Antares Nuclear Reactor Reaches Criticality, Achieves Major Energy Milestone

Los Angeles-based Antares Nuclear’s advanced microreactor design, Mark-0, has achieved criticality, signifying the resurgence of the United States’ nuclear industry, the Department of Energy (DOE) said in a June 4 statement.

For a nuclear reactor, criticality means it has achieved a perfectly stable state. Under such conditions, nuclear power plants can generate electricity around the clock.

Mark-0 has “successfully completed a zero-power fueled criticality demonstration at DOE’s Idaho National Laboratory,” the department said.

“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.”

Energy Secretary Chris Wright thanked President Donald Trump for his leadership and the scientists and entrepreneurs involved in the project for the achievement.

“It is fitting that on the eve of our nation’s 250th anniversary, we are witnessing a historic moment for American energy,” Wright said.

“For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States.”

Mark-0 is the 53rd reactor to be constructed at the Idaho National Laboratory site since 1951. The DOE termed the criticality of the reactor as one of the “most significant technological achievements” in the field of nuclear energy over the past four decades.

Once microreactors such as Antares are further commercialized, they are expected to be used in a wide range of terrestrial and space applications. The reactors will also boost readiness at military sites by supplying the necessary energy. 

Such small-sized reactors can help power human bases on the moon, according to the European Nuclear Society.

Mark-0 will undergo further testing and must be licensed by the Nuclear Regulatory Commission before it can be commercialized.

Antares is part of the DOE’s Reactor Pilot Program, which was established after Trump signed an executive order in May 2025 directing the DOE to launch a new pilot program to accelerate testing of advanced reactor designs.

The Reactor Pilot Program sets up a new pathway for reactors from the demonstration stage to fast-tracked commercial licensing. Antares is one of 10 companies selected for the program.

The initiative seeks to ensure that at least three advanced nuclear reactor concepts outside national laboratories reach criticality by July 4.

“The skeptics didn’t believe President Trump’s Reactor Pilot Program 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,” Assistant Secretary of Nuclear Energy Ted Garrish said.

Nuclear Reactor Tech

Mark-0’s criticality demonstration is one among the many nuclear energy projects being pursued by the Trump administration.

Last month, the Department of Transportation announced that the country was seeking to develop small modular nuclear reactors (SMRs) for commercial vessels to lower shipping costs.

SMRs typically have a power-generating capacity of 300 megawatts, roughly a third of that of traditionally larger nuclear plants. Meanwhile, microreactors, such as Mark-0, are typically capable of generating 20 megawatts or less of electricity. Microreactors are a subset of SMRs.

Concerns have been raised about deploying SMR tech. A December 2025 study published in Energy Research & Social Science found that SMRs have high upfront costs, potentially requiring public subsidies.

Diverting such public investment could deprive communities of funding for other priority areas, such as education and health care, according to the study.

As for employment opportunities from SMRs, “although construction and initial operation phases generate employment, these opportunities diminish over time as SMR plants typically require fewer personnel for ongoing maintenance and operation. This raises questions about whether the socio-economic benefits such projects are purported to bring are equitably distributed,” the study said.

Meanwhile, the U.S. Army praised the zero-power criticality test of the Mark-0 demonstrator reactor in a June 5 statement.

In addition to being a technical milestone, the successful test is a crucial step toward providing “energy resilience” for the country’s defense infrastructure.

“Through continued collaboration and decisive innovation, the Army and the Energy Department are transforming energy resilience into an operational reality, ensuring military installations remain powered, secure, and mission-ready for the challenges of tomorrow,” the statement said.