US builds first 4th generation nuclear reactor

Kairos Power begins construction of the Hermes low-power test reactor in Oak Ridge, Tennessee, which uses a TRISO gravel bed design and molten fluoride salt cooling for safer operation.

Design of the Hermes low-power test reactor facility. Photo: Kairos Power

Hermes is the first and only Generation 4 reactor approved for construction by the US Nuclear Regulatory Commission, Interesting Engineering reported on August 1. According to Kairos Power, it is also the first non-light water reactor licensed in the US in more than 50 years and is expected to be operational in 2027. The Hermes reactor project is supported by the US Department of Defense's Advanced Reactor Test Program.

The Hermes reactor under construction is part of a program to develop a commercial reactor that Kairos Power could deploy within the next decade, according to the Office of Nuclear Energy. Hermes will use a TRISO gravel bed design with molten fluoride as a coolant to demonstrate the ability to produce clean, low-cost heat.

The U.S. Nuclear Regulatory Commission granted a construction license for the project last December. The U.S. Department of Energy will invest $303 million to support the design, construction, and commissioning of Hermes through its Advanced Reactor Test Program. Kairos Power is working with Los Alamos National Laboratory to produce TRISO gravel fuel for the reactor. The company also has a development cooperation agreement with the Tennessee Valley Authority to provide engineering, operations, and licensing support.

Kairos Power recently completed molten salt testing of its first Engineering Test Unit (ETU) at its Albuquerque, New Mexico, production facility. The system is the first of three being built to help design, construct, and operate Harmes. The company is currently building a second ETU in Mexico and will partner with Barnard Construction on a third system located adjacent to the Hermes reactor site.

Instead of using water like a conventional nuclear reactor, Kairos Power’s reactor uses molten fluoride salt as a coolant. Molten fluoride salt has excellent chemical stability, the ability to transfer heat at high temperatures, and the ability to retain fission products that may be released from the fuel. In addition, Kairos Power’s reactor uses a fully ceramic-coated fuel that maintains structural integrity even at extremely high temperatures. This fuel will not be damaged even at temperatures much higher than the melting point of conventional metal reactor fuels.

The company says its reactor has a number of passive safety features, meaning it does not require electricity to remove heat from the core after shutdown. The reactor has an exceptionally large safety margin based on the fuel and coolant combination, allowing emergency cooling to rely on basic physics rather than engineering systems. According to data provided by Kairos Power, the KP-FHR reactor can produce 140 MW of electricity at a net efficiency of 45%.

China has also recently begun expanding the world's first fourth-generation nuclear power plant, the Shidaowan High-Temperature Gas-Cooled Nuclear Power Plant (HTGR) in eastern China's Shandong Province. Equipped with the world's first high-temperature gas-cooled reactor, the plant began commercial operation last December. When completed, it is expected to generate 20 billion kilowatt hours per year, increase the heat supply area by 20 million square meters, and benefit 600,000 local residents.