US Microreactors Hit Criticality Milestone: A New Era for Nuclear?
The United States has reached a symbolic turning point in its pursuit of advanced nuclear energy, as four microreactor projects successfully achieved criticality by the nation’s 250th birthday. This technical milestone signals a surge in momentum for next-generation nuclear startups aiming to revolutionize the energy landscape.
Breaking the Criticality Barrier
Achieving "criticality" is a fundamental technical milestone where a nuclear reactor sustains a self-sustaining chain reaction. While the goal set by the previous administration was for three companies to reach this stage, four innovators surpassed expectations. Antares Nuclear led the way in June with its Mark-0 test reactor, followed by breakthroughs from Valar Atomics, Deployable Energy, and Aalo Atomics.
The speed of these developments is particularly striking given the industry's reputation for delays. Several of these companies are remarkably young; Valar, Antares, and Aalo were all founded in 2023, while Deployable Energy began operations in 2025. This rapid progress is largely attributed to the Department of Energy’s Reactor Pilot Program, which fast-tracks prototype development by providing land and access to national laboratory resources.
Zero-Power Criticality vs. Grid Readiness
While the news is a cause for celebration, it is vital to distinguish between theoretical success and commercial viability. These reactors have achieved "zero-power criticality," meaning they can sustain a reaction without producing meaningful amounts of heat or electricity.
To transition from a scientific test to a functional power source, these companies must overcome significant engineering hurdles. This includes developing efficient cooling systems to transfer heat from the reactor core to a turbine and refining fuel designs. For example, Aalo Atomics has set an aggressive target to produce 10 megawatts of electricity to power an on-site data center by 2027, while Deployable Energy aims for commercial deployment by 2028.
The Path to Scaling: Regulation and Debate
The success of these microreactors is a critical component of the broader AI and tech infrastructure debate. As the demand for electricity from massive data centers surges, small, scalable, and emissions-free power sources like microreactors become increasingly attractive. However, two major roadblocks remain: regulatory hurdles and policy skepticism.
The Nuclear Regulatory Commission (NRC) has historically been a slow-moving gatekeeper. While a new framework for microreactor approvals was proposed earlier this year to accelerate the process, the actual speed of implementation remains uncertain. Furthermore, some policy experts, such as those at the Third Way think tank, argue that focusing on microreactors might be an "unhelpful diversion" from the larger goal of increasing massive-scale nuclear capacity.
As these startups move from testing chain reactions to powering the grid, their ability to navigate the complex intersection of advanced engineering and federal regulation will determine if microreactors become a staple of the modern energy mix.
Key Takeaways
- Milestone Achieved: Four microreactor companies—Antares Nuclear, Valar Atomics, Deployable Energy, and Aalo Atomics—successfully reached criticality, exceeding the government's original goal.
- Engineering Gap: The current success is limited to "zero-power criticality," meaning the reactors can sustain a reaction but are not yet capable of generating electricity for the grid.
- Future Outlook: Success depends on overcoming technical cooling challenges and navigating the Nuclear Regulatory Commission's (NRC) approval processes to meet ambitious 2027–2028 deployment targets.