TerraPower on April 23, 2026, officially started construction on Kemmerer Unit 1, its flagship Natrium sodium-cooled fast reactor plant in Kemmerer, Wyoming—a milestone that marks the most consequential step yet in the U.S.’s long and fitful effort to move advanced reactor designs from concept into commercial reality.
The construction start follows the U.S. Nuclear Regulatory Commission’s (NRC’s) issuance of a construction permit on March 4, 2026—the first the agency has granted for a commercial non–light water reactor in more than 40 years and the first for any commercial reactor in nearly a decade. It also comes nearly 50 years after the U.S. last ordered a nuclear plant that was ultimately completed, and more than three decades after Vogtle Units 3 and 4 became the first new reactors built from scratch on American soil in a generation.
While historic, Kemmerer 1 arrives in a different era, under a distinctly different set of pressures. The U.S. power sector is contending simultaneously with surging electricity demand from data centers and electrification, an aging generation fleet, and broad political agreement that advanced nuclear will likely be part of the country’s long-term resource mix. Congress has responded with the Nuclear Energy Innovation and Modernization Act of 2019 and the ADVANCE Act of 2024, directing the NRC to establish a more flexible, risk-informed licensing framework for non–light water technologies and to meet tighter deadlines for licensing decisions.
Founded by Bill Gates and backed by up to $2 billion in cost-shared federal support through DOE’s Advanced Reactor Demonstration Program (ARDP), TerraPower has designed Natrium to meet both grid needs and constructability requirements. The plant features a 345-MW sodium-cooled fast reactor with an integrated molten-salt energy storage system that can boost net electrical output to 500 MW when needed while maintaining constant reactor power. Its engineering, procurement, and construction contractor, Bechtel—which built Vogtle Units 3 and 4 and brings more than 70 years of nuclear construction experience—has described Natrium not as a one-off first-of-a-kind build but as an effort to establish “a delivery approach designed for repeatability,” aiming for nth-of-a-kind execution from the first unit.
POWER has delivered in-depth reporting about the project since TerraPower’s ARDP selection in 2020. The timeline below documents how Kemmerer 1 moved from concept and federal award through siting, licensing, and early works to the April 23 construction start.
2020: ARDP selection and Natrium’s public debut
September 2020—DOE selects Natrium as an ARDP flagship.
DOE selects TerraPower, in partnership with GE Hitachi Nuclear Energy, as one of two flagship awardees under its Advanced Reactor Demonstration Program (ARDP), offering up to $2 billion in cost-shared funding to build a first-of-a-kind Natrium plant on an aggressive seven-year schedule. The selection formalizes the Natrium concept first described when TerraPower and GE Hitachi announced a cost-competitive advanced nuclear system that integrates a 345‑MWe sodium fast reactor with molten-salt energy storage (see more in this POWER feature).
2020–2021—Natrium design and coal-to-nuclear siting concept advance.
Over the next year, TerraPower and GE Hitachi continue to refine the Natrium architecture, emphasizing a “compact and simple safety envelope” and a deliberate separation between the “nuclear island” and “energy island,” as later reflected in NRC documents and POWER’s coverage of the Kemmerer 1 licensing basis (see more). TerraPower and PacifiCorp also begin assessing coal plant sites in the West as candidates for a Natrium deployment, building on the coal-to-nuclear replacement concept outlined in early coverage of the Wyoming siting effort (see more).
2021–2023: Kemmerer emerges as Natrium’s first site
November 2021 — Naughton site in Wyoming identified as the preferred location.
TerraPower and PacifiCorp identify the retiring Naughton coal plant near Kemmerer, Wyoming, as the preferred site for the Natrium demonstration (see more). The choice signals a strategic intent to demonstrate how an advanced reactor with integrated storage can repower an existing coal site and support a resource‑constrained Western grid.
2022–2023 — Early fleet signals and utility interest.
In October 2022, PacifiCorp and TerraPower launch a joint study to evaluate deployment of up to five additional Natrium units across PacifiCorp’s system, framing Kemmerer 1 as the potential lead unit of a multi‑unit fleet (see more). Through 2023, TerraPower continues to present Natrium as both a demonstration project and a commercial product, tying the design’s flexible output to high-renewables grid needs and emerging large loads such as data centers.
2024: Application and non‑nuclear groundbreaking
March 2024 — TerraPower files a construction permit application.
US SFR Owner LLC (USO), a wholly owned TerraPower subsidiary, submits a construction permit application under 10 C.F.R. Part 50 for Kemmerer Power Station Unit 1. The application seeks authorization to build an 840‑MWt, 345‑MWe sodium‑cooled fast reactor with a molten‑salt energy storage system on a site near the Naughton plant, reflecting years of pre‑application engagement with the NRC and Wyoming regulators, later summarized in POWER’s Kemmerer 1 licensing milestone coverage (see more).
May 2024 — NRC accepts the Kemmerer 1 application.
NRC staff accepts the application and begins formal review, initiating one of the agency’s first technology‑inclusive, risk‑informed licensing efforts for a commercial non‑light water reactor and establishing Kemmerer 1 as a test case for the Licensing Modernization Project (LMP) methodology that underpins the project’s proposed licensing basis.
June 10, 2024 — Non‑nuclear portion of Kemmerer 1 breaks ground.
TerraPower breaks ground on the non‑nuclear portion of Kemmerer 1 (see more). The event marks the non‑nuclear portion of “a 345‑MW Natrium sodium‑cooled fast reactor (SFR) power plant” and represents the first advanced nuclear reactor project in the Western Hemisphere to move from design into construction. Work focuses on greenfield development and non‑safety‑related support facilities while the NRC’s technical and environmental reviews continue. As EPC partner, Bechtel begins early works at the site, including a Test and Fill Facility and the Kemmerer Training Center to support full‑scope component testing and operator training ahead of nuclear construction (see more).
2025: Environmental and safety reviews converge
October 2025 — NRC issues a final Environmental Impact Statement.
The NRC issues NUREG‑2268, a final environmental impact statement (EIS) for the Kemmerer site, prepared with the DOE as a cooperating agency. The EIS evaluates environmental impacts across a broad range of resource areas and alternatives and concludes that the project’s impacts are acceptable under NEPA, clearing a key prerequisite for the construction‑permit decision.
December 2025 — NRC staff issues its safety evaluation.
In December, the NRC staff completes its safety evaluation report for the Kemmerer 1 construction permit application. The document concludes that the project meets statutory and regulatory requirements for issuance of a construction permit and reflects “enhanced licensing practices,” including a core‑team structure and extensive regulatory audits, which helped accelerate review by roughly 9 months compared to initial estimates (see more analysis in POWER’s construction‑permit coverage: link).
Early 2026: Licensing firsts and an execution “sprint”
February 22, 2026 — TerraPower details a “sprint to execution.”
In partner content hosted by POWER (link), TerraPower CEO Chris Levesque describes Natrium as one of two first‑of‑a‑kind commercial projects moving in parallel with test reactors and national‑security deployments and argues that early discipline—standardized work packages, long‑lead procurement, and tight regulatory engagement—is essential if a 2030–2031 commercial operation target is to remain credible. He frames TerraPower’s cultural shift as a move “from talking about Navier–Stokes equations to asking, ‘How many tons of concrete do we need to pour per day?’”
March 4, 2026 — NRC authorizes TerraPower’s construction permit.
On March 4, the NRC unanimously authorizes the Office of Nuclear Reactor Regulation to issue a Part 50 construction permit for Kemmerer Power Station Unit 1 (see more). The decision marks the first commercial reactor the agency has approved for construction in nearly a decade and the first approval for a commercial non–light water reactor design in more than 40 years.
March 4, 2026 — KU1 sets multiple licensing precedents.
The March 4 decision establishes several notable firsts. KU1 becomes the first power reactor to receive a Part 50 construction permit for a non‑light water commercial power reactor in more than 50 years and the first power‑reactor application to employ a fully risk‑informed, performance‑based licensing basis using the LMP methodology (NEI 18‑04 / Regulatory Guide 1.233). The permit order also reflects TerraPower’s architectural separation of the “nuclear island” and “energy island” and relies on four exemptions, including provisions related to HALEU fuel handling and emergency core cooling analysis (discussed in more detail in POWER’s licensing coverage: link).
April 23, 2026: Construction officially begins on Kemmerer 1
April 23, 2026 — TerraPower announces the official start of construction.
On April 23, TerraPower announces “the official start of construction on its flagship Natrium plant, Kemmerer Unit 1” in Kemmerer, Wyoming, in a company press release. The company says Kemmerer 1 is “on track to be the first utility‑scale advanced nuclear power plant in the United States” and calls the moment “the culmination of years of innovation, rigorous engineering and disciplined site preparation.”
The plant configuration remains consistent with the design described in POWER’s technology deep dive on Natrium (link) and NRC documents: a 345‑MW sodium‑cooled fast reactor with an integrated molten‑salt energy storage system that can raise output to 500 MW when needed, equivalent to the power demand of around 400,000 homes. TerraPower emphasizes the storage system’s ability to keep reactor power steady while varying net electrical output, a capability aimed at supporting high‑renewables grids and large variable loads.
April 23, 2026 — Bechtel shifts from early works into field execution.
Bechtel, serving as EPC contractor, reports that it is “transitioning into field execution” at the site after completing early works and positions Natrium as a demonstration of a delivery model designed for repeatable, nth‑of‑a‑kind advanced reactor builds (see its Natrium project statement).
TerraPower says the construction milestone will mobilize roughly 1,600 workers to begin plant construction and notes the project is expected to employ about 250 full‑time staff once in operation. The company also underscores that Kemmerer 1 was selected under ARDP as a full‑scale demonstration and is being developed in parallel with broader commercial plans, including a data‑center‑focused agreement with Meta (link) and plans for a second commercial Natrium site in Utah (link).
Toward 2031: From construction to operation
2030–2031 — Target commercial operation window.
US SFR Owner LLC has told the NRC it expects to complete construction by Feb. 28, 2031, before applying for a Class 103 operating license that would authorize 40 years of operation under Part 50 (link). TerraPower continues to reference a 2030–2031 commercial operation target for Kemmerer 1, contingent on maintaining construction progress, securing HALEU fuel supply—efforts that include partnerships such as its HALEU deconversion pilot with Framatome (link)—and completing the operating‑license phase of NRC review.
—Sonal C. Patel is senior editor at POWER magazine (@sonalcpatel, @POWERmagazine).
