[nandomrumber]

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

[pfdietz]

Bismuth leads to the production of polonium, which is extraordinarily dangerous.

[jabl]

Yes, some lead cooled reactor designs have used LBE, others pure lead. Though AFAIU so far the only lead cooled reactors that have actually been built and operated in production have used LBE. There is a pure lead cooled reactor under construction that should be started up in a few years if the current schedule holds.