Antares Nuclear Wins the Race to Criticality

When the Department of Energy launched the Reactor Pilot Program last June, it set a July 4, 2026, target for startups to switch on their microreactors and hit criticality—when a nuclear fission chain reaction becomes self-sustaining. Antares, an LA-based space and defense-focused nuclear startup, beat that deadline by a month. Yesterday, the company reached initial criticality with its Mark-0 microreactor during a demonstration at the DOE’s Idaho National Laboratory. Going nuclear: That makes Antares the first of the ten companies in the pilot program to hit the milestone and the “first novel reactor design to undergo a fuel test in over 50 years,” CEO Jordan Bramble said in a video statement. “It is fitting that on the eve of our nation’s 250th anniversary, we are witnessing a historic moment for American energy,” Energy Secretary Chris Wright said in a statement. “For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States.” Finish line: Forget the NBA and NHL finals, the World Cup, and the tennis majors—the race to criticality is easily this summer’s most riveting competition. And, if you’ve been as glued to it as we have, it shouldn’t come as a huge surprise that Antares crossed the finish line first. “We’ve gone from a clean sheet design of a control system— basically zero—to now for putting [it] in a reactor in about nine months,” Bramble told Tectonic after the DSA approval in April. “You hear people say it’s a marathon, not a sprint, but it’s been a marathon where you’re sprinting.” Power play: That speed—and especially the successful demonstration at the DOE’s Idaho National Laboratory—is as big a win for the Pentagon as it is for Antares. The Army launched the Janus Program last October with the goal of getting a nuclear reactor up and running at a domestic US military base (at least to start) by Sept. 30, 2028. Reaching criticality sets Antares up nicely for Janus, and doing so, along with the Reactor Pilot Program’s fast-tracked pathway, represents “a key step toward deploying electricity-producing microreactors for US military installations” by the September 2028 target date, the company said in a statement. With yet another competition to work towards—this time with the world’s biggest-spending customer—Antares isn’t resting on its laurels quite yet, especially with other nuclear startups hot on its heels. “Nuclear in America has been defined for too long by delays, by companies that said they would and then didn’t,” Bramble said. “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.” Antares’ full-sprint marathon continues. Let’s see if anyone catches up.