A small nuclear reactor in Oak Ridge, Tennessee has become the first privately developed reactor in the United States to achieve a self-sustaining nuclear chain reaction in more than four decades.
The milestone, known as first criticality, was reached on June 5, 2026, at the Hermes reactor built by Kairos Power. The event marks the first time a privately funded and developed U.S. reactor has gone critical since the 1980s.
A molten salt reactor built for testing, not power generation
Hermes is not a commercial power plant. It is a 35 megawatt thermal test reactor designed to demonstrate the company's technology. Unlike conventional reactors that use solid fuel rods and water for cooling, Hermes uses a molten fluoride salt coolant and pebble shaped fuel elements. The reactor operates at low pressure, which Kairos Power says reduces complexity and safety risks.
The reactor is located at the East Tennessee Technology Park, a former government owned site that was once part of the Manhattan Project. Local officials and residents have followed the project closely because it represents a potential economic revival for a region with a deep history in nuclear research. The site has been redeveloped for clean energy industries, and the Hermes reactor is the first new reactor built there in decades.
Why this reactor matters to the town and the industry
For Oak Ridge, a city that grew up around the federal nuclear facilities of the Cold War, the Hermes reactor is a sign of renewed private sector interest in nuclear technology. The project has created construction and engineering jobs and has drawn attention from energy companies looking for carbon free power sources. The U.S. Nuclear Regulatory Commission issued a construction permit for Hermes in 2023, the first such permit for a non water cooled reactor in over 50 years.
Kairos Power plans to use data from Hermes to design a larger commercial reactor. The company has said it aims to produce affordable nuclear power that can complement renewable energy sources like wind and solar.
A long awaited step for advanced nuclear energy
The achievement comes at a time when the United States is seeking to expand its nuclear power capacity to meet climate goals. Most of the country's existing reactors were built in the 1970s and 1980s, and many are nearing the end of their operating licenses. New reactor projects have faced cost overruns and delays. The Hermes reactor is one of several advanced reactor designs that the U.S. government has supported through demonstration programs.
The reactor will now undergo a series of tests at increasing power levels. Operators will monitor the behavior of the molten salt coolant and the pebble fuel to confirm the system performs as designed. The data collected will inform the licensing and construction of future reactors.
