By It’s a huge win for America and one that will help secure American energy for decades. A privately developed advanced nuclear reactor reached criticality Thursday at the Idaho National Laboratory, marking a significant step in the Trump administration’s effort to revive the nation’s nuclear industry and accelerate the development of smaller reactors for military, commercial and remote-energy applications.
Antares Nuclear’s Mark-0 microreactor completed a zero-power criticality demonstration as part of the Department of Energy’s Reactor Pilot Program. The milestone means the reactor sustained a controlled nuclear chain reaction, validating key elements of its design and safety performance.
The test reactor is not designed to produce electricity quite yet. Instead, the Mark-0 demonstration is intended to establish a foundation for Antares’ subsequent power-generating reactors, including a planned 2027 project that would use the same fuel.
The achievement makes Mark-0 the first privately developed non-light-water reactor to reach criticality in the United States in more than four decades, according to the Energy Department. It is also the 53rd reactor built at the Idaho National Laboratory site since 1951.
The milestone arrived ahead of a July 4 deadline established under a May 2025 executive order signed by President Donald Trump. The order directed the Energy Department to create an accelerated pilot program with the goal of bringing at least three advanced reactor designs to criticality by the nation’s 250th anniversary.
Energy Secretary Chris Wright praised the development as a sign of renewed momentum for the American nuclear industry.
“For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States. Thank you to President Trump for his bold leadership and thank you to the bold scientists and entrepreneurs at Antares and Idaho National Laboratory who helped make this moment possible. I look forward to seeing continued progress in the American nuclear renaissance.”
Antares, a California-based nuclear-energy startup founded in 2023, has raised more than $140 million in private capital. The company has outlined an ambitious timetable: criticality in 2026, electricity production in 2027 and potential deployment for military customers beginning in 2028.
Antares CEO Jordan Bramble said the company’s ability to meet its first major deadline was central to the project, according to Red State.
“Hitting our commitments is everything to us. Nuclear in America has been defined for too long by delays, by companies that said they would and then didn’t. We said criticality in 2026, electricity production in 2027, and power to the warfighter in 2028. Today is the first of those commitments delivered on the schedule we set.”
The Mark-0 reactor uses high-assay low-enriched uranium and a specialized form of coated-particle fuel known as TRISO fuel. BWX Technologies, a Virginia-based defense contractor, manufactured the fuel at its facility in Lynchburg, Virginia.
The fuel specification draws on work conducted for Project Pele, a Pentagon initiative focused on developing a transportable microreactor capable of supplying resilient power for military operations. BWXT is building the 1.5-megawatt Project Pele reactor for the U.S. military.
By relying on an established fuel technology and existing domestic manufacturing capabilities, Antares was able to focus its initial test on the reactor’s control systems and physics. The approach also illustrates how federal investments made over several decades can support the faster development of new private-sector nuclear projects.
The U.S. Army has followed the Antares project closely as the military explores the use of microreactors at installations and other locations where access to the commercial power grid may be unreliable or insufficient.
Dr. Jeff Waksman, Principal Deputy Assistant Secretary of the Army for Installation, Energy and Environment, outlined the next steps:
“Antares is the first company in that program to achieve criticality. Next, we need to see a reactor generate electricity at full power. Then the Army will take the baton and deliver a full electricity-generating nuclear reactor to a military installation.”
The Mark-0 milestone comes amid a broader federal push to shorten the time required to test and deploy advanced nuclear technologies. Under the Reactor Pilot Program, companies can work with the Department of Energy to demonstrate first-of-a-kind reactor designs under an accelerated pathway. Future commercial projects would still require additional testing and licensing by the Nuclear Regulatory Commission.
Mark Peters, president of the American Nuclear Society, said other reactors participating in the pilot program are expected to reach criticality soon.
“Criticality is a starting line, not a finish line. The work ahead is scaling up, both the reactors themselves and the professors, students, and skilled workers we need to build and operate the next generation of reactors.”
For Antares, Thursday’s test represents the first major step in a longer effort to move from a laboratory demonstration to a reactor capable of producing usable electricity. The company’s next challenge will be proving that it can maintain the same pace as it advances from a controlled chain reaction to full-power operation.
If Antares stays on schedule, its technology could help move a new generation of American-built microreactors closer to deployment before the end of the decade.
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