US-based Aalo Atomics has unveiled its completed Critical Test Reactor (CTR) at the Idaho National Laboratory (INL). The event was attended by representatives from the Department of Energy (DOE), INL and supply chain partners, including Paragon Energy Solutions and Amsted Graphite. Attendees toured the facility, viewing demonstrations of the control software and hardware, shielding systems, and the reactor itself.
The CTR facility, according to Aalo, serves as the “home” for the Aalo-X advanced reactor – a 10 MWe experimental sodium-cooled reactor unit. It is a “power-producing full-scale experimental nuclear power plant” designed to validate the technology for future commercial use. Critical Test Reactor is the functional name used by the company and DOE to describe the entire experimental installation and its primary mission: achieving its first self-sustaining nuclear chain reaction (criticality).
The company said it expects the reactor to achieve criticality “well before” the 4 July deadline. Aalo was one of 11 advanced reactor projects selected by DOE in August for support through its Nuclear Reactor Pilot Program, which aims to see at least three of them achieve criticality by 4 July 2026. This was in line with President Trump’s May 2025 Executive Order (EO) 14301, Reforming Nuclear Reactor Testing at the Department of Energy.
Aalo Atomics officially broke ground on that site on the Aalo-X Experimental Site in September 2025, specifically choosing a plot at the very edge of the INL boundary. By building on the “border,” Aalo could leverage INL’s existing infrastructure and security while maintaining the flexibility to move quickly. They completed the building in just 36 days and the reactor assembly in 40 days. The Aalo-X Experimental Site serves as the physical proof-of-concept for Aalo’s “land-and-expand” strategy, where they aim to sit reactors directly next to high-demand customers such as AI data centres.
While the reactor itself falls under DOE authorisation for testing (based on EO 14301), the proximity to the lab allowed for real-time collaboration with INL scientists on fuel performance and safety data. Aalo-X was manufactured at Aalo’s pilot factory in Austin, Texas, before being transported to and installed at the INL site. The test reactor is the precursor to the Aalo Pod, a 50 MWe XMR (Extra Modular Reactor) power plant purpose-built for data centres. Each fully modular Aalo Pod will contain five factory-built, sodium-cooled, Aalo-1 reactors, using low-enriched uranium dioxide fuel. The company is targeting commercial use by 2029.
The CTR is a zero-power criticality reactor designed specifically to validate the core physics and control systems of the Aalo-X experimental reactor. Because it operates at near-zero power levels (watts or kilowatts), it generates virtually no heat, which eliminates the need for complex cooling systems and allows for a safer, more flexible testing environment.
It comprises a scaled version of the Aalo-X core, featuring the same material stack intended for the commercial Aalo Pod. These include:
- Low-enriched uranium (LEU) dioxide fuel (with an enrichment level of 5%) – the fuel rods are supplied by Global Nuclear Fuel.
- A graphite moderator used to achieve a thermal neutron spectrum.
- Precise control rods and “shim” rods developed in-house to measure shutdown margins and reactivity worth.
- Instrumentation involving movable neutron detectors and flux wires used to map the axial flux profile and calibrate in-core monitoring systems.
The facility is a strategic bridge that allows Aalo to “measure twice and cut once” before powering up the 30 MWt Aalo-X reactor. Key experiments conducted at the CTR include:
- Reactivity Insertion Tests: Observing the reactor’s prompt response to small control rod movements to validate safety margins.
- Control Rod Calibration: Measuring the exact reactivity contribution of each rod for operational manoeuvring.
- Code Validation: Qualifying neutronics and reactor physics codes against real-world data rather than just simulations.
Aalo Atomics is utilising liquid sodium metal as the primary coolant for its microreactors, shifting away from traditional water-based systems, to achieve higher power density and simpler safety profiles. Aalo’s approach follows an iterative “build-test-learn” cycle to master sodium handling before the 2026 criticality goal. This includes:
- Sodium Test Loop (STL): A portable, skid-mounted loop used in Austin and at INL to qualify components such as electromagnetic pumps and heat exchangers under actual operating temperatures.
- Aalo-0 Prototype: A plant-scale non-nuclear unit designed to circulate 60,000 lbs of liquid sodium to simulate integral effects and de-risk thermal-hydraulic systems without using nuclear fuel.
- Aalo-X Experimental Reactor: A full 30 MWt reactor that uses a hybrid loop-pool configuration. It includes a primary vessel housing the core and a secondary sodium loop to transfer heat to a steam generator, keeping radioactive sodium isolated from the water system.
The use of liquid sodium provides several technical advantages for their “extra-modular” reactors (XMRs). Sodium is roughly 100 times more effective at removing heat than water, allowing Aalo’s reactors to produce up to 10 times more energy than traditional reactors of a similar size. Unlike water reactors that require high-pressure vessels, sodium remains liquid at high temperatures at atmospheric pressure, significantly reducing the risk of explosive failures and allowing for lighter, factory-built modules. Because the system is air-cooled, it requires no external water source, enabling deployment in arid regions or directly on-site at data centres.
The high thermal conductivity of sodium enables natural circulation for decay heat removal, meaning the reactor can safely cool itself down during a power loss without operator intervention or active pumps. Aalo is specifically testing for known sodium-related issues such as sodium purity management, leak detection, and potential “plugging” of equipment due to stagnant flow or improper heat tracing.
While many liquid sodium-cooled reactors are “fast reactors, Aalo Atomics has made a specific design choice to include a graphite moderator to slow the neutrons to operate on a thermal spectrum. This allows the use of LEU fuel while most fast reactors require HALEU (high-assay low-enriched uranium) enriched to between 5% and 20%. By sticking to a thermal spectrum, Aalo can use standard, commercially available fuel pellets, avoiding the supply chain bottlenecks currently facing HALEU. The NRC and international bodies have decades of data on 5% enriched fuel, simplifying the licensing process. Thermal reactors also have well-understood feedback loops, which Aalo uses to ensure the reactor is passively safe.
In Aalo’s (XMR) design, the integration of the graphite moderator and liquid sodium coolant is handled through a unique “stack” architecture that balances thermal efficiency with structural simplicity. Unlike traditional reactors that use a large open tank, Aalo organises the core into a vertical assembly. The core is composed of high-purity nuclear-grade graphite blocks precisely machined with vertical channels. Within these graphite blocks, Aalo places stainless steel cladding tubes inside which the uranium dioxide fuel pellets are sealed. The liquid sodium flows in the narrow “annulus” (gap) between the fuel pins and the graphite walls.
The sodium acts as the “glue” that pulls heat away from the fuel pins and transfers it into the graphite and eventually to the heat exchangers. Because sodium has such high thermal conductivity, it ensures the entire “stack” maintains a uniform temperature, preventing “hot spots” that could damage the fuel. Aalo’s design effectively uses the sodium for heat transport and the graphite for physics control.
Passive safety (Negative Temperature Coefficient) is ensured. As the temperature rises, the graphite and sodium expand. In Aalo’s thermal spectrum, this expansion naturally reduces the “reactivity” (the rate of fission), causing the reactor to power down automatically without human or computer intervention. Because sodium doesn’t boil until the entire “stack” operates at atmospheric pressure the graphite blocks are not under stress from high-pressure steam, which greatly extends the lifespan of the core.
This “block and tube” approach allows Aalo to factory-fabricate the core in sections in Austin, Texas, and then ship them to the site for final assembly, rather than building a massive pressure vessel on-site.
In a post on X, Aalo cofounder and CEO Matt Loszak said the nuclear fuel for the Critical Test Reactor “will be arriving any day now”. Aalo announced earlier in March that it had signed a fuel fabrication contract with Global Nuclear Fuel (a GE Vernova-led alliance with Hitachi Ltd and affiliate of GE Vernova Hitachi Nuclear Energy) for fuel to power Aalo-X.
Loszak noted that one step remains before the reactor can start up – “the final DOE approval to turn the reactor on, which we expect to receive soon”. He said he was proud to cut the ribbon on the reactor facility with the Manager of the Idaho Operations Office Bob Boston and co-founder Yasir Arafat.
He noted in his post: “Alongside the CTR unveiling, I also discussed the mission ahead: how we will drive Aalo from criticality to a full-power reactor purpose-built for AI data centres.” He said it happens in phases:
Project First Light – the Critical Test Reactor…. proving the nuclear core and our ability to control nuclear reactions.
Project Crucible – Building all parts of the power plant except for the nuclear core: the non-nuclear power plant, Aalo-0. This hardware is all about solving the sodium coolant challenge. “Sodium engineering is our crucible, and thus Aalo-0 is the next mountain Aalo will climb”.
Project Ascension – The full-power Aalo-X demonstration reactor. This is when Aalo ascends to delivering electricity and integrates that electrical output with data centre infrastructure. “The endpoint of these three missions is the Aalo-1, the purpose-built reactor for AI data centres.”
Loszak said the AI industry’s appetite for power is growing faster than the grid can feed it. “And we are optimistic that AI will deliver incredible futures for humanity…. Nuclear is the obvious answer. It’s the only carbon-free baseload technology with the energy density to co-locate with data centres anywhere, independent of weather, water access, or gas pipeline availability. We know our nuclear physics work; now we are organising around those physics and working to master them.”
