Lead is a heavy metal that is denser than most common materials. Lead is soft and malleable, and has a relatively low melting point. Lead is widely used as a gamma shield. Major advantage of lead shield is in its compactness due to its higher density. It has high corrosion resistance, malleability, unusual electrical properties, and the ability to form useful alloys. Representative uses include: shielding agent against x-ray and gamma radiation; diecast grids for batteries; coatings that prepare surfaces for soldering. On the other hand, lead is extremely toxic and poses certain environmental hazards.
Lead forms a wide range of low-melting alloys and readily alloys with tin in all proportions, forming the tin-lead solders used widely in industry. In nuclear industry, lead and lead-bismuth alloys can be used as a reactor coolant.
Lead and Lead-bismuth Eutectic
Lead, lead-bismuth eutectic, and other metals have also been proposed and occasionally used. The lead-cooled fast reactor is a nuclear reactor design that features a fast neutron spectrum and molten lead or lead-bismuth eutectic coolant. Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead (44.5%) and bismuth (55.5%). Molten lead or lead-bismuth eutectic can be used as the primary coolant because lead and bismuth have low neutron absorption and relatively low melting points.
Melting and boiling points of lead and lead-bismuth eutectic mixture are:
- melting point – 327.5°C
- boiling point – 1749°C
- lead-bismuth – eutectic mixture
- melting point – 123.5°C
- boiling point – 1670°C
As compared to sodium-based liquid metal coolants such as liquid sodium or NaK, lead-based coolants have significantly higher boiling points, meaning a reactor can be operated without risk of coolant boiling at much higher temperatures. Lead and LBE also do not react readily with water or air, in contrast to sodium and NaK which ignite spontaneously in air and react explosively with water. Due to its denseness and high atomic number, lead and bismuth are also an excellent gamma radiation shield, while simultaneously being virtually transparent to neutrons.
On the other hand, lead and LBE coolant are more corrosive to steel than sodium or NaK eutectic alloy. This and the very high density of lead puts an upper limit on the velocity of coolant flow through the reactor due to safety considerations. Furthermore, the higher melting points of lead and LBE (327 °C and 123.5 °C respectively) may mean that solidification of the coolant may be a greater problem when the reactor is operated at lower temperatures.