Four US Microreactors Achieve Criticality Milestone
Four microreactors in the United States reached a critical technical milestone by July 4, exceeding a goal set by the Trump administration. The administration aimed for three new microreactors to achieve criticality, a state where a reactor can sustain a nuclear chain reaction, by the nation's 250th birthday. While a significant step for emerging nuclear technologies, achieving criticality does not mean these reactors are ready to supply electricity to the grid.
Four microreactors in the United States successfully achieved criticality by July 4, a benchmark that indicates a reactor can sustain a chain reaction. This accomplishment surpassed a goal set by the Trump administration, which aimed for three new microreactors to reach this milestone by the nation's 250th birthday.
The Reactor Pilot Program, which facilitated faster development for prototype reactors, provided selected projects with land and support from national labs. The program focused exclusively on microreactors, which are significantly smaller than the large light-water reactors currently dominating the US grid.
Antares Nuclear was the first to achieve criticality in June with its Mark-0 test reactor. Valar Atomics, Deployable Energy, and Aalo Atomics followed, with Aalo Atomics reaching the mark in the early hours of July 4. Valar, Antares, and Aalo were founded in 2023, while Deployable began in 2025.
All four reactors achieved "zero-power criticality," meaning they demonstrated the ability to start a nuclear chain reaction without producing meaningful power. According to Kathryn Huff, a former assistant secretary for nuclear energy and chair of the Department of Nuclear Engineering and Engineering Physics at the University of Wisconsin–Madison, this can be achieved without substantial engineering progress on fuel or design.
Following this milestone, the companies must now focus on developing their reactors to produce electricity, which presents significant technical challenges, such as integrating cooling systems. Aalo Atomics has stated plans to produce 10 megawatts of electricity for an on-site data center by 2027, while Deployable Energy aims to deploy commercial reactors by 2028.
Regulatory challenges are also anticipated. The Nuclear Regulatory Commission (NRC), responsible for civilian and commercial nuclear use in the US, has historically had a slow approval process. Although the NRC proposed a new framework earlier this year designed to accelerate microreactor approvals, the speed of future progress remains to be seen.
According to MIT Technology Review, some nuclear experts have questioned whether the NRC, under the Trump administration, might be loosening nuclear regulations excessively.
