First privately developed U.S. nuclear reactor in over 40 years achieves first criticality point

A new small nuclear reactor backed by the U.S. government achieved first criticality, making it the first such feat by a private company in more than four decades. First criticality is the point at which a nuclear reactor achieves a self-sustaining nuclear chain reaction for the first time, enabling it to produce energy in a controlled manner and paving the way for future power generation.

The Mark-0 demonstration reactor, developed by Antares Nuclear under the Reactor Pilot Programme of the U.S. Department of Energy, is the first among such multiple advanced units to go critical before July 4, when America celebrates the 250th anniversary of its independence. The Department of Energy described the milestone, achieved on Thursday (June 4, 2026), as “rebirth of America’s nuclear industry” and a “historic moment” for U.S. energy.

“For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States… I look forward to seeing continued progress in the American nuclear renaissance,” Energy Secretary Chris Wright said in a statement here. Mr. Wright thanked President Donald Trump, as well as scientists and engineers at Antares and Idaho National Laboratory, for helping make the project possible.

The demonstration is seen as a critical step toward generating electricity from advanced microreactors. Mark-0 uses a full-scale core and the same facility and fuel that will support Antares’ next reactor test scheduled for 2027.

“We’ve now become the first novel reactor design to undergo a fuelled test in more than 50 years,” Antares Chief Executive Officer Jordan Bramble said in a statement. Mr. Bramble attributed the achievement in part to executive actions taken by the Trump administration aimed at accelerating the development and testing of advanced nuclear technologies.

“The Mark-0 criticality test is a tremendous accomplishment that validates the safety and operational performance of Antares Nuclear’s fission reactor,” the Department of Energy said in a statement. “One of the most significant technological achievements in nuclear energy in more than 40 years, this test will go on to inform the design and licensing of future commercial reactor deployments,” it said.

When commercialised after further tests and licensure by the Nuclear Regulatory Commission, microreactors like those that Antares makes are anticipated to be used in a variety of terrestrial and space applications and to ensure readiness at military installations requiring reliable energy.

Assistant Secretary for Nuclear Energy Ted Garrish said the achievement demonstrated the effectiveness of the Reactor Pilot Programme. “The sceptics didn’t believe President Trump’s Reactor Pilot Programme could achieve criticality in less than a year. Today, we celebrate the first of the pilot projects to reach criticality and the people who rolled up their sleeves to shape the future of nuclear energy in the United States,” Mr. Garrish said.

Mr. Bramble said the Mark-0 project had also helped the company gain valuable experience in reactor regulation, supply chains and technology development.