Four U.S. microreactor companies have achieved nuclear criticality, a technical milestone proving their reactors can sustain a chain reaction. Antares Nuclear, Valar Atomics, Deployable Energy, and Aalo Atomics all reached this goal by early July 2026, exceeding the Trump administration's target of three reactors by the nation's 250th birthday. However, reaching criticality is just the first step; these companies now face significant engineering challenges before their reactors can actually generate electricity for data centers or the grid.

What Exactly Is Nuclear Criticality, and Why Does It Matter?

Nuclear criticality is a technical threshold that proves a reactor can initiate and sustain a nuclear chain reaction. Think of it as the moment a reactor "wakes up" and demonstrates it can control atomic splitting. All four companies achieved what's called zero-power criticality, meaning they started a chain reaction but generated no meaningful electricity in the process. It's a proof-of-concept test, not a power-generation milestone.

The speed at which these companies reached criticality is genuinely impressive. Three of the four firms, Valar, Antares, and Aalo, were founded in 2023, while Deployable Energy started in 2025. In an industry notorious for massive projects that routinely miss deadlines and budgets, these startups moved remarkably fast. Aalo even hit the milestone in the early hours of July 4, barely making the symbolic deadline.

Yet experts caution against reading too much into the achievement. "A zero-power-criticality test can be achieved without making real engineering progress on fuel or design," noted Kathryn Huff, former assistant secretary for nuclear energy and chair of the Department of Nuclear Engineering and Engineering Physics at the University of Wisconsin-Madison. In other words, starting a chain reaction is one thing; building a reactor that can reliably produce electricity is another entirely.

What Happens Next for These Microreactors?

The companies are projecting aggressive timelines for the next phase. Aalo says it has already begun work on a second reactor and plans to produce 10 megawatts of electricity to power an on-site data center in 2027. Deployable Energy claims it will deploy commercial reactors by 2028. However, these timelines come with significant caveats.

Moving from criticality to electricity production requires solving major technical and regulatory hurdles:

• Cooling Systems: Reactors must transfer heat from the core to generate steam and power turbines, requiring complex cooling infrastructure that hasn't yet been fully engineered for these new designs.
• Regulatory Approval: The Nuclear Regulatory Commission (NRC) oversees civilian nuclear projects in the U.S., and historically the approval process has been slow and burdensome, though the agency proposed a new framework for microreactor approvals earlier in 2026.
• Unforeseen Technical Challenges: Nuclear projects frequently encounter unexpected engineering problems that can delay timelines by months or years, especially for startup companies with limited operational experience.

The NRC's proposed new framework for microreactor approvals is designed to speed up the process, but it remains unclear how quickly the agency will actually move. Additionally, some nuclear experts have questioned whether the Trump administration is loosening nuclear rules too much, raising concerns about safety oversight.

Are Microreactors the Right Path Forward for Nuclear Energy?

Not everyone is celebrating the criticality milestone. Some nuclear policy experts argue that federal focus on microreactors is misguided. A memo from Third Way, a public policy think tank, characterized the Reactor Pilot Program as an "unhelpful diversion" from goals to meaningfully increase nuclear capacity. The analysis warns that "artificially accelerating project timelines is a short-term solution, not a long-term fix".

The concern is that while microreactors are smaller and potentially faster to build than traditional large reactors, they may not deliver the scale of electricity generation needed to power the growing demand from artificial intelligence data centers. Large light-water reactors that dominate the grid today are tens or even hundreds of times larger than these microreactors, meaning many more units would be needed to replace or supplement existing capacity.

Still, the milestone represents genuine progress in an emerging technology sector. The Department of Energy selected 11 reactor projects for the Reactor Pilot Program in August 2025 and offered them land and support from the national labs system. Four of those projects have now proven their core concept works, which is a necessary foundation for everything that comes next.

The real test will come in the next two to three years, as these companies attempt to move from laboratory demonstrations to actual power generation. If they succeed, microreactors could become a viable option for powering data centers in remote locations or industrial facilities. If they stumble on cooling systems, regulatory approval, or other engineering challenges, the timeline could slip significantly, and the broader case for microreactors as a climate solution may weaken.