Authored by Steffan Szumowski via The Nuclear Review,

On July 1, the DOE announced that Deployable Energy had achieved criticality with its Unity reactor design at Idaho National Laboratory (INL). That milestone meant the DOE had officially met the target of bringing three privately funded, advanced reactor projects to criticality before July 4th.

But, just meeting the goal wasn’t good enough.

     “And one for Rickover…”

Aalo Atomics has issued a statement with the DOE that they reached criticality at the last second (12:20 am on the 4th of July), bringing the final count to four reactors! What began as an aggressive federal target quickly became a demonstration that the advanced nuclear sector can move from design, to fueled testing, to sustained fission on timelines that seemed unrealistic just last year.

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The starting point was Executive Order 14301, “Reforming Nuclear Reactor Testing at the Department of Energy.” Among other changes, the order directed the DOE to create a reactor testing pathway for privately funded advanced reactor developers. 

The DOE followed with the Reactor Pilot Program (RPP), which selected 11 reactor projects for accelerated testing:

Over the next several months, the developers raced to iterate through their designs with the goal of achieving criticality before America’s birthday. With the deadline finally coming to pass and the dust settling, here’s a review of the four projects that answered the call to action:

#1 Antares Nuclear

Reactor: Mark-0

Antares was the first company to achieve criticality with their heat pipe-cooled design. The DOE described it as the first privately developed non-light-water reactor to reach criticality in the US in more than four decades. ANS also noted that it was the first novel reactor to achieve criticality at INL in more than 50 years.

Compared with light-water (NuScale), heavy-water (CANDU), gas-cooled (Valar), sodium-cooled (Aalo), and molten-salt reactors (Kairos Power), heat-pipe reactors have the thinnest historical nuclear operating record. The industry has decades of fleet experience with water reactors, meaningful legacy experience with gas reactors, hundreds of reactor-years with sodium reactors, and at least a landmark operating experiment for molten salt.

Heat-pipe reactors sit on a much thinner nuclear experience base. The underlying heat-transfer technology is proven in other industries, especially electronics and aerospace, but the reactor application is still relatively new. Antares can point to KRUSTY as an important proof-of-concept for heat-pipe-cooled fission systems, but it cannot lean on the same kind of fleet experience that supports light-water reactors, gas reactors, or sodium-cooled designs.

Antares’ CEO Jordan Bramble remarked during the announcement of their criticality milestone being reached: 

     “We were never once the first company to submit a regulatory document, but we were always the first to get it approved.”

#2 Valar Atomics

Reactor: Ward 250

Valar achieved criticality next with their TRISO-fueled, helium-cooled, graphite-moderated high-temperature gas reactor (HTGR). Unsatisfied with second place for the race to criticality, the company pushed Ward-250 higher to produce both heat and electricity before any other reactor developer.

The electricity from the reactor was used to power an NVIDIA Spark, which ran a website that the company used for hosting a small game and a page for fans to order some merchandise from, until the reactor was eventually shut down after a few days of operations.

Valar also took a step further and performed a reactor safety test on a live stream. The company had the reactor at power, producing heat, and then rapidly shut down the reactor and simulated a loss of electricity to demonstrate the reactor would safely cool itself off with no human intervention.

#3 Deployable Energy

Reactor: Unity

Who? They’re not even in the RPP?

Deployable was the third reactor to reach criticality and count towards the “three by the fourth” goal from the executive order, but it’s the first reactor to do so outside of the RPP. I wrote about the new program Deployable is operating under, the Nuclear Energy Launch Pad (NELP), when it was first announced back in March of this year:

     "The Launch Pad is the program to be permanently put in place as the successor to the Department of Energy’s reactor pilot program (RPP) and fuel line pilot program (FLPP) … Companies will be able to utilize over 2,000 acres of pre-designated land at INL for projects including “advanced reactors, fuel fabrication, recycling, enrichment and other innovations”. The program will also be operated on a national scale with companies able to work with national labs from remote sites. Applications will be available annually with the initial request for applications expected to be released in a few months."

The initial selectees for the program were announced April 27th and included General Matter, Radiant Nuclear, NuCube Energy, and Deployable Energy. The fact that Deployable Energy was able to achieve criticality ahead of some of the projects under the RPP, even though they started several months later, is remarkable.

INL director John Wagner put out a post stating Deployable achieved criticality in “roughly 150 days after project kickoff.”

#4 Aalo Atomics

Reactor: Aalo-X Critical Test Reactor

Aalo is the first liquid-sodium microreactor developer in this batch, but their critical test reactor configuration does not have any sodium flowing through it. The next reactor that they are actively constructing will have a fully operational sodium cooling loop. Aalo-X CTR is the physics testing platform and the first step towards the commercialization of their thermal spectrum, sodium-cooled, UO2-fueled power plant.

The company intends to connect five of their reactors into the Aalo Pod configuration, which will be able to supply up to 50 MWe for data centers and other applications.

From Yasir Arafat, President and CTO, Aalo Atomics:

     “The hardest problem in nuclear was never the physics, our country simply forgot how to build”

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It’s one of the biggest wins the American nuclear industry has enjoyed in decades. 

No, bringing a pilot-scale microreactor critical is not the same thing as building an AP1000 at commercial scale. The technical, financial, regulatory, and project-management challenges are different. But that distinction should not erase the significance of what has happened under the RPP and NELP. After years of the US nuclear sector being defined publicly by the painful lessons of Vogtle and VC Summer, these projects show that nuclear can still build, test, and advance new hardware when the opportunity is structured correctly.

Initial criticality is not commercialization, and it is not a guarantee that any one of these companies will ultimately succeed at scale. But it is a real milestone. It means designs have moved from paper reactors to physical systems capable of sustaining a nuclear chain reaction.

And there is still more to come!! Multiple other companies are still pursuing initial criticality, which will allow them to keep iterating, validating their designs, and preparing for eventual commercialization under the NRC. That includes companies like Oklo, Radiant, and Last Energy.

The microreactor industry has shown what structured opportunity can unlock. The large reactor industry needs its own version of that momentum.