A privately developed microreactor reached a milestone at Idaho National Laboratory on Thursday when Antares Nuclear achieved a zero-power fueled criticality demonstration of its Mark-0 reactor — the first such achievement for a privately developed non-light-water reactor in the United States in more than four decades.

A Historic Milestone at INL

Energy Secretary Chris Wright marked the occasion with a direct 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.” The demonstration positions the Mark-0 as a leading example of the Trump administration’s push to revitalize domestic nuclear energy development.

The Mark-0 was authorized by the Department of Energy under its Reactor Pilot Program. The reactor used TRISO fuel originally developed through the Department of the Army’s Project Pele program, with the fuel itself fabricated by Virginia-based BWX Technologies, Inc. The U.S. Army has been integrated as a future end user of the technology.

Race Toward the July 4 Deadline

The criticality marks the first of several advanced reactors expected to go critical before July 4 — a deadline established by President Donald Trump to coincide with the nation’s 250th anniversary. The administration framed the milestone as both a technical achievement and a symbolic declaration of American energy ambition.

Antares CEO Jordan Bramble highlighted the pace of development, noting that his company moved “from concept to a critical reactor, safely, in less than 12 months.” That timeline — concept to criticality in under a year — underscores the speed at which the Reactor Pilot Program pushed the project forward.

What the Mark-0 Represents

The Mark-0 is classified as a microreactor, a category of small nuclear systems designed for portability and deployment in locations where conventional power infrastructure is limited or unavailable. Its development and testing at Idaho National Laboratory reflects the facility’s continued role as the nation’s primary site for advanced nuclear research and demonstration.

The use of TRISO fuel — a high-temperature, ceramic-coated fuel form considered more resistant to meltdown than conventional reactor fuels — adds a safety dimension to the design that proponents argue makes microreactors well-suited for military and remote applications.

What Comes Next

With the Mark-0 now having achieved criticality, attention turns to whether additional advanced reactors will meet the July 4 deadline. The administration has signaled that multiple projects are in the pipeline, and INL is expected to remain central to those demonstration efforts.

For Idaho, the milestone reinforces the laboratory’s standing as a hub of national energy policy activity. INL has long served as the site where federal nuclear ambitions are tested and validated, and Thursday’s demonstration adds another chapter to that legacy.

The broader policy context includes growing bipartisan interest in nuclear energy as a reliable, low-emission power source — a conversation that has moved rapidly from the margins of energy debates to the center of both federal and state planning discussions. Idaho’s role in hosting and advancing that work continues to draw national attention to the state’s energy and research sectors. For more on state-level policy developments coming out of Idaho this year, see our ongoing coverage at Idaho’s Parental Choice Tax Credit Opens New Application Window.