Antares Nuclear Inc., a Torrance, California-based company, announced on Thursday, June 4, 2026, that its Mark-0 microreactor achieved initial criticality at the Idaho National Laboratory (INL). This accomplishment makes Antares the first private company to bring an advanced reactor to criticality under the U.S. Department of Energy's (DOE) Reactor Pilot Program. Criticality signifies that a nuclear chain reaction has become self-sustaining, leading to a steady release of energy. The DOE confirmed this achievement, noting it's the first privately developed non-light-water reactor to reach criticality in the U.S. in over four decades.
This milestone directly results from President Donald Trump's May 2025 executive order, which established the DOE pilot program to accelerate reactor testing. The order set an ambitious goal of achieving criticality for advanced reactor designs by July 4, 2026. Antares Nuclear was one of five companies selected by the DOE last year for support under this program. The criticality demonstration validates key reactor physics parameters and provides essential data for future reactor development and commercial licensing.
The demonstration was a collaborative effort involving Antares Nuclear, the Department of Energy, Idaho National Laboratory, and BWX Technologies, with the U.S. Army participating as a prospective end-user. Energy Secretary Wright stated that this moment marks a historic step for American energy, fittingly on the eve of the nation's 250th anniversary. Assistant Secretary of Nuclear Energy Ted Garrish highlighted that the project achieved criticality in less than a year, proving skeptics wrong about the ambitious timeline. Antares plans to begin producing electricity from the same facility in 2027 and aims to deploy electricity-generating microreactors to U.S. military installations by 2028.
