US microreactors: Four reactors reach criticality by July 4

Four US microreactor prototypes achieved zero-power criticality by July 4, meeting a Department of Energy goal that had sought three new microreactors to reach criticality by the nation’s 250th birthday. The projects that hit the milestone are Antares Nuclear, Valar Atomics, Deployable Energy, and Aalo Atomics.

Which reactors reached the milestone and when?

Four microreactor projects — Antares Nuclear, Valar Atomics, Deployable Energy, and Aalo Atomics — reached zero-power criticality around July 4, with Antares achieving the milestone first in June and Aalo hitting it in the early hours of July 4. Antares reached criticality in its Mark-0 test reactor; Aalo’s timing is explicitly noted as the early hours of July 4.

The milestone also fulfilled an objective set by the federal goal that aimed for three new microreactors to achieve criticality by the nation’s 250th birthday. The Reactor Pilot Program had previously selected 11 reactor projects in August and offered those teams land and support from the national labs system.

What does "zero-power criticality" actually mean?

Zero-power criticality is a technical test that proves a reactor can sustain a chain reaction while producing no meaningful power output. It is a first-step demonstration rather than proof a reactor can produce electricity on the grid. Kathryn Huff, a former assistant secretary for nuclear energy, said, "A zero-power-criticality test can be achieved without making real engineering progress on fuel or design." The companies will still need to add systems such as cooling equipment and complete other engineering work before producing power.

What are the companies promising next?

Some firms in the group have set concrete near-term targets: Aalo Atomics says it has 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 says it plans to deploy commercial reactors by 2028. Several of the companies reached criticality quickly: Valar, Antares, and Aalo were founded in 2023, and Deployable Energy started in 2025.

Those timelines are aggressive relative to typical nuclear schedules. The Reactor Pilot Program was designed to fast-track prototype reactors by offering land and national-lab support, but firms will still face technical hurdles and regulatory steps before the reactors can deliver grid electricity.

Why it matters

Zero-power criticality shows these teams have passed an important technical threshold and that the Reactor Pilot Program can accelerate prototype progress: four projects met the milestone rather than the three the administration had targeted. The result underscores renewed federal focus and private activity in small-scale nuclear designs. At the same time, critics warn that symbolic milestones can divert attention from scaling larger, grid-ready capacity; a memo from the Third Way think tank called federal focus on the program an "unhelpful diversion" and said, "Artificially accelerating project timelines is a short-term solution, not a long-term fix." Regulatory pace remains a major unknown. The Nuclear Regulatory Commission proposed a new microreactor approval framework earlier this year, but how quickly approvals will move is still unclear.

What to watch

Watch for demonstration steps that move each project from zero-power criticality to power-producing operation: evidence of completed heat-removal systems, fuel and design engineering beyond zero-power tests, and concrete licensing milestones from the Nuclear Regulatory Commission. Also track the program-level timeline: the Reactor Pilot Program selected 11 projects in August and provided national-lab support, and progress or setbacks on those projects will indicate whether the program yields operational microreactors or remains mainly symbolic.