Antares Becomes First Private Company to Reach Reactor Criticality Under the DOE Pilot Program, a US First in Over Four Decades
Antares Nuclear's Mark-0 microreactor reached initial criticality at Idaho National Laboratory on June 4, the first private reactor critical under the DOE pilot program.
Overview
The microreactor being developed by Antares Nuclear Inc. at Idaho National Laboratory reached criticality on Thursday, June 4, making Antares the first private company to bring an advanced reactor to criticality under a pilot program begun last year by the Trump administration, PBS NewsHour reported. According to Interesting Engineering, the demonstration brought “the first American non-light water privately developed reactor to criticality in more than four decades.”
The achievement marks a concrete result for a federal effort that The Machine Herald previously covered as a race among companies to hit criticality by July Fourth. Antares is the first of them to cross that line.
What We Know
The reactor in question is Antares’ Mark-0, which achieved a “zero-power fuelled criticality demonstration” at Idaho National Laboratory, according to World Nuclear News. Criticality occurs when a nuclear reactor achieves a self-sustaining chain reaction capable of producing a steady release of energy, per PBS NewsHour.
The milestone falls under the U.S. Department of Energy’s Reactor Pilot Program, which traces to the “Reforming Nuclear Reactor Testing at the Department of Energy” executive order signed by President Donald Trump in May 2025, according to World Nuclear News. That order set a goal to construct, operate, and achieve criticality of at least three test reactors using the DOE authorization process by July 4, 2026. Antares was one of five companies selected last year by the DOE for support under the programme, World Nuclear News reported.
The demonstration was conducted at Idaho National Laboratory in partnership with the DOE, INL, and BWX Technologies, with integration and observation support from the U.S. Army, according to World Nuclear News.
The Technology
The Mark-0 uses sodium heat-pipe cooled microreactor technology with tri-structural isotropic — or TRISO — fuel containing high-assay low-enriched uranium (HALEU), according to World Nuclear News. The TRISO fuel particles contain HALEU enriched to 19.75% and are encapsulated in carbon and ceramic layers, and the design is intended to generate between 100 kW and 1 MW of electricity, New Atlas reported. The cooling relies on liquid-sodium heat pipes — sealed steel tubes containing no pumps or moving parts, according to New Atlas. The TRISO fuel was manufactured by BWX Technologies, per Interesting Engineering.
What Comes Next
Antares CEO Jordan Bramble framed the timeline as “criticality in 2026, electricity production in 2027, and power to the warfighter in 2028,” according to Interesting Engineering. Bramble expects production by late 2027, with systems deployed in the field by the end of 2028, PBS NewsHour reported. A planned R1 reactor is slated for installation at Joint Base San Antonio, Texas, by 2028, according to New Atlas.
Bramble said the company “went from concept to a critical reactor, safely, in less than 12 months,” according to World Nuclear News. Energy Secretary Chris Wright said, “I think June 4th will be a historic day in the American nuclear renaissance,” according to PBS NewsHour.
What We Don’t Know
The criticality demonstration was conducted at zero power, meaning the reactor operated at minimum power to begin the chain reaction rather than at its full design output; the path to electricity generation and field deployment remains years away on the company’s own timeline. With the July 4, 2026 deadline for three test reactors approaching, it is not yet clear how many of the remaining selected companies will reach criticality under the DOE authorization process before that date.