Depleted Uranium, also known as DU, is nuclear waste metal. Uranium is extremely dense and extremely heavy which makes it ideal for anti-tank rounds, but it is also radioactive, and upon impact radioactive dust from a DU bullet is spread in every direction and it can take centuries to dissipate.
You might find it hard to believe, but depleted uranium is still being used by the United States military. The United States Government has tried to play ignorant as to the cause of Gulf War Syndrom, but they good and well what caused it, and it wasn't Saddam's chemical weapons. It was our chemical weapons.