Torrance, Calif.-based Antares Nuclear reached zero-power criticality at the Department of Energy’s Idaho National Laboratory (INL) Thursday, becoming the first reactor company to reach this milestone under the reactor pilot program.
Antares’ Mark-0 test reactor reached criticality exactly one month before DOE’s reactor pilot program’s deadline of July 4. The company additionally becomes the 53rd reactor to be at INL and the first reactor to be at the site since 1951.
Antares CEO Jordan Bramble said in a Thursday LinkedIn post that the company was able to move fast on reaching criticality as it focused on engineering rigor pertaining to safety requirements and “a focus on results before optics.”
Bramble did not elaborate on what “a focus on results before optics,” meant in the LinkedIn post.
“This reactor test validates our reactor physics and tests our control system,” Bramble said. “We went from a clear sheet design to a reactivity control system in a reactor in just nine months. We just debugged our entire supply chain, and we performed a rapid shakedown of the recently updated DOE 1271 authorization standard; something that has never been accomplished before.”
Antares, founded in 2023, was the first nuclear company among the 10 companies selected under the reactor pilot program to receive DOE approval for its preliminary design safety analysis and documented safety analysis. Bramble said the company now shifted its attention to the company’s Mark-1 reactor that will produce electricity.
During an April webinar hosted by the American Nuclear Society called “Path to Criticality”, Bramble said the company was in a strong position to reach the July 4 deadline. Bramble noted that the company already had a facility at INL, a pathway to fueling the reactor and had a partnership with DOE.
The company’s pathway to fueling its reactor included a partnership with BWX Technologies. BWXT has been working with Antares tri-structural isotropic – or TRISO fuel since October.
Bramble said he believes that Antares was able to move on TRISO fuel needs because it was able to leverage BWXT’s specification and fabrication that was also taking place under its fuel development for Project Pele.
Project Pele a 1.5-megawatt transportable microreactor that BWXT is constructing for the U.S. Army’s Strategic Capabilities Office. Antares modeled its reactor fuel on the TRISO fuel compacts BWXT made for Project Pele, according to a Thursday BWXT press release.
Additionally, BWXT said that it processed the high-assay low-enriched uranium (HALEU) feedstock material used to manufacture the Antares TRISO fuel compacts from scrap materials provided by DOE’s National Nuclear Security Administration.
“BWXT is proud to work with Antares and deliver the fuel necessary for this important milestone at the Idaho National Lab and for the future,” BWXT’s President for Government Operations Joe Miller said. “Antares is moving quickly to progress from concept to criticality, and we are proud to supply this team with the TRISO needed to do so.”
Over the past year, expectations for DOE’s pilot reactor program shifted a lot from the possibility one to two reactors could reach criticality by July 4 at one point to up to four reactors hitting that deadline. In Antares becoming the first to do so, DOE officials welcomed the milestone.
“For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in the United States,” Energy Secretary Chris Wright said in DOE’s press release. “I look forward to seeing continued progress in the American nuclear renaissance,” Wright went on to say.
“The skeptics didn’t believe President Trump’s Reactor Pilot Program could achieve criticality in less than a year,” said Assistant Secretary of Nuclear Energy Ted Garrish. “Today, we celebrate the first of the pilot projects to reach criticality and the people who rolled up their sleeves to shape the future of nuclear energy in the United States,”
Under President Donald Trump’s May 23, 2025 executive order “Reforming Nuclear Reactor Testing at the Department of Energy”, DOE created the reactor pilot program to give private companies an opportunity to build advanced nuclear test reactors at DOE’s national laboratories and other sites.
While Antares hit criticality first, DOE still seeks to have at least three reactors reach zero-power criticality by next month. Valar Atomics and Aalo Atomics are nuclear companies to have their documented safety analyses approved. Other companies such as Last Energy and Radiant Nuclear have gotten their preliminary design safety analyses approved.