Small modular nuclear reactor reaches criticality in first test Startup Antares has announced its test reactor at the Idaho National Laboratory has reached criticality, a self-sustaining nuclear reaction, making it the first new design to do so. This reactor utilizes an innovative TRISO fuel system designed for enhanced safety and simplicity. While the Mark 0 reactor is currently only testing its core and not generating power, full system testing including electrical generation is planned for next year.

• Antares’ test reactor at Idaho National Laboratory has reached criticality.
• This is the first new nuclear reactor design to achieve this milestone.
• The reactor uses a TRISO fuel system with multiple protective layers.
• The design aims to prevent meltdowns by containing the fuel pellets.
• A graphite sheath is used to slow down escaping neutrons.
• The current Mark 0 reactor is for testing and safety data generation, not power generation.
• Full system testing, including power generation, is expected next year.
• The company is collaborating with the Department of Defense’s Project Pele and NASA. Continue reading https://arstechnica.com/science/2026/06/first-us-test-of-modular-reactor-reaches-criticality/