Page "Nuclear reactor" Paragraph 37

:::: Using lead as the liquid metal provides excellent radiation shielding, and allows for operation at very high temperatures.

Also, lead is ( mostly ) transparent to neutrons, so fewer neutrons are lost in the coolant, and the coolant does not become radioactive.

Unlike sodium, lead is mostly inert, so there is less risk of explosion or accident, but such large quantities of lead may be problematic from toxicology and disposal points of view.

Often a reactor of this type would use a lead-bismuth eutectic mixture.

In this case, the bismuth would present some minor radiation problems, as it is not quite as transparent to neutrons, and can be transmuted to a radioactive isotope more readily than lead.

The Russian Alfa class submarine uses a lead-bismuth-cooled fast reactor as its main power plant.