Aalo Atomics’ Aalo-X Critical Test Reactor (CTR)—dubbed “Project First Light”—has reached criticality at Idaho National Laboratory (INL), marking the fourth Department of Energy (DOE)–authorized advanced reactor startup under the federal push to accelerate reactor testing and demonstration.
The U.S. Department of Energy (DOE) said July 6 that Aalo’s test reactor, which DOE referred to as Aalo-X, “successfully completed a zero-power fueled criticality demonstration” at INL under DOE’s Reactor Pilot Program. Aalo told POWER the Critical Test Reactor reached criticality at 12:20 a.m. MT on July 4, allowing DOE to exceed the target in Executive Order 14301, which directed the department to approve at least three reactors to reach criticality by July 4, 2026.
“President Trump asked for three advanced reactors to be authorized and achieve criticality by the 250th anniversary of our great country,” Energy Secretary Chris Wright said in DOE’s July 6 statement. “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!”
Aalo follows Antares Nuclear’s Mark-0 reactor, which reached criticality at INL on June 4, and Valar Atomics’ Ward 250 reactor, which reached criticality at the Utah San Rafael Energy Lab on June 18, as the third Reactor Pilot Program developer to reach the milestone. Deployable Energy’s Unity reactor reached zero-power criticality at INL on June 30 through DOE’s Nuclear Energy Launch Pad initiative, making Aalo the fourth DOE-authorized advanced reactor criticality in a month.
As Yasir Arafat, Aalo president and chief technology officer, explained in a July 2 post on X, after Aalo had begun fuel loading, “Criticality is not the birth of fission. It’s the moment when the reactor no longer depends on an external neutron source to sustain the chain reaction.” In reactor-physics terms, Aalo’s CTR crossed from subcritical operation, where k < 1 and each neutron generation produces fewer fissions than the last, to critical operation, where k = 1 and each generation sustains the next.
“The hardest problem in nuclear was never the physics, our country simply forgot how to build,” Arafat said in DOE’s July 6 announcement. “The success of the Department of Energy Reactor Pilot Program is proof America can execute again.” Arafat also noted that Aalo went “from breaking ground to a sustained chain reaction in just eight months,” calling it “one of the fastest reactor builds in modern American history.”
A Full-Scale Reactor Physics Campaign for Aalo-X
During a June 24 tour of Aalo’s two-acre INL site, Arafat told POWER the CTR campaign is geared to test four key execution questions: whether the company can build a nuclear facility quickly, factory-fabricate reactor hardware, establish a repeatable fuel pathway, and stand up an operating organization under DOE authorization. The criticality milestone marks a significant technical step in Aalo’s broader Aalo-X program, which is moving in stages from zero-power reactor physics to sodium systems testing, full-power operation, electricity production, and eventual deployment of Aalo’s commercial Pod configuration.
“Reaching criticality is our most significant milestone to date, as it paves the way for the deployment of the Aalo Pod to power commercial data centers once it receives authorization from the Nuclear Regulatory Commission,” Matt Loszak, Aalo’s CEO, said in the company’s July 6 release. “More importantly, the Aalo-X Critical Test Reactor has the same full-scale core components as our commercial reactors. The Aalo-X’s 10 MWe reactor design positions it as the premier power provider for the modern data center.”
“Criticality is just the beginning,” Arafat noted on July 6. “In the coming months, we will continue building and testing multiple reactors, including the commercial Aalo Pod design, which in the next 18 months will provide a scalable and affordable power option to data centers and enterprises.”
Company materials shared during the INL tour describe the CTR as a full-scale zero-power physics reactor built to test reactor physics, control systems, and the nuclear core before Aalo moves to power operation. The CTR applies Aalo’s staged development approach to the fueled core, graphite, control rods, instrumentation, fuel loading, and operating procedures at near-zero power.
The CTR’s fuel supply was also a crucial component test. Aalo said the fuel rods for the CTR were fabricated by Global Nuclear Fuel (GNF), GE Vernova’s nuclear fuel arm, and delivered to the site in early April. However, while the CTR uses 4.95% enriched uranium dioxide fuel, the commercial reactor is expected to use standard uranium oxide fuel enriched between 5% and 8%, rather than high-assay low-enriched uranium (HALEU), Arafat told POWER.
Next Steps: From CTR to Aalo-0 to Aalo-X to Aalo Pod
Last week at the CTR, the final startup sequence kicked off following a series of DOE authorization milestones, including a final readiness assessment. After Energy Secretary Chris Wright signed the final approval to load fuel, the company was able to “flip the switch” and achieve criticality. “Aalo’s celebration was intentionally reminiscent of Chicago Pile-1 (CP-1), the first self-sustaining, controlled nuclear chain reaction that occurred 84 years ago on December 2, 1942,” the company noted. “CP-1 represents the birth of nuclear reactors, and today INL is the leading U.S. site for developing and testing them.”
Now that the CTR has achieved criticality, the campaign is slated to conduct controlled reactivity insertion tests, axial flux profiling with movable neutron detectors and flux wires, control rod calibration, shutdown-margin tests, and abnormal-configuration tests involving fuel or moderator changes. Those experiments are intended to qualify neutronics codes, verify reactivity margins, calibrate in-core instrumentation, and establish control behavior before Aalo-X operates at power, as Arafat explained in a February blog.
Near the CTR, Aalo is assembling Aalo-0, a full-scale, non-nuclear prototype designed to circulate 60,000 pounds of sodium under operating conditions and test heat-transport and power-conversion systems, including heat exchangers, plugging meters, cold traps, heat tracing, instrumentation and controls, welds, modules, and operating procedures. Modules for Aalo-0 were built at the company’s Austin manufacturing facility and shipped to Idaho, and Aalo says the system is slated for commissioning in fall 2026.
Loszak told reporters during the June 24 tour that Aalo intentionally split the test program into two full-scale campaigns—a fueled reactor-physics campaign in the CTR and a separate sodium campaign on the same INL site—because the approach allows Aalo to test “the nuclear fuel portion” and “the coolant, the sodium coolant” in isolation before combining them in a power-producing system.
“We’ve been testing the nuclear fuel at full scale in that building right there, and we’re going to be testing the sodium at full scale at this site as well,” he said. “Those learnings will all come together, and we’re going to be putting the sodium through the nuclear fuel, taking the heat away, turning a turbine, producing electricity,” Loszak said.
The next major undertaking is Aalo-X, the full-power 30-MWth/10-MWe sodium-cooled demonstration nuclear power plant at INL authorized under DOE’s Reactor Pilot Program. Aalo says the plant will use data from the CTR and Aalo-0 to support construction and licensing in 2027, followed by demonstrations of operations and safety mechanisms in 2028. The demonstration is intended to prove the 10-MWe Aalo-1 reactor for the company’s commercial 50-MWe Pod, which would use five 10-MWe reactors connected to a shared turbine.
In February, Arafat wrote that Aalo seeks Technology Readiness Level 9 by running Aalo-X at full 30-MWth power, generating 10 MWe, accumulating fuel burnup, demonstrating safety systems, handling transients such as rapid load changes and pump trips, and exercising refueling procedures. Aalo expects the reactor to run at 100% power for sustained periods, including a 100-hour endurance run, and to produce “an exhaustive dataset on neutronics, thermal performance, fuel behavior, and operations and maintenance activities.”
This week, Aalo announced it had already begun work on its second nuclear reactor, located next to the Aalo-X test reactor at the INL campus. Dubbed “Project Ascension,” the test commercial-scale system will “produce electricity and power for an on-site data center in the coming months,” the company said. “In addition, Aalo recently announced a collaboration with Microsoft and Nvidia to lay the groundwork for an automated co-piloting system that could transform how a fleet of nuclear reactors operates safely.”
Aalo’s ultimate commercial product will be the Aalo Pod, a 50-MWe sodium-cooled nuclear plant comprising five 10-MWe reactors connected to a shared turbine. Company materials describe the Pod as a factory-fabricated, data-center-focused system with a 40-year initial system life, phased refueling, redundant architecture for high uptime, a roughly 5-acre footprint, and no need for local water resources, given that the design uses sodium cooling and air-cooled condensers. Aalo is positioning the Pod as a “bring your own power” option for AI data centers that would not draw power from the grid.
Aalo also used the CTR campaign to test its factory-first manufacturing model under the executive order’s compressed schedule. Loszak said Aalo began site work in January and built the CTR structure in a stunning 36 days, then installed reactor hardware manufactured at the company’s Austin, Texas, factory and shipped to Idaho on standard highways. The transportation considerations pushed Aalo to design a reactor large enough to produce customer-relevant power but small enough to move through repeatable factory and road transport channels, he noted.
Arafat said Aalo built the CTR hardware in its 40,000-square-foot Austin manufacturing facility, subjected the facility and its processes to DOE review, and rapidly installed most of the reactor hardware after it arrived in Idaho. The CTR campaign, he said, tested whether Aalo could construct a reactor building, fabricate reactor hardware in a factory, assemble fuel bundles, install the reactor in the field, and stand up the operating programs required for DOE-authorized nuclear work. Aalo said it is now expanding into a one-million-square-foot factory to apply assembly-line manufacturing to reactor production and support mass production of the Aalo Pod.
Four DOE-Authorized Criticalities, Four Different Reactor Cases
Aalo’s criticality capped a month in which DOE-authorized projects reached criticality across different reactor concepts, sites, fuels, and authorization pathways. Antares Nuclear’s Mark-0 reactor reached criticality at INL on June 4, becoming the first advanced reactor to do so under DOE’s Reactor Pilot Program. Mark-0 is a high-assay low-enriched uranium (HALEU), TRISO-fueled, sodium heat-pipe-cooled microreactor built as a test platform for Antares’ deployable nuclear power concept.
Valar Atomics’ Ward 250 reactor reached criticality at the Utah San Rafael Energy Lab in Emery County on June 18, becoming the second Reactor Pilot Program project to reach the milestone and the first outside the national laboratory system. Ward 250 is a TRISO-fueled modular high-temperature gas reactor (HTGR) using helium coolant, and Valar began power ascension after reaching criticality.
Deployable Energy’s Unity reactor reached zero-power criticality at INL on June 30 through DOE’s Nuclear Energy Launch Pad initiative rather than the Reactor Pilot Program. Unity is a water-moderated, helium-cooled microreactor using 4.95% enriched low-enriched uranium dioxide fuel and commercially available materials. Deployable moved from project kickoff to a delivered reactor, delivered fuel, and readiness for criticality in roughly 150 days, using an existing neutron radiography space at INL’s North Beam Station for a zero-power campaign focused on validating the physics basis for its 1-MWe-class Unity Nuclear Battery platform.
—Sonal C. Patel is a POWER senior editor (@sonalcpatel, @POWERmagazine).
Editor’s note: This is a developing story and will be updated as more details become available or are confirmed/clarified.
