The US Department of Energy’s Reactor Pilot Program has achieved a notable, albeit early, technical milestone: four American microreactor startups have successfully reached zero-power criticality. This sequence of achievements, which began in June with Antares Nuclear and rapidly continued through Aalo Atomics, Valar Atomics, and Deployable Energy, signals a genuine acceleration in the development timeline for advanced nuclear reactors, even as it underscores the vast technical distance remaining between a working prototype and a grid-connected power plant.
What the Reactor Pilot Program Actually Accomplished
The program, initiated by the DOE in August, was designed to remove two of the most significant bottlenecks for novel reactor designs: access to suitable land and the deep technical resources of the national laboratory system. By selecting 11 microreactor projects—units a fraction of the size of the light-water reactors that provide the bulk of today’s nuclear capacity—the program effectively created a fast-track lane for experimental designs. The four companies that achieved criticality are among the most aggressive in the cohort, with Valar, Antares, and Aalo having been founded as recently as 2023, and Deployable Energy only starting operations in 2025. The speed at which these very young companies reached their first major technical milestone is a genuine departure from the norms of an industry better known for projects that run years late and billions over budget.
Understanding Zero-Power Criticality: A Test, Not a Power Plant
It is important to be precise about what these four companies have actually demonstrated. The milestone they reached is known as zero-power criticality. This is a test in which a reactor sustains a controlled nuclear chain reaction, but at a power level so low that it produces no meaningful thermal or electrical output. The primary purpose is to confirm the neutron physics of the core design: that the fuel arrangement, the moderator, and the control systems can initiate and maintain a stable reaction. As Kathryn Huff, a former assistant secretary for nuclear energy and a chair at the University of Wisconsin–Madison, noted on the Catalyst podcast, this test can be achieved without having made real engineering progress on the fuel itself or on the overall system design. It is a necessary and valuable validation step, but it is the start of a long process, not the finish line.
For readers trying to contextualize this achievement in the broader landscape of energy technology, a simple question and answer is useful. What does zero-power criticality mean for electricity production? It means very little directly. The reactor is not producing power, and most of the components needed to convert heat into electricity—such as the primary cooling loop, the heat exchangers, and the turbine or power conversion system—are typically not even installed during this test phase.
The Engineering Gaps Between Criticality and Commercial Power
With the zero-power milestone behind them, the four companies now face the far more complex task of engineering a complete, reliable power system. The most immediate challenge is thermal management. A reactor that is confirmed to be neutronically stable still requires a fully integrated cooling system to extract heat from the core and transfer it to a power generation system. This involves significant additional equipment, detailed thermal-hydraulic analysis, and extensive safety testing. The path from a zero-power test to a system that can safely and consistently deliver even a fraction of its rated output is measured in years of iterative design, component testing, and regulatory review. The DOE program has provided the initial catalyst, but the companies themselves must finance and execute the full development cycle.
Aggressive Commercial Timelines and What They Imply
The companies are not hiding their ambitions. Aalo Atomics has announced that it has already begun work on a second reactor and plans to generate 10 megawatts of electricity to power an on-site data center by 2027. Deployable Energy is targeting the commercial deployment of its reactors by 2028. These timelines are extremely aggressive by conventional nuclear standards, but they reflect the design philosophy behind microreactors: smaller size, factory fabrication, simplified safety systems, and faster installation. Whether these projections hold will depend on the companies’ ability to solve the thermal and integration challenges ahead, as well as on the regulatory path set by the Nuclear Regulatory Commission. The fact that these are untested designs from very young companies introduces a significant degree of risk, but the program’s structure is deliberately designed to accept a high rate of failure in exchange for a faster discovery of what works.
What This Means for Developers and Decision-Makers Now
For professionals in energy, infrastructure, and AI—where data center power demand is a rapidly growing constraint—the immediate takeaway is that advanced nuclear is moving from theoretical slides to physical hardware faster than at any point in the last two decades. While none of these four reactors is ready to sell power, the fact that they exist and have passed a fundamental nuclear physics test is a signal worth tracking. Developers and corporate energy buyers should monitor the technical progress of these and other microreactor projects closely, specifically looking for evidence of sustained power output and integrated system testing, not just further criticality milestones. The companies now move from proving the physics to proving the engineering.
