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The First New INL Novel Nuclear Reactor in 50 Years Achieves Criticality.
This came in on one of my news feeds; it took place over a month ago:
Antares Mark-0 Becomes First Advanced Nuclear Reactor to Achieve Criticality Under DOE Pilot Program
Antares Nuclear Inc.’s Mark-0—a sodium heat-pipe-cooled microreactor fueled by high-assay low-enriched uranium (HALEU) tri-structural isotropic (TRISO) fuel compacts—has achieved zero-power criticality at Idaho National Laboratory’s (INL’s) Reactor and Critical Experiment (RACE) facility, becoming the first advanced reactor to reach that milestone under the Department of Energy’s (DOE’s) Reactor Pilot Program.
The development, announced on June 4, also marks the 53rd reactor built at the INL site since 1951 and the first novel reactor design to achieve criticality at the laboratory in more than 50 years, according to INL Laboratory Director John Wagner. The much-watched DOE Reactor Pilot Program, established under President Trump’s May 2025 Executive Order 14301, directs the DOE to accelerate reactor testing and to target at least three advanced-reactor criticalities by July 4, 2026.
“Criticality is the condition at which a nuclear fission chain reaction becomes self-sustaining,” Wagner explained in a LinkedIn post. “What Antares achieved is specifically zero-power criticality—the chain reaction was sustained at essentially no measurable energy output. This is not electricity generation. It is not full-power operation. It is proof that the system works: the scientific and engineering validation that every subsequent step depends on,” he wrote. “That distinction matters for context. It should not diminish what happened.”
From Startup to Criticality in Two Years
The zero-power criticality milestone marks a significant step for Torrance, California–headquartered Antares, which was founded in 2023 and has raised more than $140 million in private capital, including a $96 million Series B round that closed in December 2025. The company announced Jan. 26 that DOE had approved its Preliminary Documented Safety Analysis for Mark-0, calling the approval a key step toward fabrication, assembly, installation, and operation under the Reactor Pilot Program...
The development, announced on June 4, also marks the 53rd reactor built at the INL site since 1951 and the first novel reactor design to achieve criticality at the laboratory in more than 50 years, according to INL Laboratory Director John Wagner. The much-watched DOE Reactor Pilot Program, established under President Trump’s May 2025 Executive Order 14301, directs the DOE to accelerate reactor testing and to target at least three advanced-reactor criticalities by July 4, 2026.
“Criticality is the condition at which a nuclear fission chain reaction becomes self-sustaining,” Wagner explained in a LinkedIn post. “What Antares achieved is specifically zero-power criticality—the chain reaction was sustained at essentially no measurable energy output. This is not electricity generation. It is not full-power operation. It is proof that the system works: the scientific and engineering validation that every subsequent step depends on,” he wrote. “That distinction matters for context. It should not diminish what happened.”
From Startup to Criticality in Two Years
The zero-power criticality milestone marks a significant step for Torrance, California–headquartered Antares, which was founded in 2023 and has raised more than $140 million in private capital, including a $96 million Series B round that closed in December 2025. The company announced Jan. 26 that DOE had approved its Preliminary Documented Safety Analysis for Mark-0, calling the approval a key step toward fabrication, assembly, installation, and operation under the Reactor Pilot Program...
I am not familiar with this reactor type. New reactor designs are being put forward faster than I can keep up with them. I am familiar with TRISO and sodium coolants, but unaware of the combination being developed. To my mind, lacking any knowledge, this type of reactor should have an interesting neutron spectrum, shy of a full fast reactor, but not thermal either; something more at epithermal.
I would think the materials science issues would be interesting, TRISO compatibility with sodium metal. It would be something about which learning would be a worthy exercise.
It is a sign of our times that INL built 52 reactors of different types in less than 20 to 30 years, and has only built one since. One hopes that the data and reports on these 52 early reactors is still available. The need is urgent.