A chemical element is an atom with a specific number of protons in its nucleus. 

The number of protons in the nucleus is the defining property of an element, and is referred to as its atomic number.  All atoms with the same number of protons are the same element with the same chemical properties.  Atoms with the same number of protons but different number of neutrons are different isotopes of the element.

The following elements are important for nuclear power:   Hydrogen (H), Helium (He); Lithium (Li), Beryllium (Be), Boron (B), Iron (Fe), Tungsten (W), Uranium (U), Plutonium (Pu).