TerraPower, the Bellevue, Washington-based nuclear company now under full construction at its first commercial sodium-cooled reactor site in Kemmerer, Wyoming, completed a 7-billion-won ($4.67 million) technology transfer last year from South Korea's Korea Atomic Energy Research Institute, acquiring the intellectual property and manufacturing know-how behind STELLA-2 — the most advanced sodium-cooled fast reactor safety-testing facility in the world. South Korea's Ministry of Science and ICT and KAERI disclosed the deal publicly this past Sunday, with TerraPower engineers completing their hands-on training at the KAERI facility between May 11 and 15.
The disclosure lands two months after the US Nuclear Regulatory Commission issued TerraPower its construction permit — the first ever granted by the agency for a commercial non-light-water reactor. That permit authorizes work on the Natrium plant but does not authorize operation; to run the reactor, TerraPower must obtain a separate operating license, for which the STELLA-2 safety data and in-house testing infrastructure will form a critical evidentiary base.
What STELLA-2 Tests and Why TerraPower Needed It
An SFR uses liquid sodium — not water — to transfer heat from the reactor core to the steam turbines that generate electricity. Sodium boils at roughly 880°C, far above the reactor's operating range, meaning the primary loop runs at near-atmospheric pressure rather than the high pressures required in conventional light-water reactors. That design eliminates one category of risk but introduces another: liquid sodium reacts violently with water and ignites on contact with air, which means any coolant leak becomes a potential fire.
STELLA-2, whose full name is Sodium Thermal-hydraulic Experiment Loop for Limitless Advanced Reactor, is a purpose-built facility that replicates the core safety systems of a prototype SFR at full operating conditions without using nuclear fuel. KAERI has used it to map heat transfer and coolant flow behavior across a range of normal and emergency scenarios, accumulating a detailed library of thermal-hydraulic data that no other institute outside Russia and France has assembled at comparable scale. The technology TerraPower acquired covers the IP behind that facility's design, manufacturing specifications, and the operational dataset. The goal, according to KAERI, is for TerraPower to build its own equivalent facility in the United States to support Natrium safety verification under NRC frameworks.
"This training is an intensive full-cycle technology course related to SFRs, ranging from understanding the physical and chemical properties of sodium to measurement and safety management, sodium handling practice, and practice at the SFR prototype reactor comprehensive effect test facility," KAERI said in a statement following the engineers' departure.
Jeong Ik, head of KAERI's Nuclear Education Center, said the training represented "a field-based international education program that shares sodium system design and operation experience on an actual facility basis" and that the institute expected it to "contribute to strengthening continuous technical exchange and cooperation networks between the Institute and TerraPower."
The Natrium Reactor: What Is Under Construction in Wyoming
The Natrium system TerraPower is building at Kemmerer — at the site of a retiring PacifiCorp coal-fired plant — pairs a 345-megawatt sodium-cooled fast reactor with a molten-salt energy storage system capable of boosting total output to 500 MW for several hours at a time. That storage capability allows the plant to absorb surplus renewable energy and dispatch power on demand — a feature that BloombergNEF nuclear analyst Chris Gadomski has described as pairing "very nicely with renewables."
Construction is expected to complete by 2030, with grid power targeted for 2031. The estimated cost is approximately $4 billion, with the US Department of Energy covering roughly half and TerraPower's investors — including Bill Gates and South Korean conglomerate SK Innovation, the company's second-largest shareholder — funding the remainder. In January 2026, TerraPower signed a deal with Meta to supply up to eight Natrium reactors, the first two delivering power by 2032.
Alan Ahn, deputy director for nuclear at the think tank Third Way, described the NRC construction permit as a development that "positions TerraPower firmly as one of the leaders of the pack" among advanced reactor developers at a time of intense commercial interest in new nuclear capacity.
A Safety Record That Demands Transparency
Sodium-cooled fast reactors have a functional but turbulent operational history. Russia's BN-600 — one of only two commercially operating SFRs in the world — suffered 14 sodium fires in its first 17 years of operation but has now generated electricity for 45 years. Japan's Monju prototype suffered a sodium fire before it ever reached the grid in the 1990s and was eventually decommissioned without contributing meaningful commercial output.
Ed Lyman, director of nuclear power safety at the Union of Concerned Scientists, raised a specific concern about the NRC's handling of TerraPower's application: the safety review was originally scheduled to conclude in August 2026 but was completed in December 2025, following a Trump administration executive order that imposed 18-month deadlines on all NRC permit decisions. Lyman argued that "the only way the staff could finish its review on such a short timeline is by sweeping serious unresolved safety issues under the rug or deferring consideration of them until TerraPower applies for an operating license, at which point it may be too late to correct any problems."
TerraPower CEO Chris Levesque has responded by noting the company has worked with sodium systems for 18 years and has designed the Natrium plant so that sodium and water are in separate buildings — the primary engineering mitigation against coolant fires. Katy Huff, former assistant secretary of nuclear energy at the Department of Energy, has explained that sodium fires result from slow leaks rather than high-pressure ruptures, meaning they are "rarely catastrophic" and arguably present a more manageable hazard profile than the pressurized water in conventional reactors.
South Korea's Program Sold What It Could Not Fully Use
The KAERI deal is a commercial success for Seoul's nuclear research establishment — and an uncomfortable illustration of the cost of inconsistent political support for advanced reactor development. South Korea's government launched a public-private SFR development program originally budgeted at 29 billion won to run from 2025 through 2028. During the 2025 National Assembly budget deliberations, that allocation was slashed by 90 percent — from 7 billion won to 700 million won — as part of broader cuts tied to anti-nuclear policy priorities under the prior government.
The 2025 budget was subsequently restored to 7 billion won, but the disruption had already stalled the program, pushing its completion timeline from 2028 to 2029. KAERI is also developing SALUS, a fourth-generation reactor designed to burn spent nuclear fuel from South Korea's existing pressurized-water reactors while generating electricity, but that program's timeline and commercialization path remain uncertain.
KAERI President Joo Han-gyu first disclosed the deal in February during a ministry briefing, describing it as having "produced results of around $5 million." Industry observers told the Korea Herald that the transaction underscores South Korea's "strong reactor research and testing capabilities, while also exposing how inconsistent government support has weakened the country's momentum in commercializing advanced nuclear technologies."
The deal also reflects a deepening Korea–US nuclear partnership. South Korean conglomerate SK Innovation holds a stake in TerraPower alongside Korea Hydro and Nuclear Power, which acquired part of SK Innovation's TerraPower position in January 2026. Doosan Enerbility, South Korea's primary nuclear equipment manufacturer, has signed a separate component-supply contract with TerraPower. HD Hyundai Heavy Industries signed a partnership agreement with TerraPower in March 2025.
TerraPower's Natrium design also entered the UK's Generic Design Assessment process in April 2025, and the company is exploring potential reactor sites in Kansas and Utah. If the Wyoming plant performs as designed, it will be the first commercial demonstration that a sodium-cooled fast reactor can pass the full US regulatory gauntlet — a result the entire advanced nuclear sector is watching.
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