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Antares Nuclear, Inc. announced Thursday that its Mark-0 microreactor achieved criticality at Idaho National Laboratory, becoming the first advanced reactor to reach the milestone under a U.S. Department of Energy pilot program established after President Donald Trump’s 2025 executive order aimed at accelerating nuclear development.
The Torrance, California-based company said the reactor reached initial criticality under DOE authorization, making Antares the first private company to bring an advanced reactor to criticality through the Department of Energy’s Reactor Pilot Program.
“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,” Antares CEO Jordan 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.”
Criticality occurs when a reactor achieves a self-sustaining nuclear chain reaction, a major milestone in reactor development. Antares said the demonstration validated key reactor physics parameters and produced testing data and control system performance information that will support future reactor development.
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Energy Secretary Chris Wright said Antares Nuclear’s Mark-0 microreactor became the first privately developed non-light-water reactor to achieve criticality in the U.S. in more than four decades under the Department of Energy’s Reactor Pilot Program. (F. Carter Smith/Bloomberg)
The Department of Energy confirmed the achievement Thursday, describing it as the first privately developed non-light-water reactor to reach criticality in the U.S. in more than four decades.
“Today’s achievement is a historic moment for American nuclear energy,” Energy Secretary Chris Wright said in a statement. “By bringing the first American non-light water privately developed reactor to criticality in more than four decades, Antares has shown what is possible when American innovation is unleashed.”
The milestone comes just over a year after Trump signed four executive orders directing the federal government to accelerate reactor testing, expand domestic nuclear fuel production and streamline pathways for advanced nuclear technologies.
President Donald Trump holds a signed executive order in the Oval Office. Antares Nuclear said its Mark-0 microreactor achieved criticality under a Department of Energy pilot program created after Trump’s 2025 executive orders aimed at accelerating advanced nuclear reactor development. File photo. (Anna Moneymaker/Getty Images)
One of those orders, Executive Order 14301, directed the Department of Energy to establish a pilot program designed to speed testing and demonstration of advanced reactor designs. The administration set a goal of achieving criticality for advanced reactor concepts by July 4, 2026.
“The President and DOE set an ambitious timeline for reactor testing, and we met that challenge,” Bramble said. “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.”
Antares said the criticality demonstration was conducted in partnership with the Department of Energy, Idaho National Laboratory and BWX Technologies, while the U.S. Army participated as a future end user of the technology.
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The company said the Mark-0 used TRISO fuel fabricated by BWXT and benefited from fuel technology developed through Project Pele, a Defense Department effort to build transportable microreactors for military applications.
DOE officials said the achievement demonstrates the potential of the Reactor Pilot Program.
“The skeptics didn’t believe President Trump’s Reactor Pilot Program could achieve criticality in less than a year,” Assistant Secretary of Nuclear Energy Ted Garrish said. “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.”
The company said engineers gained critical insight into reactor physics, control systems and supply chain performance during the demonstration. The data will be used to support future reactor development and eventual commercial licensing.
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“We went from concept to a critical reactor, safely, in less than 12 months. That doesn’t happen by accident. The team treated the schedule as non-negotiable,” Bramble said. “For the American nuclear renaissance to succeed, we need efficient, iterative reactor testing, not a decade per design.”
Antares said it expects to begin producing electricity from the same facility in 2027 and remains on track to deploy electricity-generating microreactors to U.S. military installations by 2028.
