WASHINGTON, D.C. — Antares Nuclear has completed a fueled criticality demonstration of its Mark-0 microreactor at Idaho National Laboratory, a milestone that could accelerate the deployment of advanced nuclear technologies and support federal efforts to bring new reactor designs online later this decade.
The test was conducted under the U.S. Department of Energy’s Reactor Pilot Program and marks the first time in more than four decades that a privately developed non-light-water reactor in the United States has reached criticality, according to the Energy Department.
Criticality occurs when a nuclear reactor achieves a self-sustaining chain reaction, a key step in demonstrating that a design can operate as intended. The Mark-0 test was conducted at zero power and did not generate electricity.
Federal officials indicated the demonstration provides operational data that could support future reactor development, licensing efforts and commercial deployment. The Energy Department stated that subsequent reactors based on the design could begin producing electricity as early as 2027, subject to additional testing and regulatory approvals.
The milestone comes as the Trump administration seeks to accelerate domestic nuclear development through a series of executive actions and regulatory initiatives aimed at expanding U.S. energy production and shortening deployment timelines for advanced reactors.
“For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States,” Energy Secretary Chris Wright said in a statement.
The Mark-0 reactor is the first project expected to reach criticality under the Reactor Pilot Program, which was established to speed the demonstration of first-of-a-kind reactor technologies using Department of Energy authorities.
Assistant Secretary for Nuclear Energy Ted Garrish characterized the achievement as evidence that advanced reactor developers can move more quickly under the pilot program. “Today, we celebrate the first of the pilot projects to reach criticality,” he said.
Antares Chief Executive Officer Jordan Bramble noted that the demonstration provided experience beyond reactor performance, including lessons related to regulatory processes and supply-chain development.
Information gathered during the test is expected to inform future reactor designs and licensing applications. If approved by the Nuclear Regulatory Commission for commercial use, microreactors could serve remote communities, military installations, industrial facilities and potential space-based applications.
The Mark-0 becomes the 53rd reactor constructed at Idaho National Laboratory since 1951, joining a site that has long served as a testing ground for U.S. nuclear technologies.
The demonstration also represents an early test of the administration’s broader effort to revive domestic nuclear construction. Building on the pilot program’s initial results, the Energy Department recently launched its Nuclear Energy Launch Pad initiative, which aims to further accelerate the development and deployment of advanced reactor technologies.
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