The United States has marked a significant step in its nuclear energy resurgence as Antares Nuclear successfully demonstrated fuelled criticality for its Mark-0 advanced reactor at Idaho National Laboratory (INL). The achievement was completed under the Department of Energy’s Reactor Pilot Program, a federal initiative designed to accelerate next-generation nuclear technologies. The criticality test represents the first time the Mark-0 advanced reactor has reached a self-sustaining nuclear chain reaction under controlled conditions. The milestone confirms that the reactor’s core design functions as intended and provides an important validation point for future licensing and commercial development. As a result, the demonstration lays the groundwork for follow-on reactors capable of generating electricity later this decade. It also positions the Mark-0 advanced reactor among the first new-generation nuclear systems expected to meet the accelerated deployment targets outlined in President Donald Trump’s May 2025 executive order to expand US nuclear capacity. Commenting on the landmark achievement, Energy Secretary Chris Wright said. “For the first time in more than four decades, a new privately developed non-light-water reactor has reached criticality in …