Lead has some nice benefits: Provides shielding or reflection (depends on the reactor spectrum) High boiling point Doesn't have the same chemical reactivity

Lead has some nice benefits: Provides shielding or reflection (depends on the reactor spectrum) High boiling point Doesn't have the same chemical reactivity as other liquid metals like sodium or NaK But, it needs to be balanced with the drawbacks: High density. Liquid sodium is less than a tenth of the density of liquid lead. This becomes more of a problem the larger the reactor gets due to seismic loading. Poorer heat transfer qualities than sodium (which is why liquid metals have mostly stuck to sodium) Must be designed to flow with low velocities due to corrosion/erosion* *This can drastically affect the size of the piping or flow passages required for cooling. Flowing above a certain velocity will erode and destroy pumps, piping, etc. A low performance reactor won't be as highly impacted as a high performance reactor that needs to flow as much coolant as fast as possible. The Russians have done a lot of work in this area.