Overview
- Antares confirmed on Thursday that its Mark-0 prototype reached initial criticality at Idaho National Laboratory, producing a controlled, self-sustaining fission chain reaction for the first time.
- The Mark-0 uses liquid sodium instead of water for cooling, which raises thermal-efficiency benefits and avoids steam-overpressure failure modes while creating distinct safety needs because sodium reacts violently with air and water.
- The reactor test was conducted under a DOE special authorization as part of the Reactor Pilot Programme that selected 11 microreactor projects to push several designs to criticality by July 4.
- Antares says it will use lessons from Mark-0 in further tests and aims to field microreactors for U.S. military sites by September 2028, but commercial deployment requires formal NRC licensing.
- Officials call the milestone a major step in reviving U.S. advanced nuclear work, marking the first privately developed non-light-water reactor to reach criticality in decades and offering potential for more resilient power at bases and strategic sites.