Published: June 06, 2026, 00:49 UTC
A small nuclear reactor the size of a shipping container achieved a self-sustaining chain reaction for the first time this week, passing the first major test for a new generation of American nuclear power. The Antares Mark-0 reached zero-power criticality at the Idaho National Laboratory on June 4, becoming the first privately developed non-light-water reactor to do so in the United States in more than four decades.
The reactor is a microreactor designed by Antares Nuclear, a Torrance, California-based startup founded in 2023 by CEO Jordan Bramble. The company has raised over $140 million in private capital, including a $96 million Series B round closed in December 2025, with backing from partners including Battelle, BWX Technologies, Idaho National Laboratory, and the Department of Defense.
What “criticality” actually means
Criticality is the point at which a nuclear fission chain reaction becomes self-sustaining. Each split atom releases enough neutrons to continue the reaction without an external neutron source.
The Mark-0 achieved this at “zero power” – meaning the chain reaction runs at essentially no measurable energy output. The test reactor has no steam turbine or generator attached. As INL Laboratory Director John Wagner put it, “This is not electricity generation. It is not full-power operation. It is proof that the system works.”
The purpose of the Mark-0 test is to validate reactor physics, control systems, and safety parameters before building a full-power version. The next step is the Mark-1 prototype, expected to demonstrate actual electricity generation sometime in 2027.
How it works
The Antares design is a sodium heat-pipe microreactor using TRISO fuel – small pellets made of a uranium oxide core surrounded by multiple layers of carbon and ceramic, each designed to contain fission products. These pellets are embedded in a prismatic graphite core that acts as a moderator, slowing neutrons to sustain the chain reaction.
Sodium heat pipes transfer heat passively from the core to a heat exchanger. There are no pumps in the primary cooling loop. The heat then drives a closed Brayton cycle turbine using pressurized nitrogen as the working fluid instead of steam. The design produces between 100 kW and 1 MW of electricity, depending on configuration, and each fuel load lasts over six years of continuous operation.
The production version, called the R1, is designed to be factory-produced and transportable by truck. Antares Nuclear plans to deploy it at remote sites, military bases, and potentially in space applications, where its compact, passive design and long fuel life are significant advantages.
The regulatory path
The Mark-0 was tested under the Department of Energy’s Reactor Pilot Program, a fast-track regulatory pathway established by President Trump’s Executive Order 14301 in May 2025. The program aims to demonstrate advanced reactor technologies on an accelerated timeline.
In January 2026, the DOE approved the Preliminary Documented Safety Analysis for the Mark-0 under DOE-STD-1271 – the first such approval for an advanced reactor in the Reactor Pilot Program. Full civilian deployments will eventually require Nuclear Regulatory Commission licensing, but military customers operate under the Army Reactor Regulatory Office’s independent statutory authority.
The U.S. Air Force’s ANPI program has already selected Antares for deployment at Joint Base San Antonio, with operational nuclear power expected by 2028.
Why it matters
The Antares Mark-0’s criticality represents something the US nuclear industry has not seen in a long time: velocity. From founding in 2023 to criticality in 2026 is roughly three years. By comparison, NuScale’s light-water SMR design took over a decade from company founding to NRC certification, and still has no deployed units.
Microreactors like the Antares R1 fill a specific niche that large light-water reactors cannot reach: remote installations, military forward operating bases, disaster recovery sites, and off-world habitats. They trade raw power output – a large plant might produce 1 GW, compared to the R1’s 1 MW – for portability, passive safety, and factory production economics.
The reactor is the 53rd built at INL since 1951 and the first novel design to achieve criticality there in over 50 years. It is also the first to meet the July 4, 2026 deadline set by the executive order that created the Reactor Pilot Program, validating the regulatory model itself.
NASA has expressed interest in the design for space applications. The company also has support from the Defense Innovation Unit and the U.S. Space Force.
For now, the Mark-0 sits at INL running a self-sustaining chain reaction at zero power. The engineering question has been answered. The next test – whether the design can generate meaningful electricity reliably and economically – starts now.
Sources: Ars Technica (June 5, 2026); DOE Press Release (June 4, 2026); World Nuclear News (June 5, 2026); Power Magazine (June 5, 2026); US Army (June 5, 2026)