First privately developed U.S. nuclear reactor in over 40 years achieves first criticality point

Key points

• A new small nuclear reactor backed by the U.S. government achieved first criticality, making it the first such feat by a private company in more than four decades.
• First criticality is the point at which a nuclear reactor achieves a self-sustaining nuclear chain reaction for the first time, enabling it to produce energy in a controlled manner and paving the way for future power generation.
• The Mark-0 demonstration reactor, developed by Antares Nuclear under the Reactor Pilot Programme of the U.S. Department of Energy, is the first among such multiple advanced units to go critical before July 4, when America celebrates the 250th anniversary of its independence.
• The Department of Energy described the milestone, achieved on Thursday (June 4, 2026), as "rebirth of America’s nuclear industry" and a "historic moment" for U.S. energy.
• "For the first time in more than four decades, a new privately developed non-light-water reactor has reached...