What the compositions could be potentially interesting as promising media in the role of molten salt coolants? The nuclear industry assumes the use of the lightest nuclei . In addition, it is obliged to possess significant thermodynamic stability, hence high mixing entropy. Using such conditions of the problem it is not difficult to propose a composition of a mixture of six salts of fluorides and chlorides of lithium, sodium and potassium. At this composition there can arise certain technological advantages and economy on lithium salts, because especially sodium and potassium chlorides are cheap enough, safe and not hygroscopic.