Four US Microreactors Achieve Criticality, Marking a Key Milestone

The United States has marked a significant milestone in its pursuit of advanced nuclear energy, with four new microreactors achieving "criticality" just in time for the nation's 250th birthday on July 4th. This achievement surpasses the Trump administration's ambitious goal set last year for three microreactors to reach this technical benchmark. The successful completion of this phase by Antares Nuclear, Valar Atomics, Deployable Energy, and Aalo Atomics is seen as a positive indicator for emerging nuclear technologies, particularly as the world grapples with increasing electricity demands and the urgent need for emissions-free solutions to combat climate change. Criticality, in nuclear terms, signifies a reactor's ability to sustain a nuclear chain reaction. This is a crucial technical step, but it is important to clarify that reaching "zero-power criticality" does not equate to generating electricity for the grid. These reactors, developed under the Department of Energy's Reactor Pilot Program, were fast-tracked as prototypes, receiving land and support from national labs. The program, which selected 11 microreactor projects, focuses on units significantly smaller than the large light-water reactors currently dominating the power grid. The speed with which these relatively new companies—founded mostly in 2023 and 2025—hit this milestone is particularly noteworthy in an industry often plagued by delays and budget overruns. However, the journey from criticality to commercial power generation is fraught with technical challenges. As experts like Kathryn Huff point out, zero-power criticality tests can be achieved without substantial engineering progress on fuel or design. To produce electricity, these companies must now undertake significant work, including adding essential equipment such as complex cooling systems to transfer heat from the reactor core. Despite these hurdles, the companies are projecting aggressive timelines: Aalo Atomics aims to produce 10 megawatts of electricity for an on-site data center by 2027, while Deployable Energy targets commercial reactor deployment by 2028. Such ambitious timelines, especially in the nuclear sector, are often met with skepticism due to the inherent complexity of the technology and potential regulatory obstacles. The Nuclear Regulatory Commission (NRC), responsible for civilian nuclear use, has historically had a slow approval process for reactors. While the NRC recently proposed a new framework designed to expedite microreactor approvals, its actual impact on accelerating development remains to be seen. There are also concerns among some nuclear experts that the agency, under the previous administration, might be loosening crucial safety regulations too much. Furthermore, not all nuclear proponents view this microreactor milestone as an unmitigated success. Critics, such as the public policy think tank Third Way, argue that the federal focus on this program represents an "unhelpful diversion" from more substantial goals of increasing overall nuclear capacity. They suggest that "artificially accelerating project timelines is a short-term solution, not a long-term fix." While achieving criticality is undoubtedly a significant first step, a considerable amount of work, innovation, and regulatory navigation will be required before these microreactors can come online and become a meaningful source of electricity for the US grid.