Comment by jabl
4 hours ago
There have been some sodium cooled designs that have used a closed cycle gas turbine using nitrogen as the working fluid for the secondary circuit, in order to avoid any issues with sodium-water reactions with a traditional steam Rankine secondary circuit.
There are also fast reactor designs using lead as the coolant rather than sodium. These are interesting, but less mature than sodium cooling. Sodium is better from a cooling and pumping perspective though.
Lead-bismuth eutectic.
A eutectic is an alloy that has a lower melting point than any of its components.
Lead-bismuth eutectic or LBE is a eutectic alloy of lead (44.5 at%) and bismuth (55.5 at%) used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative. It has a melting point of 123.5 °C/254.3 °F (pure lead melts at 327 °C/621 °F, pure bismuth at 271 °C/520 °F) and a boiling point of 1,670 °C/3,038 °F.
https://en.wikipedia.org/wiki/Lead-bismuth_eutectic