Small modular nuclear reactor reaches criticality in first test

Antares Nuclear completed a zero-power criticality test for its Mark-0 demonstrator reactor at Idaho National Laboratory on June 4, 2026, becoming the first company in the Energy Department's Reactor Pilot Program to successfully complete a fueled criticality test. This milestone occurs when a nuclear chain reaction becomes self-sustaining enough to produce a steady release of energy. The successful test confirms the reactor can operate safely and establishes a basis that would allow subsequent reactors to produce electricity in 2027 and beyond.

Source: Ars Technica | 2026-06-05