Antares Mark-0 Becomes First Advanced Nuclear Reactor to Achieve Criticality Under DOE Pilot Program
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Home Nuclear 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.
Antares began machining the Mark-0 graphite core on Jan. 12 at its Antares Prime facility, and fuel fabrication for its first reactors has been underway through BWX Technologies since October 2025 using HALEU secured through a DOE allocation. Antares says it holds agreements with the U.S. Air Force, Space Force, NASA, and the Defense Innovation Unit, and is advancing toward initial deployments for defense and space customers in 2028.
“Now that Mark-0 is critical, the real work is just beginning,” said Antares CEO Jordan Bramble in a LinkedIn post on June 4. “I want to reiterate how this fits into our larger roadmap to mature our technology to its commercial potential. This should be obvious, but the goal of a reactor is to sell electricity to customers.”
Following reactor physics experiments, Antares will execute “the next phase of our roadmap—sustained electricity production,” Bramble said. Antares is “able to move fast towards this milestone because we’ve already completed over 6 months of full-power thermal testing in an electrical prototype. We will perform version 2.0 of this in 2026. This is an easier, more iterative way to test, because there is no regulatory process, and you can disassemble to examine material effects.”
He added: “All of our iterative testing sets us up to produce electricity for 6+ months. Hundreds of days, not hundreds of hours. We’re able to test for longer and faster because we’ve designed our reactor around a proven, fully qualified fuel spec developed under Project Pele.”
From Mark-0 to Mark-1 to Power Warfighter in 2028
The Mark-0 is a small, high-temperature, sodium heat-pipe reactor configured specifically for zero-power criticality testing, according to a DOE Idaho Operations Office categorical exclusion determination. Unlike a power-producing prototype, the Mark-0 version, while “not equipped with power conversion or heat removal systems,” is designed to serve as a platform for validating reactor physics, reactivity control behavior, and system-level safety performance in operation, while producing no measurable thermal output.
But Mark-0 is only Antares’ first iterative step. Speaking during a March 31 American Nuclear Society webinar, Antares CEO Jordan Bramble said the first criticality test was “a stepping stone” toward the company’s “North Star” of an electricity-producing prototype reactor. He...