The Department of Energy announced that Antares Nuclear’s Mark-0 reactor completed a zero-power fueled criticality demonstration at Idaho National Laboratory on June 4. The milestone marks the first privately developed non-light-water reactor to go critical in the United States in more than 40 years.
The achievement stems from President Donald Trump’s May 2025 executive order that set a July 4 deadline for reactors to reach criticality. Assistant Secretary of Nuclear Energy Ted Garrish noted the progress, stating that skeptics had doubted the Reactor Pilot Program could succeed in less than a year. Energy Secretary Chris Wright described the timing as fitting, given the nation’s upcoming 250th anniversary. He highlighted it as a historic moment for American energy. The Mark-0 reactor is the 53rd built at the Idaho National Laboratory site since 1951. It provides the foundation for later reactors expected to generate electricity in 2027 and could eventually power military sites, terrestrial uses, and space missions.