Antares Successfully Tests Nuclear Microreactor

Antares Nuclear Inc said it has achieved "initial criticality" for a microreactor design that uses TRISO (tri-structural isotropic) fuel, the first to reach that stage under a Trump administration program.

"This test confirms that the reactor can operate safely and establishes a basis that would allow subsequent reactors to produce electricity in 2027 and beyond", the Department of Energy (DOE) said in an online 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", said Energy Secretary Chris Wright. According to DOE, criticality means a reactor has been demonstrated to be capable of operating.

"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", DOE said. "When commercialized 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".

Antares said separately, "This reactor validates key reactor physics parameters for Antares' reactors and contributes verification and validation data back to the Department of War's Project Pele".

The design, Mark-0, "benefited from using the same nuclear fuel as the Project Pele program, an initiative to design, build, and demonstrate a prototype of a transportable micro nuclear reactor for military use", Antares said.

Mark-0, demonstrated at the Idaho National Laboratory, used TRISO fuel manufactured by Virginia-based BWXT Technologies Inc, which is building the 1.5-megawatt Project Pele for the United States Army. "That TRISO fuel specification, developed within DOE's Advanced Gas Reactor program over the past several decades, paired with BWXT’s decades of TRISO development at its Specialty Fuels Fabrication facility in Lynchburg, helped accelerate Antares’ path to a successful criticality test, and demonstrates the value of mature, scalable U.S. fuel manufacturing infrastructure", BWXT said separately.

Antares expects the technology to deliver electricity to U.S. military installations by September 2028.

The project is among 11 initial selections announced by DOE under President Donald Trump's Reactor Pilot Program on August 12, 2025. The reactor program implements a presidential order that seeks to reform the national lab process for reactor testing, establish a pilot program for reactor construction and operation outside of national labs and streamline environmental reviews for reactor facilities. The March 23, 2025 order aims to have at least three advanced reactors achieve "criticality" by July 2026 to help grow the U.S.' nuclear energy capacity from about 100 gigawatts (GW) to 400 GW by 2050. 

The other projects under the program are Aalo Atomics Inc, Atomic Alchemy Inc, Deep Fission Inc, Last Energy Inc, Natura Resources LLC, Oklo Inc (2 projects), Radiant Industries Inc, Terrestrial Energy Inc and Valar Atomics Inc.

"Beyond meeting the objectives of Executive Order 14301, the demonstration produced the testing data, model validation, and control system performance that will advance Antares' commercial reactor toward full-power electricity production", Antares said. Engineers gained direct insight into the physics of the Mark-0 core, the behavior of its control drums, and the maturity of its supply chain. These lessons will have compounding benefits across every reactor Antares builds".

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