"The Rebirth Of America's Nuclear Industry": Antares Microreactor Goes Critical

Antares just did something that has been painfully rare in American nuclear energy: it took a privately developed advanced reactor from concept to actual criticality on an aggressive, publicly stated schedule.

On June 4th at Idaho National Laboratory (INL), the company’s Mark-0 microreactor achieved initial zero-power fueled criticality under the Department of Energy’s Reactor Pilot Program. 

When a reactor goes critical, it is experiencing a self-sustaining chain reaction of fissioning uranium atoms inside of its core. A zero-power fueled criticality means the reactor was taken critical at an extremely low power level to prevent any heat production or significant radiation and facilitate data collection. 

It is the first advanced reactor to hit that mark in the program and the first privately developed non-light-water reactor to reach criticality in the United States in more than four decades.

Today, an advanced non-lightwater nuclear reactor reached criticality in the United States for the first time in more than four decades.

Thanks to President Trump unleashing the American Nuclear Renaissance, this was made possible. 🇺🇸 pic.twitter.com/UwWxdDd0ms

— U.S. Department of Energy (@ENERGY) June 4, 2026

This is not another rendering or licensing milestone:

• Fission happened
• Ahead of schedule

We have been tracking the microreactor industry’s rapid evolution across multiple articles. The handful of developers positioning under the new DOE fast-track authorities created by President Trump’s May 2025 executive orders. 

In early April we detailed Antares securing the first-ever Documented Safety Analysis approval for an advanced reactor under DOE-STD-1271, which is a regulatory green light viewed as equivalent to an NRC license for their test reactor. 

We covered their selection for the Air Force’s Advanced Nuclear Power for Installations program at Joint Base San Antonio. And just last week we reported on their groundbreaking multi-year commercial HALEU supply agreement with Urenco, the first long-term commercial contract of its kind, securing fuel for scale beyond limited government allocations.

Today’s criticality is the concrete payoff from that string of updates.

The Mark-0 demonstration validates key reactor physics and overall system performance for Antares’ broader R1 transportable microreactor design. The unit is sized for 100 kWe to 1 MWe, with a targeted refueling interval of more than six years, factory-fabricated modularity, and high-temperature heat pipes. 

It uses TRISO fuel fabricated by BWX Technologies, drawing directly on the fuel specification and manufacturing work matured under the Department of Defense’s Project Pele military microreactor program. The U.S. Army was integrated throughout as a future end user.

Energy Secretary Chris Wright called it fitting on the eve of the nation’s 250th anniversary: the first new privately developed non-light-water reactor criticality in America in over 40 years. Assistant Secretary Ted Garrish noted the skeptics who doubted the Reactor Pilot Program could deliver criticality in less than a year. ANS President Mark Peters congratulated the team but correctly framed criticality as “a starting line, not a finish line.”

U.S. Chief Technology Officer Dr. Ethan Klein was a little more enthusiastic...

AMERICAN ENERGY DOMINANCE.
AMERICAN NUCLEAR RENAISSANCE.
A HISTORIC MILESTONE.
🇺🇸🇺🇸🇺🇸 https://t.co/28wFYWYN7U

— U.S. CTO Ethan Klein (@USCTO47) June 5, 2026

Antares CEO Jordan Bramble on making history: “Hitting our commitments is everything to us. Nuclear in America has been defined for too long by delays, by companies that said they would and then didn’t. We said criticality in 2026, electricity production in 2027, and power to the warfighter in 2028. Today is the first of those commitments delivered on the schedule we set.”

The company went from concept to a critical reactor safely in less than 12 months. 

Criticality is the starting line. But for the first time in a long time, that line just got crossed on a credible, aggressive timeline rather than a bureaucratic one. The microreactor race has a clear early leader in execution. The question now is whether the rest of the industry and the policy apparatus treat this as the new baseline, or simply another headline before the next round of delays sets in.