Sodium in Nuclear Reactor Cooling

In its elemental state it has been used as a molten coolant in nuclear reactors and is currently under research for sodium/sulfur batteries. Sodium acts as a heat transfer fluid in some nuclear reactors and inside hollow valves of high-performance internal combustion engines. When the reactor is consistently running, a pure sodium coolant is used because sodium melts at about 98 °C. However, when the reactor is shut down, the sodium can melt the cooling pipes. Therefore, if the reactors need to be shut down often, the alloy of sodium and potassium is used because the compound melts at -11 °C so the cooling pipes will not freeze at room temperature.