Five Critical Reactors, Zero Watts to the Grid

Five reactors, nine weeks, one deadline

On May 23, 2025, President Trump signed Executive Order 14301, creating the Department of Energy's Reactor Pilot Program with a specific, public target: get at least three privately developed test reactors, sited outside the national laboratories, to criticality by July 4, 2026. What followed was a sprint. Antares Nuclear's Mark-0 unit reached criticality at Idaho National Laboratory on June 4. Valar Atomics' Ward 250 followed on June 18 at the Utah San Rafael Energy Lab. Deployable Energy's Unity reactor reached zero-power criticality at Idaho National Laboratory on June 30. Aalo Atomics closed out the group on July 4, 2026, the deadline itself, also at Idaho National Laboratory.

Oklo's Groves Isotope Test Reactor came fifth, reaching criticality on August 6, 2026, at a site in Lockhart, Texas, five weeks past the original deadline but under the same program. It is also the first Reactor Pilot Program unit built on private land rather than federal or state property. Oklo chief executive Jacob DeWitte called reaching criticality in under a year 'an incredible milestone,' and DOE Assistant Secretary for Nuclear Energy Ted Garrish credited the executive order directly, saying Groves is 'part of the revival of America's nuclear energy industry.'

What criticality actually means here

Criticality is the point at which a nuclear reactor sustains its own fission chain reaction without an outside neutron source. It is a real engineering achievement and a necessary step toward producing power, but it is not the same claim as producing power, and none of these five reactors is doing that. All five were authorized directly by the Department of Energy under its own statutory siting powers, a route created specifically to bypass the years-long Nuclear Regulatory Commission Part 50 or Part 52 licensing process that any commercial, grid-connected reactor in the United States still has to complete before it can sell electricity.

Groves illustrates the gap precisely. It is authorized for up to 100 megawatts electrical but has never been operated anywhere near that output, and Oklo says explicitly that the unit will not produce commercial isotopes; its job is to establish the technical and operational foundation for a future commercial version. Deployable Energy's Unity reactor went further in its own description, reaching what the company called zero-power criticality. None of the five has a power purchase agreement in place, and none is connected to any electricity grid.

The separate, slower project that actually matters for a data centre

Oklo's commercial power design, called Aurora, is a distinct project from Groves, running on the ordinary Nuclear Regulatory Commission review track rather than the Department of Energy pilot shortcut. An Aurora unit is under construction at Idaho National Laboratory, and Oklo has separately proposed Aurora projects tied to a Meta-backed campus in Ohio and to Eielson Air Force Base in Alaska. That is the reactor design that would eventually power a data centre or a grid connection, and it is moving on a conventional, longer regulatory calendar that the Groves criticality date does not change.

The same separation applies to the other four pilot companies. Valar Atomics is still marketing the commercial reactor fleet behind its billion-dollar Series B raise, reported by Servola on August 3, and Antares Nuclear, Aalo Atomics and Deployable Energy each still have their own path to a licensed commercial unit ahead of them. A test reactor reaching criticality is evidence the underlying physics and hardware work at small scale. It is not evidence of a construction permit, a signed power contract, or a delivery date.

What a European buyer should actually check

For an EU or UK company weighing a small modular reactor vendor's pitch for on-site or dedicated nuclear power, a rising number of them tied to AI compute demand, 'our reactor achieved criticality' is a genuine but narrow claim. The useful question is which regulatory track the specific unit belongs to: a Department of Energy pilot test reactor, a Nuclear Regulatory Commission Part 52 commercial design certification, or a design under review by a national regulator inside the EU. A second question matters just as much: is the reactor that went critical the same design and the same site contracted to eventually supply your power, or a separate demonstration unit built to hit a different deadline entirely.

It is also worth stating plainly what does not transfer. The Reactor Pilot Program's authority rests on US federal siting law and an executive order that applies to US federal and, in Groves' case, private land inside the United States. None of it shortens licensing in Germany, France, or any other EU member state, where a national regulator, its own review timeline, and its own public consultation process apply regardless of how quickly the same company's US demonstration reactor went critical.

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