US Antares Achieves Criticality for Privately Developed Microreactor, First in 40 Years

The microreactor 'Mark-0' developed by US-based Antares Nuclear has reached criticality. A reactor's critical state means its nuclear fission chain reaction can sustain itself, and it is considered a significant milestone in reactor development. Antares Nuclear explained that achieving criticality validates key reactor physics phenomena and produces data that will contribute to future reactor development.

Antares is one of the first private companies to bring an advanced reactor to criticality through the DOE's reactor pilot program. The reactor reached initial criticality under DOE approval, following the establishment of the DOE pilot program after a 2025 executive order. The Department of Energy also confirmed the achievement, stating that Antares Nuclear's Mark-0 microreactor is the first privately developed non-light water reactor to achieve criticality in the United States in over 40 years. This achievement was made under the DOE pilot program established following President Donald Trump's 2025 executive order, which aims to accelerate nuclear development.

Antares CEO Jordan Bramble stated that the company has met the first of its 2026 criticality achievement commitments on schedule. He added that this is an example of government and industry collaborating to achieve significant results. Antares Nuclear indicated that this criticality achievement will support future commercial reactor licensing processes.