A coolant, or heat transfer fluid, is a fluid which flows through a device in order to prevent its overheating, transferring the heat produced by the device to other devices that utilize or dissipate it.

The use of lead coolant avoids the potential for fires and for water-coolant reactions associated with sodium. These designs offer temperatures substantially above those available from current water-cooled reactors, increasing thermodynamic efficiency. They take advantage of this feature as well as of passive safety features to reduce capital and operating costs, and they have lower fuel-cycle costs than current water-cooled reactors. They can operate as breeders or near-breeders to increase resource utilization and can ease the management of radioactive waste by consuming plutonium and transmuting minor actinides into stable isotopes.

Lead Coolant are used where high temperature stability is required, eg. some fast breeder nuclear reactors.