Three US Microreactors Reach Criticality in One Month — A Nuclear Milestone
In June 2026, the United States achieved something no other country has done before: three unique advanced microreactor designs all reached criticality within a single month, marking a turning point for nuclear energy innovation.
The milestone was driven by President Trump's Executive Order 14301, which directed the Department of Energy to authorize three advanced reactors with the goal of achieving criticality in each by July 4, 2026. All three met the deadline.
The Three Reactors
Antares Nuclear Mark-0 — On June 4, the Torrance, California-based startup achieved zero-power criticality at Idaho National Laboratory's RACE facility. The Mark-0 is the first privately developed non-light-water reactor to go critical in the United States in 40 years. It is a helium-cooled, high-temperature microreactor designed to be factory-produced and transported by truck, plane, or ship.
Valar Atomics Ward 250 — The Utah-based startup achieved criticality at the San Rafael Energy Lab in Emery County, Utah, on June 18. The Ward 250 is a 250-kilowatt microreactor small enough to be airlifted by a C-17 military transport aircraft. Valar has already demonstrated powering an Nvidia Blackwell desktop AI workstation with the reactor and proposed a 30-megawatt closed-loop AI factory powered by its design.
Deployable Energy Unity — The third reactor reached criticality at Idaho National Laboratory on July 1, completing the trifecta. Unity is the first project under the DOE's Nuclear Energy Launch Pad initiative, which leverages national laboratory resources to expedite advanced reactor demonstrations.
Why This Matters
These microreactors represent a fundamentally different approach to nuclear power. Unlike traditional gigawatt-scale plants that take a decade to build and cost billions, microreactors are designed to be factory-manufactured, transported to site, and installed within months. They produce 1 to 20 megawatts of electricity — enough to power a small town, a military base, a data center, or an industrial facility.
The DOE's Reactor Pilot Program authorized all three projects in record time. The department says the program's fast-track certification model could become the template for future advanced nuclear deployments.
Key Facts
- Antares Mark-0 achieved criticality on June 4, 2026 — the first private non-light-water reactor in the US in 40 years
- Valar Atomics Ward 250 was airlifted by C-17 to its Utah test site and reached criticality on June 18
- Deployable Energy Unity completed the trifecta on July 1, fulfilling the executive order by the July 4 deadline
- All three designs are "microreactors" under 20 megawatts — small enough for factory production and rapid deployment
None of the reactors have yet sold a single watt of electricity commercially. The DOE says the next phase will focus on grid connection, regulatory approval for commercial operation, and scaling up production.