How is sodium used as a coolant in some nuclear reactors?

Molten sodium is used as an effective heat-transfer fluid in liquid-metal fast breeder reactors because of its excellent thermal properties. In these systems, the sodium flows through a tightly packed bundle of fuel in the reactor core to extract heat.12

Because sodium becomes radioactive when exposed to the neutron flux within a reactor, it is used in a multi-loop configuration to ensure safety. The primary sodium, which circulates through the reactor core, transfers its heat to a secondary, nonradioactive sodium loop via a heat exchanger. This secondary loop then serves as the heat source for a steam generator, which powers a turbine.32

To maintain the system, the molten sodium must be kept pure, as impurities can accelerate corrosion. Operators use continuous purification processes, such as filtration and cold trapping, to remove oxides and other contaminants from the circulating liquid metal. These systems are designed to operate reliably under both normal and abnormal conditions to prevent the reactor fuel from melting.45