| Section3 = }} NaK (rhyming with "sack")'s a sodium (Na) - potassium (K) alloy usually liquid at room temperature. Usage As coolant One notable use's 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's used in many other heat transfer app… (More on Nak)