Antares Mark-0 achieves initial criticality, 1st private non-light-water US reactor
Big news for American energy: the Antares Nuclear Mark-0 test reactor just became the first privately developed, non-light-water reactor in the US to reach "initial criticality" in over 40 years.
This means it's officially able to sustain a nuclear chain reaction, a major step for testing its safety and design.
The milestone happened on June 4, 2026, at Idaho National Laboratory.
Antares uses HALEU TRISO fuel
Antares uses advanced HALEU fuel formed into TRISO fuel particles and has a cooling system with zero moving parts, making it reliable even under tough conditions.
It's part of a US Department of Energy program to boost innovation in nuclear technology.
Next up: Antares is set for deployment at Joint Base San Antonio by 2028, with Energy Secretary Chris Wright calling this a "Today's achievement is a historic moment for American nuclear energy," for US nuclear innovation.