A privately developed nuclear reactor in the United States has achieved first criticality for the first time in more than four decades, marking a significant milestone for the country's advanced nuclear energy ambitions and opening the path for future commercial power generation.
First criticality is the stage at which a reactor achieves a self-sustaining nuclear chain reaction for the first time, allowing it to generate energy in a controlled manner.
The Mark-0 demonstration reactor, developed by Antares Nuclear under the US Department of Energy's Reactor Pilot Programme, became the first among several advanced reactor projects to reach criticality ahead of July 4, when the US marks the 250th anniversary of its independence.
The Department of Energy hailed the achievement as the "rebirth of America’s nuclear industry" and a "historic moment" for the country's energy sector.
"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.
Wright also thanked President Donald Trump, along with scientists and engineers at Antares Nuclear and Idaho National Laboratory, for helping make the project possible.
The demonstration is considered a key step toward the future deployment of advanced microreactors capable of generating electricity. Mark-0 uses a full-scale reactor 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 over 50 years," Antares Chief Executive Officer Jordan Bramble said in a statement.
Bramble said the milestone was aided by executive actions taken by the Trump administration to accelerate the development and testing of advanced nuclear technologies.
Describing the test as a major breakthrough, the Department of Energy said the Mark-0 criticality demonstration validated the safety and operational performance of Antares Nuclear's fission reactor.
"One of the most significant technological achievements in nuclear energy in over 40 years, this test will go on to inform the design and licensing of future commercial reactor deployments," it said.
Following further testing and licensing by the Nuclear Regulatory Commission, microreactors such as those being developed by Antares are expected to be deployed across a range of terrestrial and space applications, including at military installations requiring reliable energy supplies.
Assistant Secretary for Nuclear Energy Ted Garrish said the achievement underscored the success 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," Garrish said.
Bramble added that the Mark-0 project had also provided the company with valuable experience in reactor regulation, supply chain development and advancing next-generation nuclear technology.