NaK Coolant

NaK Coolant is an alloy of sodium (Na) and potassium (K) used as a coolant in nuclear reactors. It is particularly useful in these situations because it remains liquid at room temperature and is commercially available in various grades. NaK is highly reactive with air or water, and as a consequence must be handled with special precautions. Quantities as small as one gram can be a fire or explosion risk. 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 absolute vacuum of space. Some of the coolant has leaked and these NaK droplets constitute a significant space debris hazard.

Physical properties

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.