Four US Microreactors Reached Criticality by July 4. That’s the Easy Part.

Four microreactor startups in the United States have achieved criticality, the point at which a reactor can sustain a nuclear chain reaction, meeting a federal target tied to the country's 250th birthday, according to MIT Technology Review.

The Trump administration had set a goal last year for three new microreactors to reach the milestone by July 4, 2026. Four companies did. Antares Nuclear got there first, hitting criticality in June with its Mark-0 test reactor. Valar Atomics, Deployable Energy, and Aalo Atomics followed, with Aalo clearing the bar in the early hours of July 4.

The pace stands out in an industry defined by decade-long timelines and budget overruns. Three of the four companies, Valar, Antares, and Aalo, were founded in 2023. Deployable started in 2025.

What criticality actually proves

The milestone is narrower than it sounds. All four reactors reached what is known as zero-power criticality, a test that confirms a reactor can start a chain reaction without generating any meaningful power.

"A zero-power-criticality test can be achieved without making real engineering progress on fuel or design," said Kathryn Huff, a former assistant secretary for nuclear energy who now chairs the Department of Nuclear Engineering and Engineering Physics at the University of Wisconsin–Madison, speaking on the Catalyst podcast earlier this year.

Getting from that point to a reactor that feeds electricity onto the grid is a separate engineering problem. In some cases, the companies still need to install major hardware, including cooling systems to move heat out of the reactor core.

The program behind the sprint

The reactors came out of the Department of Energy's Reactor Pilot Program, which was built to fast-track prototypes. In August, the DOE selected 11 reactor projects, offering them land and support from the national laboratory system.

All are microreactors. The large light-water reactors that supply most of the US grid today are tens to hundreds of times their size, which is part of the appeal for developers targeting distributed power and on-site industrial loads.

The timelines to watch, and to doubt

The companies are projecting aggressive schedules. Aalo says it has begun building a second reactor and aims to produce 10 megawatts to power an on-site data center in 2027. Deployable Energy says it plans to deploy commercial reactors by 2028.

Startup timelines in nuclear tend to slip, and not only because the machines are complex. Regulatory approval remains a bottleneck. The Nuclear Regulatory Commission oversees commercial nuclear use in the US, and its approval process has historically been slow. The agency proposed a new framework for microreactor approvals earlier this year intended to speed things up, though it is not yet clear how much faster the process will move. Some nuclear experts have also questioned whether the NRC under the current administration is loosening rules too far.

Not everyone in the nuclear camp is cheering. Federal attention on the pilot program is an "unhelpful diversion" from the goal of meaningfully expanding nuclear capacity, according to an analysis from Third Way, a public policy think tank. The memo argued that "artificially accelerating project timelines is a short-term solution, not a long-term fix."

Why it matters

Criticality is a real first step, and reaching it this quickly signals momentum for a class of reactors that could eventually help meet rising electricity demand from data centers and other loads. But there is a long distance between a lab-scale chain reaction and a small reactor delivering steady power to customers. For now, the four companies have proven their designs can turn on. Proving they can produce power, clear regulators, and scale is the work that remains.