Liquid metal cooled reactor Clementine was the first liquid metal cooled nuclear reactor and used mercury coolant, thought to be the obvious choice since it is liquid at room temperature. However, because of disadvantages including high toxicity, high vapor pressure even at room temperature, low boiling point, producing noxious fumes when heated, relatively low thermal conductivity, and a high neutron cross-section, it has fallen out of favor. Sodium and NaK (a eutectic sodium-potassium alloy) do not corrode steel to any significant degree and are compatible with many nuclear fuels, allowing for a wide choice of structural materials. They do, however, ignite spontaneously on contact
Liquid fuel Some common properties of liquid fuels are that they are easy to transport, and can be handled with relative ease. Physical properties of liquid fuels vary by temperature, though not as great as for gaseous fuels. Some of these properties are: flash point, the lowest temperature at which a flammable concentration of vapor is produced; fire point, the temperature at which sustained burning of vapor will occur; cloud point for diesel fuels, the temperature at which dissolved waxy compounds begin to coalesce, and pour point, the temperature below which the fuel is too thick to pour freely. These properties affect