On June 4, 2026, the U.S. Department of Energy’s Reactor Pilot Program reached its defining early milestone. Antares Nuclear Inc.’s Mark-0 microreactor achieved zero-power fueled criticality at Idaho National Laboratory’s Reactor and Critical Experiment facility in Idaho Falls. It makes Antares the first private company to bring an advanced reactor to criticality under the DOE Reactor Pilot Program using the DOE authorization process. The DOE characterized the achievement as one of the most significant technological achievements in nuclear energy in more than 40 years.
What Zero-Power Criticality Means
Zero-power criticality is a foundational experimental milestone at which a nuclear fission chain reaction becomes and remains self-sustaining. In the case of Antares specifically, power levels are so low that no meaningful heat is produced and no heat-removal system is required.
“What Antares achieved is specifically zero-power criticality—the chain reaction was sustained at essentially no measurable energy output,” INL Laboratory Director John Wagner explained following the announcement. “This is not electricity generation. It is not a full-power operation. It is proof that the system works: the scientific and engineering validation that every subsequent step depends on.”
Mark-0 Microreactor: Design and Technical Specifications
The Mark-0 is the 53rd reactor built at INL since 1951 and, according to INL Laboratory Director John Wagner, the first novel reactor design to achieve criticality at the laboratory in more than five decades.
The Mark-0 is a full-scale, zero-power demonstration microreactor designed to validate the physics and core performance of Antares’ planned commercial R1 reactor design. It is a high-temperature sodium heat-pipe-cooled reactor that uses entirely passive, redundant liquid metal heat pipes to transport heat away from the reactor core, eliminating the high-pressure liquid pumps and active cooling systems that characterize conventional light-water reactor designs.
The Mark-0 is fueled by high-assay, low-enriched uranium, enriched to just under 20% U-235, and formed into Tristructural Isotropic (TRISO) fuel compacts. The Mark-0 is distinct from Antares’ commercial product line in one critical respect: it is equipped with neither a power conversion system nor a heat removal system, as neither is required for zero-power criticality testing. The commercial R1 microreactor is sized for 100 kilowatts electric to 1 megawatt electric output, with a targeted refueling interval exceeding six years and factory-fabricated modularity enabling rapid deployment at distributed sites.
Why DOE Reactor Pilot Program Matters for Advanced Nuclear Energy
The Reactor Pilot Program was established pursuant to Executive Order 14301, signed by President Trump in May 2025, titled Reforming Nuclear Reactor Testing at the Department of Energy.
The program’s central regulatory innovation is its use of DOE authorization under the Atomic Energy Act in place of Nuclear Regulatory Commission licensing for the initial testing phase. Participating companies fund their own testing activities and operate under DOE oversight, proceeding through a sequence of safety analysis approvals, the Preliminary Documented Safety Analysis (PDSA), and the Final Documented Safety Analysis, rather than through the NRC’s standard licensing pathway.
DOE authorization does not substitute for eventual NRC licensing when a company moves toward commercial operation, but it creates a substantially faster pathway for physics validation and early demonstration.
Technical Roadmap Toward Commercial Deployment
The Mark-0 criticality demonstration does not conclude Antares’ development program. The company’s next milestone is Mark-1, which Antares plans to operate at INL’s Materials and Fuels Complex MFC-793 test facility in 2027.
While the Mark-0 was configured exclusively for zero-power criticality testing, the Mark-1 will operate at full power and be integrated with a nitrogen-closed Brayton cycle power conversion system. Mark-1 represents what Antares has described as its ultimate development milestone before the R1 enters commercial deployment.
Milestones in the DOE Reactor Pilot Program
As of the Mark-0 achievement date, the DOE program requires two additional criticality events to meet the Executive Order’s July 4, 2026, target. Energy Secretary Wright has stated publicly that DOE expects to meet that goal.
Among the remaining program participants with approved safety analyses and active construction or testing programs, Radiant Industries received Preliminary Documented Safety Analysis (PDSA) approval from DOE in February 2026 for its Kaleidos 1-MW helium-gas-cooled microreactor, fueled by HALEU TRISO.
Valar Atomics received PDSA approval in March 2026 for its Ward 250, a 100-kilowatt-thermal high-temperature gas-cooled, graphite-moderated reactor using HALEU TRISO fuel, and has been among the most advanced participants in the program. Aalo Atomics received PDSA approval in March 2026 for the Aalo-X experimental power plant, a precursor to the company’s planned 10 MW sodium-cooled Aalo-1 reactor using low-enriched uranium dioxide fuel.
PDSA approval in the DOE authorization framework allows a company to transition from concept validation into execution-phase activities, including final design completion, site preparation, and early construction, while advancing in parallel toward the Final Documented Safety Analysis required for authorized reactor operation.
Conclusion
The Mark-0 criticality has regulatory and commercial implications that extend beyond Antares. The DOE Reactor Pilot Program was designed, in part, to demonstrate that advanced reactor designs can be validated at speed, and the United States retains the institutional capacity to authorize and operate novel reactor technology outside the decades-long NRC licensing cycle when appropriate authorization mechanisms are in place. Whether the program reaches criticality events before July 4, 2026, will determine how policymakers, investors, and international competitors monitor the pace of U.S. advanced nuclear development.
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