Antares Industries Moves Closer to Microreactor Criticality Test at Idaho National Laboratory
IDAHO FALLS, Idaho — Advanced nuclear energy company Antares Industries cleared a major regulatory milestone this week, receiving U.S. Department of Energy approval of the Documented Safety Analysis for its Mark-0 demonstration microreactor at Idaho National Laboratory. The approval puts the California-based company on track to achieve criticality with the reactor before July 4, a target tied directly to a Trump administration executive order designed to accelerate advanced reactor development in the United States.
The DOE’s acceptance of the Documented Safety Analysis — approved April 6 — reflects the federal agency’s formal acceptance of the final design for the Mark-0 reactor and the safety case supporting it. Antares will now enter the DOE Readiness Review process, the final phase before the agency authorizes startup of the reactor at INL’s Materials and Fuels Complex.
A Presidential Priority at INL
Tom Mancinelli, head of federal strategy and policy for Antares Industries, said the company’s accelerated path to this point began when President Trump signed an executive order last May directing the Department of Energy to create a pilot program to fast-track commercial licensing for advanced reactors and microreactors. The order set a goal of achieving criticality in at least three advanced reactors by July 4.
“When that executive order came out, we as a microreactor developer, decided this was perfect for us,” Mancinelli said. “We applied for the program and in August 2025, we were selected as one of the 11 companies to advance.”
That selection placed Antares into a formal working relationship with the Department of Energy, which took on the role of primary regulator for the accelerated timeline. The company began detailed technical meetings with experts from DOE and INL in mid-September, followed by additional sessions with representatives from the Department of Army and the Nuclear Regulatory Commission.
“After about 3,000 pages of documentation and 150 or so detailed technical questions, we were working our way through the DOE’s review process,” Mancinelli said. The DOE approved a preliminary safety analysis document in January before granting full Documented Safety Analysis approval this month.
Jason Andrus, Antares’ licensing director, met with Dr. John C. Wagner, Idaho National Laboratory director, at Antares’ test facility at INL in March, underscoring the close coordination between the company and the nation’s premier nuclear research institution.
What Criticality Means and What Comes Next
For a microreactor, criticality — as defined by the Nuclear Regulatory Commission — is the normal operating state in which the nuclear chain reaction becomes self-sustaining. In practical terms, it means the number of neutrons produced equals the number lost or absorbed, creating a stable, steady-state level of power generation where each fission event produces enough neutrons to cause exactly one more fission.
The Mark-0 demonstration at Building 793 of INL’s Materials and Fuels Complex, a specialized facility used for advanced defense and space-related reactor technologies, will validate reactor physics, neutronics models, and the instrumentation and control systems planned for use in the company’s next-generation Mark-1 electricity-producing reactor, which is targeted for 2027.
Fuel fabrication for the company’s first reactors has been underway through BWX Technologies since October 2025, using high-assay low-enriched uranium — known as HALEU fuel — secured through a DOE allocation.
“We will immediately gather the data from this criticality test which will show how neutrons move to the core of our reactor,” Mancinelli said. “It’ll show the physics of our design to help us understand how our control drums operate.”
Jordan Bramble, CEO and founder of Antares, described the moment as a culmination of sustained effort and partnership. “We are entering the final innings, and that’s incredibly exciting,” Bramble said. “Getting here was only possible with strong support from our partners at Idaho National Laboratory and BWXT, and leadership at DOE, along with relentless work from the Antares team.”
Antares maintains two full-time employees at its office in the Rogers Building in Idaho Falls, with a broader staff of approximately 10 that Mancinelli said will continue to grow as the criticality date approaches and final checks and procedures are completed.
Following the Mark-0 demonstration, Antares plans to use the same test facility and fuel batch for its Mark-1 reactor development in 2027, with initial deployments for defense and space customers targeted for 2028. Antares is headquartered in Torrance, California.
What Comes Next
Antares has entered the DOE Readiness Review process — the final regulatory phase before the department authorizes the Mark-0 reactor startup. The company has an internal target date ahead of July 4 that it has shared with INL partners but has not yet made public. Officials said they expect to disclose that date in the near future. For ongoing coverage of energy development at Idaho National Laboratory and statewide energy policy, visit idahonews.co and IdahoNewsNetwork.com.