Antares Nuclear Inc.'s Mark-0 microreactor achieved zero-power criticality at Idaho National Laboratory on June 4, the company and lab confirmed. The milestone makes it the first advanced reactor to reach a self-sustaining chain reaction under the Department of Energy's Reactor Pilot Program. "It is proof that the system works," said INL Laboratory Director John Wagner.
The chain reaction was sustained at essentially no measurable energy output. This was not electricity generation. It was a physics validation—the first novel reactor design to go critical at the Idaho site in more than 50 years.
The Mark-0 is the 53rd reactor built at INL since 1951. Wagner emphasized the distinction in a LinkedIn post. "Criticality is the condition at which a nuclear fission chain reaction becomes self-sustaining," he wrote. "What Antares achieved is specifically zero-power criticality." He added that the distinction should not diminish what happened. The test proves the reactor's control system and core physics function as designed.
The timeline is aggressive. President Trump's May 2025 Executive Order 14301 established the Reactor Pilot Program, directing the DOE to accelerate testing and target at least three advanced-reactor criticalities by July 4, 2026. Antares cleared the first hurdle with six weeks to spare.
The company began machining the Mark-0's graphite core on January 12 at its Antares Prime facility. BWX Technologies had been fabricating the fuel since October 2025. The fuel itself is a story of engineering inheritance.
Antares used TRISO fuel compacts—uranium kernels coated in carbon and silicon carbide—built to a specification developed under Project Pele, the US Army's program for a transportable 1.5-MW microreactor. "Building on a proven fuel specification developed through Project Pele let our team focus on what we had to prove ourselves: our control system and reactor physics," said Antares CEO Jordan Bramble in a press release. That fuel choice removed years of development risk. BWX Technologies President and CEO Rex D.
Geveden called the milestone a demonstration of the company's advanced fuel fabrication capabilities. Joe Miller, president for Government Operations at BWXT, noted Antares is "moving quickly to progress from concept to criticality." The company will continue supplying TRISO fuel as the program advances. Bramble was direct about what comes next. "Now that Mark-0 is critical, the real work is just beginning," he wrote on LinkedIn. "The goal of a reactor is to sell electricity to customers." Antares has already completed over six months of full-power thermal testing on an electrical prototype.
A second version of that prototype will run in 2026. Bramble said the company's iterative approach—testing without a regulatory process, disassembling to examine material effects—allows faster progress than traditional nuclear development. He set a clear target. "All of our iterative testing sets us up to produce electricity for 6+ months.
Hundreds of days, not hundreds of hours."
The Mark-0 itself is a stripped-down machine. A DOE Idaho Operations Office categorical exclusion determination describes it as a small, high-temperature, sodium heat-pipe reactor configured specifically for zero-power criticality testing. It is "not equipped with power conversion or heat removal systems." The reactor was installed below grade inside a pit on the east side of the high bay at INL's Sodium Components Maintenance Shop, a steel-framed building chosen for its existing crane and ventilation infrastructure.
That heat-pipe design is central to Antares's strategy. Rather than pumping coolant through an active loop, the Mark-0 passively transfers heat through sealed, sodium-filled pipes. Sodium vaporizes near the fuel, migrates to the cooler end, condenses, and returns.
The passive cooling eliminates active-component failure modes that burdened earlier reactor safety analyses. The same architecture applies directly to space nuclear power systems, which cannot rely on active cooling infrastructure. Bramble has said the company was designed from inception around both terrestrial microreactor and space nuclear applications.
Antares holds agreements with the US Air Force, Space Force, NASA, and the Defense Innovation Unit. Initial deployments for defense and space customers are targeted for 2028. The company has moved fast since its 2023 founding.
It has raised more than $140 million in private capital, including a $96 million Series B round that closed in December 2025. The DOE approved its Preliminary Documented Safety Analysis for Mark-0 on January 26, clearing the way for fabrication, assembly, installation, and operation under the pilot program. Fuel supply remains a bottleneck for the entire advanced reactor sector.
No US commercial enricher currently produces high-assay low-enriched uranium at commercial scale. The Mark-0 runs on HALEU—uranium enriched to less than 20% U-235—loaded at less than 120 kilograms total for the reactor's operational life. Most advanced non-light-water designs require HALEU because their neutron physics demand higher enrichment than the roughly 4% U-235 used in conventional reactors.
For the Mark-0 test, DOE and the National Nuclear Security Administration provided government-held scrap material. BWX Technologies processed it at its Specialty Fuels Fabrication facility in Lynchburg, Virginia, and fabricated it into finished TRISO compacts. Centrus Energy's 16-machine demonstration cascade in Piketon, Ohio, has produced just over 920 kilograms under a DOE contract—enough for early demonstration work but not commercial deployment.
Antares has already locked in a longer-term supply. In May 2026, the company signed what Urenco described as "the world's first multi-year" commercial HALEU supply contract. Urenco will provide enrichment services from its Advanced Fuels Facility at Capenhurst in the UK.
That facility is planned to come online in 2031 at an initial output of up to 27 metric tons per year—enough to supply up to 30 advanced reactors, Power Magazine reported. What this actually means for your family. A microreactor that can operate for hundreds of days without active cooling and fit on a flatbed truck changes the math for remote communities, military bases, and disaster response.
The policy says one thing. The reality says another. The technology is validated.
The fuel supply chain is not. Why It Matters: The Antares milestone proves that private companies can move from reactor concept to criticality in under three years when paired with government fuel stockpiles and a proven fuel specification. That speed has implications for every advanced reactor developer waiting on HALEU supply chains that will not mature until the 2030s.
If the model holds, defense and space customers get operational microreactors by 2028. Everyone else waits for Urenco's 2031 production target. The zero-power test is a beginning, not an endpoint.
Antares will now execute reactor physics experiments, then move toward sustained electricity production. Bramble's roadmap calls for the version 2.0 thermal prototype to complete testing this year. The company's agreements with the Air Force, Space Force, and NASA suggest the first paying customers will wear uniforms, not pay utility bills.
Key Takeaways: - Antares Nuclear's Mark-0 achieved zero-power criticality at INL on June 4, the first advanced reactor to do so under the DOE Reactor Pilot Program. - The test validated the reactor's control system and physics, not electricity production—the machine has no power conversion or heat removal systems. - The TRISO fuel specification came from the Army's Project Pele, allowing Antares to skip years of fuel qualification work. - Antares targets sustained electricity production for hundreds of days, with defense and space deployments planned for 2028. Watch the fuel supply chain. Urenco's UK enrichment facility will not produce commercial HALEU until 2031.
Between now and then, every advanced reactor developer competes for a limited pool of government-held material. The DOE's pilot program demands two more criticalities by July 4, 2026. The next company to cross the finish line will reveal whether Antares's speed was unique—or the new normal.
Key Takeaways
— Antares Nuclear's Mark-0 achieved zero-power criticality at INL on June 4, the first advanced reactor to do so under the DOE Reactor Pilot Program.
— The test validated the reactor's control system and physics, not electricity production—the machine has no power conversion or heat removal systems.
— The TRISO fuel specification came from the Army's Project Pele, allowing Antares to skip years of fuel qualification work.
— Antares targets sustained electricity production for hundreds of days, with defense and space deployments planned for 2028.
Source: Power Magazine
