Earlier today, on a site in Oak Ridge, Tenn., that was formerly home to the K-33 Gaseous Diffusion Plant, Kairos Power marked the start of construction on its low-power demonstration reactor. Named Hermes, the 35-MWt test reactor claims status as the first Gen IV reactor to be approved for construction by the Nuclear Regulatory Commission and the first non–light water reactor to be permitted in the United States in more than 50 years.