On July 4, four microreactor prototypes achieved zero‑power criticality under the US Department of Energy’s Reactor Pilot Program. Antares Nuclear reached the milestone first in June with its Mark‑0 test reactor, followed by Valar Atomics, Deployable Energy, and Aalo Atomics.

Zero‑power criticality confirms a reactor can sustain a chain reaction, but it does not supply usable power. The test involves no significant heat output, so the companies must add cooling systems and fuel assemblies before generating electricity. The achievement signals progress in fast‑tracking microreactor design, yet it remains an early technical step.

The startups plan aggressive deployment. Aalo Atomics aims to supply 10 MW to an on‑site data center by 2027, while Deployable Energy targets commercial deployment by 2028. Regulatory approval from the Nuclear Regulatory Commission will be decisive; the agency’s new microreactor framework could accelerate licensing, but industry analysts warn it may still lag behind the project timelines.

While the milestone illustrates engineering capability, it does not yet translate into grid‑scale power. Success hinges on overcoming cooling, fuel, and regulatory hurdles; without them, microreactors may remain niche solutions rather than a major contributor to the national electricity mix.