Sodium. (Source: Wikipedia.org)

A chemical element with atomic number 11. Sodium is a grey-white soft metal that reacts violently with water and air. It is a widespread element in the Earth’s crust, where it occurs in many compounds. Many sodium salts are readily soluble in water; thus sodium is abundant in the oceans. Sodium is a very important element for living organisms.

The fast breeder reactor uses molten sodium as a coolant because it has excellent thermal properties.