Aalo Atomics’ Test Reactor Reaches Criticality at INL, Fourth DOE

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. In a video tour posted in May 2026, Yasir Arafat, Aalo’s co-founder, president, and chief technology officer, walks through Aalo’s Critical Test Reactor (CTR) building, describing the 60-by-60-by-60-foot structure, reactor vessel, shielding, control rod drive mechanisms, reactor trip system, and neutron monitoring system (NMS). He also explains Aalo’s approach to the criticality process, in which fuel assemblies are added incrementally and neutron detector readings are compared against model predictions until the core reaches k = 1, the point at which neutron production and neutron loss are balanced. Courtesy: Aalo Atomics 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. This week, Aalo announced it had already begun work on the second nuclear reactor on the Aalo-X campus at INL. 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 a video released with the criticality announcement, Loszak said excavation and earthwork for the second full-scale reactor had been completed the week before the milestone and that the company was preparing to pour first concrete. Aalo expects to finish the reactor by the end of 2026 and “make electrons at commercial scale” in 2027, he said. The data center pairing is also tied to Aalo’s broader digitalization plan. In March, Microsoft said it was working with NVIDIA and Aalo on AI-for-nuclear tools intended to streamline permitting, accelerate design, and optimize operations across site permitting, design, construction, and continuous operations. Microsoft said its Generative AI for Permitting solution reduced Aalo’s time-intensive permitting process by 92%, with estimated savings of $80 million a year, and described the collaboration as focused on making complex nuclear project work “repeatable, traceable, secure, and predictable.” 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,” he said. Supply Chain, Factory Fabrication, and Field Assembly As Aalo explained, CTR’s fuel supply was also a crucial component of the 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. While the CTR uses 4.95% enriched uranium dioxide fuel, Arafat said Aalo built enough low-enriched uranium dioxide fuel assemblies “to ultimately produce 30 MWth,” from which the company can extract 10 MWe. 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. 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, built the CTR structure in 36 days, and then installed reactor hardware manufactured at the company’s Austin, Texas, factory and shipped to Idaho via 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 at 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 the hardware 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. The next Reactor Pilot Program candidate poised to achieve criticality is Oklo Isotopes’ Groves Isotope Test Reactor in Texas, an isotope-production test reactor that will support Oklo’s isotope business and is intended to help establish a domestic supply chain for critical isotopes used in cancer diagnosis and treatment, advanced manufacturing, scientific research, space exploration, and national security applications. Oklo said on July 1 that the DOE had approved the Documented Safety Analysis (DSA) for Groves, moving the project from the documentation phase into DOE’s final pre-startup review. Remaining steps entail the DOE’s readiness review and startup approval, after which the facility would be authorized to receive and load nuclear fuel, conduct startup testing, and proceed toward first criticality. Oklo said it is targeting first criticality for Groves in July 2026. Radiant Nuclear is also approaching fueled testing at INL. Radiant took possession of INL’s Demonstration of Microreactor Experiments (DOME) facility—the repurposed Experimental Breeder Reactor-II containment structure, capable of handling up to 20 MWth—on April 1, 2026, for a one-year fueled test campaign. DOE approved Radiant’s Demonstration Authorization Request for Kaleidos (DARK), the second of three safety submittals in DOE’s authorization pathway and designed to meet the intent of a preliminary documented safety analysis, in February 2026, which Radiant described as the first full-power test approval granted under the program. On July 1, Radiant said it had received its first tranche of tri-structural isotropic (TRISO) fuel at DOME, fabricated by Standard Nuclear in Oak Ridge, Tennessee, to Radiant’s specifications. The fuel will power Radiant’s Kaleidos reactor through a five-phase test program this summer, progressing through zero-power criticality, 1 MW thermal, full power, and full heat before at least 150 hours at full power without operator intervention. Radiant’s Kaleidos Demonstration Unit is a 1-MWe helium-cooled, TRISO-fueled high-temperature gas reactor packaged in a single shipping container, and the company says the campaign is intended to support manufacturing and customer delivery by 2028. —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.