NaK (often pronounced as such, rhyming with "sack") is an alloy of sodium (Na) and potassium (K), and particularly one that is liquid at room temperatures. It is a commercially available material in various grades. NaK is highly reactive with air or water, and must be handled with special precautions. Quantities as small as one gram can be a fire or explosion risk.
Alloys with between about 40% and 90% potassium by weight are liquid at room temperature. The mixture with the lowest melting point (the eutectic mix), consisting of 78% potassium and 22% sodium, is liquid from −12.6 to 785 °C, and has a density of 866 kg/m³ at 21°C and 855 kg/m³ at 100°C.
One notable use is 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. NaK is used in many other heat transfer applications for similar reasons.
The Soviet RORSAT radar satellites were powered by a NaK-cooled reactor. Apart from the wide liquid temperature range, NaK has a very low vapor pressure, important in the vacuum of space. Some of the coolant has leaked and these NaK droplets constitute a significant space debris hazard.
NaK is also used as a catalyst for many reactions, including precursors of ibuprofen.
Both sodium and potassium are used as desiccants in drying solvents prior to distillation. However, without heating, the solid metal is only able to react at the surface. Formation of crusts of oxide also helps to reduce the reactivity. As a liquid metal alloy at room temperature, the use of NaK as a desiccant helps to avoid these problems.