Russia has had sodium-cooled reactors [0] for some time. America experimented with them at least back to the 1950s, with the USS Seawolf (SSN 575, not SSN 21) h

Russia has had sodium-cooled reactors [0] for some time. America experimented with them at least back to the 1950s, with the USS Seawolf (SSN 575, not SSN 21) having a sodium-cooled reactor (S2G) for her first few years. Russia also had lead-bismuth reactors, at least militarily, with the Alfa-class. [0]: https://www.neimagazine.com/news/mox-use-at-russias-bn-800-reactor-confirms-reliability-of-the-technology-11278025/ https://www.neimagazine.com/news/mox-use-at-russias-bn-800-r...