The Mark-0 is one of 11 microreactor projects selected by the US Energy Department as part of an initiative to bring several models to criticality by this year’s Jul 4 celebrations marking the country’s 250th anniversary.
US Energy Secretary Chris Wright hailed an “important milestone”.
The “achievement is proof that the US still leads the world in innovation – and that America’s nuclear renaissance is well underway”, Wright wrote on X.
Antares said it conducted the test at the Idaho National Laboratory research and testing facility.
The Mark-0 does not use water to cool the reactor like most US commercial power plants, but rather liquid sodium.
That improves efficiency and avoids the risk of overpressure and steam explosions like those seen in the 1986 Chernobyl disaster.
Sodium does require specific safety measures, however, because it ignites on contact with air and explodes on contact with water.
Antares said it planned to have operational reactors ready for US military sites by September 2028.
“We went from concept to a critical reactor, safely, in less than 12 months,” Antares CEO Jordan Bramble said.
To be commercially viable, the Antares reactor, which currently has a special authorisation from the Energy Department, requires approval from the US Nuclear Regulatory Commission.
Only one reactor design, by the start-up NuScale, has been given that green light so far.
The Energy Department said it expected other companies to reach criticality by Jul 4.
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