Sodium-cooled fast reactors (SFRs) have the most extensive operational history of any advanced reactor type, with dozens of experimental and prototype reactors built and operated worldwide since the 1950s. Liquid sodium is an excellent coolant: it transfers heat efficiently, operates at atmospheric pressure (eliminating the need for thick pressure vessels), and has a boiling point of 883 degrees Celsius — well above typical operating temperatures. The fast neutron spectrum enables both breeding of new fissile material and burning of long-lived nuclear waste.
TerraPower's Natrium reactor is the highest-profile SFR under active development. Backed by Bill Gates and supported by DOE cost-sharing, Natrium pairs a 345 MW sodium-cooled fast reactor with a molten salt energy storage system that can boost electrical output to 500 MW during peak demand. The project, sited in Kemmerer, Wyoming at a retiring coal plant, is one of the most advanced SMR construction efforts in the United States. GE Hitachi's PRISM reactor is another well-known SFR design.
The primary disadvantage of sodium cooling is sodium's vigorous chemical reactivity — it burns when exposed to air and reacts explosively with water. This requires careful engineering of secondary systems and leak detection. Historical SFR projects like France's Superphenix and Japan's Monju experienced sodium leak incidents that set back the technology's reputation. Modern designs incorporate improved materials, leak detection systems, and intermediate heat transfer loops to mitigate these risks while retaining sodium's excellent thermal properties. For deeper coverage, see DeepTechIntel's nuclear section.