What is a substance and what are the two types of substances?

The two types of substances are elements and compounds.

Explanation:

A pure substance is one in which the composition is definitive and constant throughout, and that has distinct chemical properties. These substances are comprised of only one entity, not a mixture of two or more.

Elements are pure substances, assuming they are free of imperfections, because they are made of only one type of atom.

Compounds are also pure substances, like water, table salt, table sugar, etc. because they are comprised of one type of compound (H2O, NaCL, C6H1206,etc)

water is used to store them because it would keep the radiation from getting out in the open and causing harm to the workers