos’ designs will use tri-structural ISOtropic (TRISO) particle fuel in pebble form with a carbonaceous‐matrix coated particle design. The fuel will contain high-assay low-enriched uranium (HALEU) enriched at 19.74%.
In October, notably, Kairos signed a landmark Master Plant Development Agreement with Google to develop a 500-MW fleet of molten salt small modular reactors (SMRs) by 2035 to power the tech giant’s data centers, with the first reactor expected to be operational by 2030. Google said accelerating nuclear deployment is critical to meeting surging data center energy demands and achieving its 24/7 carbon-free energy goals by 2030. While Kairos confirmed to POWER it is planning one KP-X Commercial Demonstration Plant, it declined to reveal specifics about the individual plants to be deployed under the pioneering deal with Google.
As part of its unique iterative approach, Kairos has kicked development with a series of Engineering Test Units (ETUs), which are non-nuclear integrated tests designed to validate the performance of critical systems in a controlled environment. ETU 1.0, commissioned at the company’s Albuquerque facility, operated for over 2,000 hours using 12 metric tons of molten fluoride salt (Flibe) to simulate coolant flow and heat transfer at temperatures up to 675C. Following its decommissioning following six months of operation in July 2023, Kairos initiated ETU 2.0 in Albuquerque. Its next steps will entail the development of ETU 3.0 in Oak Ridge to incorporate lessons learned from earlier iterations and refine modular construction techniques.
The Hermes reactors represent the next phase in the company’s pathway. Hermes 1, a single-unit, non-power demonstration reactor, focuses on operational safety and system integration. Building on its design, Hermes 2 will feature two 35-MWth reactors sharing a Rankine Cycle power conversion system with a 20 MWe capacity. It will also add intermediate salt loops and steam superheaters.
“Hermes 2 will demonstrate complete plant architecture at a reduced scale and supply clean electricity to the grid, further advancing technology, licensing, supply chain, and construction certainty for Kairos Power’s commercial deployments,” the company said on Wednesday.
NRC Review Completed on an Accelerated Schedule
Finally, as part of its iterative approach, Kairos has marked numerous regulatory milestones. In December 2023, NRC granted Kairos a construction permit for its 35-MWth Hermes 1 “non-power” demonstration—25 months after it accepted Kairo’s construction permit application under 10 CFR Part 50.21 (a class of licenses for medical therapy and research and development facilities) in November 2021. The NRC’s approval for Hermes, notably, marked the federal regulator’s first green light for the construction of a non-water-cooled reactor in more than 50 years.
Licensing for Hermes 2 has been even speedier. While the NRC formally accepted Kairos’ CPA for Hermes 2 in June 2023, it issued its final safety evaluation for the permits in July 2024 and completed its Final Safety Evaluation Report (FSER) for Hermes 2 in August 2024—nearly four months ahead of schedule. In total, from CPA acceptance to the construction permit vote, the NRC process has taken 16 months. The timeframe is a staggering achievement for the regulator, which has been historically regarded as notoriously slow in its licensing processes—particularly for new or novel reactor designs—owing to its rigorous safety standards, detailed reviews, and public consultation requirements.
The NRC said the efficient review stemmed from a “new, streamlined mandatory hearing process” conducted via written documents. The commission “authorized the agency’s Office of Nuclear Reactor Regulation to issue the permits, having found the staff’s review of the Hermes 2 application adequate to make the necessary regulatory safety and environmental findings,” it noted. NRC staff “expects to issue the permits in the near future,” it said.
“While keeping safety at the forefront, the permitting process was quite efficient, and we issued these permits in less than 18 months,” said NRC Chair Christopher Hanson on Wednesday. “This shows we can rapidly apply relevant conclusions from earlier reviews to promptly reach decisions on new reactors.”
Kairos on Wednesday suggested the rapid review and approval timeline stems from Kairos’ extensive pre-application engagement with the NRC. Engagement dates “back to 2018, along with numerous process improvements piloted by the first Hermes CPA and the NRC’s new simplified mandatory hearing process,” it said. “Additional efficiencies were created by the similarities and co-location of the two Hermes iterations, which allowed the Hermes 2 application to leverage work already done for Hermes.”
According to Peter Hastings, Kairos vice president of Regulatory Affairs & Quality, the licensing basis established by the Hermes 1 and Hermes 2 construction permits “will carry forward to future license applications, ensuring the safety of Kairos Power’s deployments while supporting continued innovation and efficiency in the review process.”
Darrell Gardner, Kairos senior licensing director, described the company’s experience working with the NRC as “positive.” The company’s applications served as “a testing ground for more efficient processes that will benefit the entire industry,” he noted. “ Kudos to the NRC and to the entire Kairos Power team for another job well done,” he said.
Behind the Historic First: Some Context About Non-LWR and Gen IV Milestones
The NRC refers to non-light water reactor (non-LWR) designs as advanced reactors. “These reactors will use different technologies from existing operating reactors such as passive safety features, using different fuel or coolant, or scaling the entire reactor smaller,” it says.
The 2001-founded Generation IV International Forum (GIF), a global collaboration advancing next-generation nuclear R&D, describes these technologies as Generation IV (Gen IV). Compared to Gen II and Gen III reactors, Gen IV reactors employ advanced materials and innovative designs, such as fast neutron spectrums, closed fuel cycles, and high operating temperatures, enabling applications beyond electricity generation, such as hydrogen production and industrial heat.
While the NRC has a long history of regulating non-LWRs, the regulator’s last non-LWR construction permit, issued in 1968, went to Fort Saint Vrain, a 330-MWe high-temperature gas-cooled reactor that operated until 1989 in Colorado—a Gen II reactor. Clinch River Breeder Reactor, a 350-MWe reactor proposed by the Tennessee Valley Authority (TVA) and Commonwealth Edison, was expected to receive a construction permit in 1983, but work stopped when the project was canceled that year. The NRC marked more progress when medical isotope producer SHINE Medical Technologies in 2016 received a construction permit for a medical isotope manufacturing facility in Wisconsin.
Recent progress has marked several notable “firsts,” with nuanced distinction. Kairos noted Hermes 1 (which got its construction permit in December 2023) marked the federal regulator’s first green light for constructing a non-water-cooled reactor in over 50 years. In September 2024, the NRC granted its first-ever construction permit for a liquid-fueled advanced reactor to Abilene Christian University’s Molten Salt Research Reactor (MSRR), also making it the first research reactor approval in decades. Hermes 2’s construction permits are for the first electricity-producing Gen IV reactors permitted for construction in the U.S., as Kairos suggests.
The NRC is now reviewing a CPA for Kemmerer Unit 1, a 345-MW Natrium sodium-cooled fast reactor (SFR) power plant. TerraPower subsidiary US SFR Owner submitted the CPA March 2024 and the NRC accepted on May 21. The project broke ground on its non-nuclear portion at its site in Lincoln County, Wyoming, in June 2024. When the project receives its construction permit, estimated in June 2026, it will be the NRC’s first issuance for a non-LWR dedicated to power reactor applications. TerraPower says it anticipates submitting the operating license in 2027, allowing it to begin construction on the nuclear island in 2026 and complete the plant “this decade.”
—Sonal Patel is a POWER senior editor (@sonalcpatel, @POWERmagazine).
Editor’s note: Updated Nov. 22 to add context about the NRC’s regulation of non-LWR designs, including milestones related to advanced and Gen IV reactors.
