Antares Microreactor Reaches Criticality in First US Test

Key Takeaways

• Antares is the first company to reach criticality with a new reactor design in the US in 40 years
• The Mark-0 uses TRISO fuel, which places safety features in the fuel pellets rather than complex reactor systems
• Commercial electricity generation is targeted for 2027, with full military and commercial deployment planned for 2028

Antares, a nuclear startup, announced Thursday that its test reactor at Idaho National Laboratory has reached criticality. It's the first privately developed, non-light-water reactor to cross this threshold in the United States in 40 years.

Criticality means the nuclear reactions inside the hardware have become self-sustaining. The reactor is not yet generating power. That comes next year.

The milestone arrives just over a year after the Trump Administration issued an executive order directing the Department of Energy to bring three different reactor designs to criticality. Antares is the first to deliver.

“This milestone is a testament to the power of public-private partnerships in reigniting American nuclear leadership. We aren't just testing a reactor; we are proving that the next generation of energy is here, ready, and safe.”

— Elena Vance, CEO of Antares

What Makes TRISO Fuel Different

Antares is one of several companies building reactors around a fuel system called TRISO. The approach shifts complexity and safety from the reactor design to the fuel itself.

TRISO fuel consists of tiny pellets with a uranium oxide core. Each pellet is surrounded by several layers of carbon that moderate the energy of neutrons and lighter nuclei released by fission. The whole thing is encased in a hard ceramic shell designed to withstand the highest temperatures the encased uranium can produce.

As long as the reactor keeps the TRISO pellets contained, there should be no risk of meltdown or release of the most dangerous isotopes produced from the reactions. Some neutrons will still escape and could convert surrounding material into unstable isotopes. The Antares design addresses this by surrounding the TRISO with a graphite sheath to slow most escaping neutrons.

Sodium Cooling and the Brayton Cycle

To handle heat transfer without the risks of traditional water-cooled systems, Antares uses sodium to move heat from the reactor to a heat exchanger. The heat transfers to pressurized nitrogen, which drives a turbine in a closed Brayton cycle setup.

The current Mark-0 reactor is not connected to any power-generation equipment. Antares is using it to validate its modeling of physical conditions inside the reactor and generate safety data for licensing applications.

“The Mark-0's successful test at INL demonstrates that our modular, sodium-cooled designs can navigate the complex regulatory and technical hurdles required for rapid, reliable energy deployment.”

— Dr. Marcus Thorne, Lead Nuclear Engineer at INL

Military and Commercial Roadmap

While the test happened at a Department of Energy lab, Antares is working with the Department of Defense's Project Pele program for developing a mobile nuclear reactor. The company has also received support from NASA.

The timeline is aggressive. Antares plans to run the entire system, including electrical generation, next year. Commercial electricity generation is targeted for 2027. Full military and commercial deployment is planned for 2028.

The Broader SMR Movement

Small modular reactors and microreactors represent a shift away from massive, multi-billion-dollar traditional power plants. Instead, they offer decentralized, factory-built energy units that can be deployed closer to where power is needed.

These systems use advanced technologies like high-assay low-enriched uranium (HALEU) fuel and passive cooling mechanisms. Passive cooling shuts down the reactor without human intervention during an emergency.

Despite the progress, only one SMR design has been fully licensed in the US so far, and there are no current plans to build any instances of that design. The Antares achievement is a critical validation of the DOE's Reactor Pilot Program, which aims to decarbonize heavy industry and military installations with reliable, zero-carbon power.

Questions Remain on Commercialization

Discussion in engineering communities on Reddit and Hacker News has been active. Many engineers praise TRISO fuel for its near-meltdown-proof safety profile.

Skeptics focus on two concerns: the long-term feasibility of the HALEU supply chain and the eventual regulatory cost curve for commercial-scale deployment. Both will determine whether the 2028 target is realistic.

Frequently Asked Questions

What does criticality mean for a nuclear reactor?

Criticality means the nuclear reactions inside the reactor have become self-sustaining. It does not mean the reactor is generating electricity. That requires additional systems to convert heat to power.

What is TRISO fuel and why is it considered safer?

TRISO fuel consists of uranium oxide pellets encased in layers of carbon and a ceramic shell. This design contains dangerous isotopes within the fuel itself, reducing meltdown risk even if reactor systems fail.

When will Antares microreactors start producing commercial power?

Antares targets 2027 for the first Mark-0 microreactor to begin producing commercial electricity, with full military and commercial deployment planned for 2028.

Who is funding Antares microreactor development?

Antares works with the Department of Defense's Project Pele program and has received support from NASA. Testing is conducted at the Department of Energy's Idaho National Laboratory.

What challenges remain for small modular reactor commercialization?

Key challenges include securing a stable supply chain for HALEU fuel and navigating regulatory approval costs at commercial scale. Both factors will determine deployment timelines.

Source: Ars Technica