This article has been updated and corrected.

Antares Nuclear Inc., a Torrance-based nuclear power company, hit a landmark milestone for American power in early June.

The company, which is developing small modular nuclear reactors, achieved criticality with its Mark-0 reactor in a lab in Idaho Falls, Idaho. Criticality allows engineers to prove that its reactor can maintain a self-sustaining nuclear chain reaction – in other words, allowing nuclear power to exist.

It’s the first privately developed non-light-water reactor to reach criticality in the U.S. in more than four decades, marking a significant turning point for energy goals stateside. The achievement brings the U.S. closer to developing a fully operational small modular nuclear reactor. Currently, the only two countries that can boast that claim are China and Russia.

“We’re not a legacy prime that has a deep history of landing contracts with the Pentagon,” said Tom Mancinelli, head of federal strategy and policy at Antares. “But what we do think is that the product we’re building – micro-reactors – can provide resilient energy for assets here on Earth that provide critical functions for our national defense.”

The Mark-0 reactor is being incubated under the U.S. Department of Energy Reactor Pilot Program, which set the goal of having three nuclear companies achieve criticality by July 4. Only 10 companies are part of the program, including El Segundo-based Valar Atomics Inc. and Radiant Industries Inc.

“The Mark-0 demonstration will continue to generate learning in a safe, rapid setting to improve our reactor design,” Jordan Bramble, the chief executive of Antares, said in a statement. “Beyond reactor physics, we’ve gained a profoundly greater mastery of our regulatory pathway and our supply chain.”

The future of nuclear power

Small modular reactors, like Antares is developing, are a rather new invention in the nuclear energy space. While nuclear power plants take up miles of land and power large swathes of city homes and businesses, these smaller reactors are portable and provide around-the-clock energy in rural, less-connected environments, or in areas that lost power due to weather changes. Rural hospitals, hard-to-access airports, remote mines or – in Antares’ case – military hubs in unconnected areas could use small modular reactors. These reactors, which are transportable to these distant corners of the world, lessen reliance on the power grid.

“Think about places like remote austere environments where our military is operating, where the grid occasionally goes down, where there’s occasionally power outages due to weather,” said Mancinelli. “So, things like the northern slope of Alaska or island bases in the Indo-Pacific region.”

Achieving criticality does more than just check a box. Criticality allows companies like Antares to validate their models by observing the reactor’s control drums as the chain reaction loops.

What’s next for Antares?

Now that the company has achieved criticality, it can begin to build an electricity producing reactor, which it hopes to get done by the end of 2027. According to a 2025 executive order, President Donald Trump’s administration set a goal of having a reactor on a U.S. military base by September 2028.

“We very much want to be among the first companies to do that,” said Mancinelli.