Note: Single-source report; awaiting corroboration.
Antares Nuclear’s Mark-0 advanced reactor has successfully completed a zero-power fueled criticality demonstration at the U.S. Department of Energy’s Idaho National Laboratory, confirming the reactor’s capability to operate safely, according to DOE. This milestone, part of the DOE Reactor Pilot Program, provides a foundation for future reactors to begin producing electricity as early as 2027.
The Mark-0 is the first privately developed non-light-water reactor to achieve criticality in the United States in over four decades. The demonstration is considered among the most significant technological advancements in nuclear energy in the past 40 years and will support the design and licensing of future commercial advanced reactors.
Once commercialized, microreactors like the Mark-0 are expected to serve terrestrial energy needs, space missions, and military sites requiring reliable power. The reactor is the 53rd built at the Idaho National Laboratory since 1951, adding to a legacy of test reactors that have shaped the U.S. reactor fleet.
The DOE’s Reactor Pilot Program, initiated under President Trump’s May 2025 executive order with a deadline for advanced reactor criticality by July 4th, has been credited with accelerating nuclear development. Assistant Secretary of Nuclear Energy Ted Garrish highlighted the program’s role in exceeding expectations by achieving criticality in less than a year, emphasizing collaboration among national labs and experts.