goingcritical:antares-mark-0United States · Microreactor

Antares Mark-0

Developer: Antares Nuclear

Site: Idaho National Laboratory, Idaho

Status: Critical since June 4, 2026 critical

Summary:

First reactor to reach criticality under the DOE Reactor Pilot Program (EO 14301); zero-power fueled criticality — no power conversion or heat removal systems

Details

DesignZero-power unit of the R1 sodium heat-pipe-cooled, HALEU TRISO-fueled microreactor
CapacityUndisclosed

Milestone history

  1. Announced August 12, 2025
  2. Construction start —
  3. Fuel load —
  4. First criticality June 4, 2026

References

  1. DOE, first advanced reactor criticality under Reactor Pilot Program
  2. ANS Nuclear Newswire, Antares achieves zero-power criticality at INL (June 4, 2026)
  3. World Nuclear News, first criticality for US microreactor under DOE programme
  4. DOE, initial Reactor Pilot Program selections (Aug 12, 2025)