Antares Achieves Initial Criticality of Mark-0 Nuclear Microreactor

June 10, 2026

Antares Nuclear completed a zero-power criticality test for its Mark-0 demonstrator reactor at Idaho National Laboratory on June 4, 2026. This milestone advances the deployment of next-generation microreactors and reflects strong collaboration between the U.S. Army and the Department of Energy to support military applications of advanced nuclear technologies.

Antares is the first company in the Energy Department’s Reactor Pilot Program (RPP) to receive agency authorization and successfully complete a fueled criticality test. As directed by Presidential Executive Order (EO) 14301, "Reforming Nuclear Reactor Testing at the Department of Energy," the RPP aims for at least three advanced reactors to reach criticality by July 4, 2026. The RPP offers foundational support to the Army’s Janus Program, a strategic initiative to deploy advanced nuclear microreactors for installation and operational nuclear energy.

The Janus Program is the Army’s answer to Presidential Executive Order 14299, "Deploying Advanced Nuclear Reactor Technologies for National Security." The Army coordinated with the Energy Department to ensure fuel was fabricated in time to support the criticality timeline and also provided expertise to support the safety review of the Antares criticality test.

"This is a historic moment for advanced nuclear energy in the United States," said Energy Secretary Chris Wright. "The Antares team has achieved a major milestone in the development of micro-reactor technology. These technologies will play a critical role in strengthening U.S. energy security for both military and civilian applications."

Jeff Waksman, Principal Deputy Assistant Secretary of the Army for Installations, Energy and Environment, said, "The criticality test of Antares Nuclear’s Mark-0 reactor is an important step toward meeting the president’s goals for nuclear power resiliency for the U.S. Army. A microreactor is now generating neutrons."

"Next, we need a microreactor to generate electrons, and we need to do so with sufficient reliability and maintainability to improve energy resilience at military installations," Waksman added. "The tight teamwork demonstrated here between the Army and Department of Energy teams sets us up in a good place to deliver on nuclear power at a military installation in 2028."

Antares Nuclear’s testing at Idaho National Laboratory's Materials and Fuels Complex — the former site of the Army’s Mobile, Low Power-1 mobile reactor — highlights historical ties and ongoing Army and Energy Department cooperation to develop and regulate advanced microreactor nuclear power solutions. The ongoing partnership will enable authorizations and associated achievements under the RPP to influence and mature permitting and regulatory frameworks within the Army Reactor Regulatory Office for the Janus Program.

The Janus Program is the Army’s next-generation nuclear power program, aiming to deliver secure, resilient, and reliable energy to support national defense installations and critical missions in accordance with EO 14299. In partnership with the Defense Innovation Unit (DIU), the program will build commercial microreactors through a milestone-based contracting model to accelerate delivery of advanced energy solutions to the warfighters.

The Army Reactor Regulatory Office has statutory authority to independently regulate and permit nuclear reactors deployed to support military missions. The alignment between the Army and the Energy Department in permitting allows the Army to leverage the Energy Department's safety frameworks and operational lessons learned.

Ultimately, the successful Mark-0 criticality test represents more than a technical milestone; it is a critical step toward providing energy resilience for national defense infrastructure.

"Hitting our commitments is everything to us. Nuclear in America has been defined for too long by delays, by companies that said they would and then didn't," Jordan Bramble, CEO of Antares, said in a statement about the achievement. "We said criticality in 2026, electricity production in 2027, and power to the warfighter in 2028. Today is the first of those commitments delivered on the schedule we set. The President and DOE set an ambitious timeline for reactor testing, and we met that challenge. I want to thank our partners at the Department of Energy, Idaho National Lab, BWXT, and the U.S. Army. This is what happens when industry and government work together to accomplish big things."

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