With all of the medical and environmental concerns over DU, why hasn't DoD found a substitute for DU?

Each weapon system that uses DU has undergone extensive developmental testing and evaluation. As part of that process, DoD evaluates possible alternative metal alloys considering operational requirements and medical/environmental impacts. As improvements have been made in the "hardness" of armored vehicles, tests have demonstrated that DU offers superior performance to all other alloys.

DoD must also evaluate the environmental and medical consequences of exposure to any new alloy. Uranium has an advantage in this arena over several candidate materials because of the extensive database on radiological and chemical properties of uranium. While some candidate replacement alloys may not be radioactive, they are not necessarily less toxic to humans.