Criticality for fourth US microreactor to meet 4 July deadline

Austin, Texas-based Aalo was named in August last year by the Department of Energy (DOE) as one of 11 advanced reactor projects initially selected for support through its Nuclear Reactor Pilot Program, which aimed to see at least three of them achieve criticality by 4 July this year. The Reactor Pilot Program leverages DOE authorisation to expeditiously certify and construct first-of-a-kind advanced reactor designs for demonstration. The initiative is part of the Reforming Nuclear Reactor Testing at the Department of Energy executive order signed by President Donald Trump in May 2025. 

Two weeks after being selected, the company broke ground on a plot of land at the border of Idaho National Laboratory (INL) to start construction of its first experimental extra modular nuclear reactor, the Aalo-X - a low-enriched uranium–fueled, sodium-cooled reactor.

Aalo announced that its Critical Test Reactor (CTR) achieved criticality at 00:20 (local time) on 4 July.

Criticality is the point at which a nuclear reactor sustains a controlled, self-supporting chain reaction. Although the initial criticality was achieved with a full-scale core load, this was a zero-power criticality.

"Our CTR went from groundbreaking to a sustained chain reaction in less than eight months - one of the fastest reactor builds in 80 years - and our company has gone from founding to fission in less than three years," Aalo said. "The CTR includes a full-scale core, demonstrating the nuclear components of our 10 MWe reactors, which will be deployed in 50 MWe Aalo Pods to power AI data centres.

"Criticality has validated our supply chain, reactor physics, control systems, and fueling procedures at commercial scale. We are now expanding into a one-million-square-foot factory to apply assembly-line manufacturing to reactor production, which will open the door to mass-producing the Aalo Pod, our fully modular nuclear plant purpose-built for AI data centres."

The company said it has already begun work on its second nuclear reactor for Project Ascension, a commercial-scale system located on the Aalo-X Campus at INL. The plan is for the new reactor to produce 10 MWe of electricity and power an on-site data centre in 2027.

"The hardest problem in nuclear was never the physics, our country simply forgot how to build. The success of the Department of Energy Reactor Pilot Program is proof America can execute again," said Yasir Arafat, President and CTO, Aalo Atomics. "We are proud to play a major role in America's nuclear renaissance."

Welcoming the milestone, US Energy Secretary Chris Wright said: "Last month I toured the Aalo facility at Idaho National Laboratory and was impressed by the company's determination to successfully demonstrate their technology by the Fourth of July. President Trump asked for three advanced reactors to be authorised and achieve criticality by the 250th anniversary of our great country. I'm pleased to share that through the dedication and hard work of Aalo, INL and DOE, we have surpassed that ask and delivered four."

Antares Nuclear's Mark-0 reactor became the first to reach initial criticality in early June, closely followed by Valar Atomics' Ward 250 reactor. Deployable Energy's Unity demonstration reactor achieved criticality on 1 July.