Four experimental microreactors achieved criticality — the ability to sustain a nuclear chain reaction — in a rapid-fire sequence that concluded just before the Trump administration's July 4, 2026 deadline, marking an unprecedented pace for nuclear development in the United States.
Antares Nuclear was first, reaching criticality in its Mark-0 test reactor in June. Valar Atomics and Deployable Energy followed, and Aalo Atomics hit the milestone in the early hours of July 4 — an inspiring example of just barely meeting the deadline. All four companies were founded in 2023 or later, underscoring the speed at which private capital and startup culture are moving into nuclear energy.
The achievements came through the Department of Energy's Reactor Pilot Program, which selected 11 microreactor projects in August 2025 and offered them land and support from the national labs system. These are all microreactors — dramatically smaller than the large light-water reactors that dominate the grid today.
But experts caution that zero-power criticality is a far cry from generating electricity. "A zero-power criticality test can be achieved without making real engineering progress on fuel or design," said Kathryn Huff, a former assistant secretary for nuclear energy and chair of the Department of Nuclear Engineering at the University of Wisconsin–Madison. The reactors still need cooling systems to transfer heat out of the reactor core, power generation equipment, and — critically — regulatory approval from the Nuclear Regulatory Commission.
The companies are projecting aggressive timelines. Aalo says it plans to produce 10 megawatts of electricity to power an on-site data center in 2027. Deployable Energy targets commercial reactors by 2028. But nuclear startups have a long history of missing deadlines, and the NRC's approval process — while potentially streamlined under a new microreactor framework proposed this year — remains a significant hurdle.
Some nuclear policy experts are not celebrating. Third Way, a public policy think tank, called the federal focus on the Reactor Pilot Program an "unhelpful diversion" from goals to meaningfully increase nuclear capacity. "Artificially accelerating project timelines is a short-term solution, not a long-term fix," the organization wrote.
The milestone does demonstrate that private companies can move quickly in nuclear development when given government support and streamlined access to national lab resources. Whether that speed translates to grid-ready power plants — and whether the regulatory framework can keep pace — remains the open question.
