Antares Nuclear’s Mark-0 Microreactor Achieves Criticality
Antares Nuclear announced on Thursday that its Mark-0 microreactor has reached criticality at the Idaho National Laboratory. This marks it as the first new reactor to hit this milestone under the U.S. Department of Energy’s pilot program, which was established following President Trump’s 2025 executive order aimed at speeding up nuclear development.
Based in Torrance, California, Antares revealed that this accomplishment was achieved with the approval of the Department of Energy, making it the first private company to bring a new reactor to criticality through the DOE’s Reactor Pilot Program.
“Keeping our promises is crucial for us. Nuclear power in the U.S. has faced long delays, with many companies making claims but not following through,” stated Antares CEO Jordan Bramble. “We set our timeline—criticality in 2026, generating electricity in 2027, and powering warfighters by 2028. Today, we’ve met that first goal.”
Criticality is when a nuclear reactor achieves a self-sustaining nuclear chain reaction, a key milestone in reactor development. Antares noted that the demonstration validated crucial parameters of the reactor and provided essential test data to aid future reactor endeavors.
The Department of Energy applauded this step, calling it the first privately developed non-light water reactor in the U.S. to reach criticality in over 40 years. Energy Secretary Chris Wright commented on this achievement: “Today’s success is a historic moment for America’s nuclear energy landscape, showcasing what can happen when innovation is unleashed.”
This milestone came just a year after Trump enacted four executive orders aimed at enhancing nuclear reactor testing and promoting domestic nuclear fuel production.
One such executive order, 14301, specifically called for the formation of a pilot program to expedite the testing of advanced reactor designs, with a goal of achieving criticality by July 4, 2026.
“We took on the ambitious testing schedule set by the President and the Department of Energy, and we met it,” Bramble added. He expressed gratitude towards partnerships with the Department of Energy, Idaho National Laboratory, BWXT, and the U.S. Army, emphasizing the power of collaboration in achieving significant milestones.
Antares conducted the criticality demonstration in conjunction with the Department of Energy, Idaho National Laboratory, and BWX Technologies, with potential military use also in focus.
The Mark-0 microreactor utilizes TRISO fuel produced by BWXT and benefits from technology developed through Project Pelle, a Department of Defense initiative aimed at creating portable microreactors for military purposes.
DOE officials noted the results indicate promise for the reactor pilot program. Ted Garrish, assistant secretary of nuclear energy, remarked, “Skeptics doubted that President Trump’s reactor test program could achieve criticality so quickly. Today, we recognize this pilot project’s success and the dedicated people behind it.”
During the demonstration, engineers gained crucial insights into reactor physics and supply chain efficiency, which will be vital for future development and eventual commercial licensing.
Bramble noted, “We transitioned from concept to critical reactor in just 12 months. This wasn’t a coincidence; we treated the timeline as essential. For a successful nuclear renaissance in America, we need a streamlined testing process, not a decade for each design.”
Looking ahead, Antares plans for the Mark-0 to start generating power in 2027, with the microreactors being deployed to U.S. military facilities by 2028.
