Antares hits criticality at Idaho National Laboratory, but power generation still isn’t on

On Thursday, Antares said the test reactor it placed at Idaho National Laboratory reached criticality. For decision-makers watching the small modular reactor race, this is a meaningful checkpoint with a frustrating catch: “criticality” means the nuclear reactions became self sustaining, not that the reactor had started generating power.

So what did Antares actually do, and why does it matter right now? It crossed a threshold that only a small set of reactor designs have been able to demonstrate, and it is the first new design to reach criticality in this specific regulatory and program context. Criticality is the physics green light that the chain reaction can maintain itself inside the hardware. It does not automatically translate to “grid-ready.” In other words, Antares cleared step one of a long staircase, and it also clarified that the stair exists.

This milestone lands in the middle of a broader push to accelerate US nuclear development. Just over a year ago, the Trump Administration issued an executive order aimed at speeding nuclear power in the United States. An ecosystem of startups has grown around different, typically smaller reactor designs, but only one reactor design has been fully licensed so far. Even more important for timelines, the source notes there are no plans to actually build instances of that licensed design. The executive order directed the Department of Energy to get three different reactor designs to reach criticality in a bit over a year.

Antares is now positioned as the first to clear that DOE-directed goal with a new design. That is strategically significant because “reach criticality” is a measurable, objective marker. It is also a marker that can change how capital markets and internal corporate boards think about schedule risk. When firms are competing for funding and government attention, the difference between “prototype exists” and “chain reaction runs” is huge. It means the core technical barrier has at least been approached successfully in test conditions, even if the reactor is not yet producing power.

The design story behind Antares is also a window into how the SMR industry tries to compress risk and complexity. Antares, like a number of companies, is basing its approach on a new fuel system called TRISO. The source is explicit about the logic: this fuel system takes some complexity and safety considerations out of the reactor design and pushes them into the fuel design. That matters because reactor safety and licensing are often driven by how systems behave under extreme conditions. If the fuel can be engineered to handle them, then the rest of the system may be simpler. In practice, TRISO is built around tiny pellets with a uranium oxide core.

Those pellets are surrounded by multiple layers of carbon. The carbon’s job is to moderate the energy of both neutrons and lighter nuclei released by fission reactions. Put plainly, it helps manage the reaction environment inside the fuel, not just the fuel chemistry at rest. Then the entire fuel stack is encased in a hard ceramic shell designed to withstand the highest temperatures produced by the encased uranium. This is the second-order engineering bet embedded in the milestone. If you can put more of the burden on the fuel materials, you may reduce the number of reactor-specific mechanisms you must perfect before the system can pass regulatory and safety scrutiny.

For investors and boards, there are two practical implications to keep front and center. First, the timeline attached to the DOE objective is now being recalibrated around “who can reach criticality and when.” Second, criticality is only the first technical proof point. Antares “isn’t ready to generate power yet,” per the source. That means next stages will still test whether the reactor can translate self-sustaining reactions into controlled output, with systems that can support power conversion and operational stability. Boards should treat this as momentum, not completion.

For peers running similar programs, the Antares announcement also clarifies what “progress” looks like in a world where only one design has been fully licensed and no plans exist to build it yet. The executive order’s goal is not about commercial deployment. It is about compressing the demonstration timeline. Antares reaching criticality suggests the industry can clear some foundational barriers faster than historic nuclear buildouts, at least in test settings. The strategic stakes are straightforward: whoever proves the next steps with the same credibility will attract better odds in both funding cycles and regulatory attention, while everyone else remains stuck selling potential without the physics receipts.