Four US microreactors reach criticality

A US nuclear pilot program aimed at accelerating prototype reactor development reached a symbolic goal by July 4, when four microreactors achieved criticality. The Trump administration had set a target for three new microreactors to reach the milestone by the nation’s 250th birthday, and Antares Nuclear, Valar Atomics, Deployable Energy and Aalo Atomics met or exceeded that mark.

The Department of Energy selected 11 reactor projects for the Reactor Pilot Program in August, offering land and support from the national labs system. Antares Nuclear reached criticality first in June with its Mark-0 test reactor, followed by Valar Atomics, Deployable Energy and Aalo Atomics. The speed stood out because Valar, Antares and Aalo were founded in 2023, while Deployable started in 2025.

The milestone remains limited. The reactors achieved zero-power criticality, meaning they demonstrated the ability to sustain a nuclear chain reaction without producing meaningful power. Further work may require major systems, including cooling equipment to move heat out of the reactor core. Aalo plans to produce 10 megawatts of electricity for an on-site data center in 2027, while Deployable Energy plans commercial reactors by 2028.

Regulatory approval remains a major uncertainty. The Nuclear Regulatory Commission has proposed a new framework for microreactor approvals, but the pace is unclear, and some experts have questioned whether rules are being loosened too much. Third Way called the federal focus on microreactors an “unhelpful diversion” from broader nuclear capacity goals.