Lanthanides versus actinides
The heaviest fission fragments belong to the lanthanide family (shown in green on the distribution graph of abundance). In the periodic table, the lanthanides are elements with very similar chemical properties (the family begins at lanthanum and ends at lutetium). The lanthanides also have very similar chemical properties to a second family present in spent fuels, the actinides: uranium and plutonium, but also the minor actinides, neptunium, americium and curium. Partitioning the minor actinides from the lanthanides is therefore difficult.