Idaho lab showcases new generation of small nuclear reactors

Antares Nuclear’s Mark-0 reached zero-power fueled criticality at Idaho National Laboratory on June 4, putting a privately developed non-light-water reactor into a category the United States had not seen in more than four decades. The Department of Energy called the test one of the most significant technological achievements in nuclear energy in more than 40 years, and said it was the first of multiple advanced reactors expected to reach criticality by the July 4 deadline in President Donald Trump’s May 2025 executive order.

The timing matters because the federal government is trying to compress a process that normally stretches over years. Antares was one of five companies selected last year for the Reactor Pilot Program, which is meant to speed testing through DOE authorization rather than the usual Nuclear Regulatory Commission licensing route. DOE said the Mark-0 result could help establish a basis for reactors to produce electricity in 2027 and beyond. World Nuclear News said the pilot program is intended to achieve criticality of at least three test reactors by July 4.

Idaho National Laboratory is built for this kind of work, and the setting underscores how much is riding on it. The site was established in 1949 as the National Reactor Testing Station, covers 890 square miles in southeast Idaho, and has hosted 52 previous reactors. Mark-0 is the 53rd reactor built there since 1951, when Experimental Breeder Reactor I produced the first usable quantity of electricity from nuclear fission. The laboratory’s role shifted after the 1970s, then returned to a stronger nuclear-energy focus in 2005, making Idaho a natural stage for the current revival.

The work has unfolded under heavy security, with soldiers, checkpoints and warning signs reflecting both the promise and sensitivity of advanced nuclear development. DOE said the demonstration involved the department, Idaho National Laboratory, BWX Technologies and the U.S. Army, a lineup that shows how quickly civilian energy policy is merging with defense priorities. The reactors are being pitched as faster and cheaper to build than conventional plants, with potential uses at military bases and energy-hungry data centers.

That promise still has to survive the market. Antares’ milestone came as Aalo Atomics was expected to follow in Idaho within days, and Valar Atomics reached the same criticality step in Utah on June 18, suggesting the push is broader than one company or one state. Yet the economics remain unproven at commercial scale, and skeptics still question whether small modular reactors can compete with wind and solar.

The historical shadow is also hard to miss. Nuclear Regulatory Commission records show Three Mile Island Unit 2 suffered severe core damage in the March 28, 1979 accident, and debris fuel from the plant is stored at the Idaho Nuclear Technology and Engineering Center at INL. In Washington and Idaho, the milestone was celebrated as proof that the industry has moved from PowerPoint to hardware, but the national test will be whether these reactors can clear the remaining regulatory steps and deliver electricity on schedule.