As part of the US Department of Energy (DOE) Reactor Pilot Program, US start-up Antares Nuclear’s Mark-0 reactor design has completed a zero-power fuelled criticality demonstration at DOE’s Idaho National Laboratory (INL). The demonstration was conducted in partnership with DOE, INL, and BWX Technologies (BWXT), with integration and observation support from the US Army.
Mark-0 is the first several advanced reactors racing to go critical by the 4 July deadline set by President Trump in his May 2025 Executive Order (EO)14301, Reforming Nuclear Reactor Testing at the Department of Energy. The EO created the DOE Reactor Pilot Program and mandated that the Secretary of Energy approve a streamlined pathway to achieve criticality for at least three advanced test reactors by the US 250th Independence Day.
The Antares R1 Mark-0 is a full-scale, zero-power demonstration microreactor built to validate the physics and core performance of the company’s planned commercial R1 design. It is a high-temperature sodium heat-pipe reactor, utilising entirely passive, redundant liquid metal heat pipes instead of high-pressure liquid pumps. The core uses graphite and boron carbide control drums turned by independent actuator motors, a physical setup heavily inspired by historical space reactor designs.
Mark-0 runs on high-assay low-enriched uranium (HALEU), enriched to just under 20% U-235. It employs Tristructural Isotropic (TRISO) coated particle fuel packed into a structural prismatic graphite core, which inherently prevents meltdown scenarios. The fuel was manufactured at the BWX Technologies (BWXT) specialty facility in Lynchburg, Virginia. The project transitioned from a design concept to a critical reactor in less than 12 months by using the fast-tracked regulatory and testing pathway.
Because the Mark-0 was built strictly as a physics validation platform, it has no power conversion loop or heat-sink systems installed. Achieving initial criticality means the Mark-0 sustained a controlled, self-supporting nuclear chain reaction. As a zero-power test, it operated at essentially no measurable energy output and did not generate electricity.
The operational data verified by the Mark-0 will directly anchor the upcoming commercial Mark-1 microreactor, designed to scale output fluidly between 100 kWe and 1 MWe. It integrates a simple, recuperated Nitrogen (N2) closed Brayton cycle operating at low pressures to ensure high reliability with virtually zero corrosion risk. It is optimised to run uninterrupted on a single fuel core for 4 to 6 years without the need for on-site refuelling.
Antares has already initiated design work on the Mark-1 prototype. The company remains on an aggressive development track, aiming for electricity production by 2027 and initial power delivery to US military installations by late September 2028
“It is fitting that on the eve of our nation’s 250th anniversary, we are witnessing a historic moment for American energy,” said Energy Secretary Chris Wright. “For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States.… I look forward to seeing continued progress in the American nuclear renaissance.”
Antares CEO Jordan Bramble noted: “DOE’s Reactor Pilot Program has served as a transformative catalyst for American nuclear energy, enabling feats many believed were impossible less than a year ago. We partnered with experts throughout the DOE and national labs to accelerate our time to commercial impact. The Mark-0 demonstration will continue to generate learning in a safe, rapid setting to improve our reactor design. Beyond reactor physics, we’ve gained a profoundly greater mastery of our regulatory pathway and our supply chain. Perhaps most of all, this programme has instituted a camaraderie within our team, DOE, and our partners at Idaho National Laboratory, that will continue to grow from here.”
He added: “This is a high-safety, high-information test that closes the loop between design prediction and physical realisation. It’s also a forcing function. It forced us to build an organisation that can design, license, build, and test reactors on a schedule, and it forced DOE’s licensing pathway to run at the pace the country needs. For the American nuclear renaissance to succeed, we need efficient, iterative reactor testing, not a decade per design. Criticality is the first step. Electricity from this same facility, using the same TRISO fuel, is expected in roughly a year. Power for military installations follows in two.”
The Mark-0 validates key reactor physics parameters for Antares reactors and contributes verification and validation data for the Department of War’s (DOW’s) Project Pele. The Mark-0 was authorised by DOE under the Reactor Pilot Program with the US Army integrated throughout as a future end user.
Project Pele is a DOW initiative to design, build, and deploy the first transportable, advanced nuclear microreactor managed by the DOW’s Strategic Capabilities Office (SCO). Its primary objective is to eliminate or drastically reduce the military’s reliance on vulnerable diesel fuel logistics lines in remote or contested environments.
It specifies a 1-5 MWe helium-cooled high-temperature gas-cooled reactor (HTGR) requiring zero water cooling, making it ideal for desert or arctic conditions. The system is engineered to fit within four standard 20-foot shipping containers and can be transported globally by air (C-17 aircraft), rail, or truck. It is designed to safely shut down, cool, and be dismantled within one week, and fully operational within 72 hours of arriving at a new site.
Development of the prototype is funded under a cost-type contract valued at roughly $300m. Major industry partners include BWXT (contracted to manufacture the reactor core and fabricate the fuel); Rolls-Royce LibertyWorks (responsible for developing the advanced closed-loop power conversion system); and Northrop Grumman (designing the ruggedised reactor control and instrumentation module). Assembly of the prototype hardware is being completed at BWXT’s Lynchburg, Virginia facility. The reactor is scheduled for shipment to the INL Critical Infrastructure Test Range Complex later this year. Project Pele is on track for full operational demonstration, hitting the hard regulatory deadline mandated by the May 2025 EO 14299, Reforming Nuclear Reactor Testing at the Department of Energy.
Mark-0 benefited from using the same nuclear fuel as Project Pele. “BWXT is proud to work with Antares and deliver the fuel necessary for this important milestone at the Idaho National Lab,” said Joe Miller, BWXT’s President for Government Operations. “Antares is moving quickly to progress from concept to criticality, and we are happy to supply this team with the TRISO needed to do so. The fuel specification and manufacturing expertise we matured through the Strategic Capabilities Office’s Project Pele directly underpin this milestone, and the data from this demonstration will emphasise the strategic importance of the nuclear industry.”
The demonstration and the licensing pathway it establishes represent a key step toward deploying electricity-producing microreactors for US military installations by 30 September 2028. “The DOE Reactor Pilot Program is a critical step necessary to deliver on President Trump’s executive orders,” said Dr Jeff Waksman, Principal Deputy Assistant Secretary of the Army for Installation, Energy and Environment. “Antares is the first company in that programme 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.”
“BWXT’s TRISO fuel supported our path to criticality,” said Jordan Bramble. “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. We’re grateful for a partnership that continues as we move from neutrons to electrons.”
Antares Nuclear (Antares Industries) was founded in 2023 to mass-manufacture micronuclear reactors. It is staffed primarily by aerospace, defence, and nuclear experts recruited from disruptive hardware environments like SpaceX, Relativity Space, and the US Air Force. Antares has raised over $140m in total private capital alongside millions in government defence contracts.
In Torrance, California, Antares operates a 145,000-square-foot facility optimised to mass-produce up to 10 factory-sealed microreactor units a year. It also has offices in Idaho Falls and Aiken, South Carolina. The company bypasses conventional commercial timelines by framing its reactors as vital defence assets. In co-operation with DOW and the US Army, Antares is a leading vendor for the Janus Program and was selected under the Advanced Nuclear Power for Installations (ANPI) programme to construct and operate a prototype reactor at Joint Base San Antonio (JBSA) by 2028. For NASA it is developing kilowatt-class space-fission propulsion systems and designing hardware for NASA’s Fission Surface Power initiative to place a 100 kWe reactor on the Moon.
