NaK has been used as the coolant in experimental fast neutron nuclear reactors. Unlike commercial plants, these are frequently shut down and defuelled. Use of lead or pure sodium, the other materials used in practical reactors, would require continual heating to maintain the coolant as a liquid. Use of NaK overcomes this. The Dounreay Fast Reactor is an example.
The Soviet RORSAT radar satellites were powered by a BES-5 reactor, which was cooled with NaK. In addition to the wide liquid temperature range, NaK has a very low vapor pressure, which is important in the vacuum of space.
An unintended consequence of the usage as a coolant on orbiting satellites has been the creation of additional space debris. NaK coolant has leaked from a number of satellites, including Kosmos 1818 and Kosmos 1867. The coolant self-forms into droplets of sodium–potassium of up to several centimeters in size. These objects are space debris.
The Danamics LMX Superleggera CPU cooler uses NaK to transport heat from the CPU to its cooling fins.