Depleted uranium

[Rennhack]

Uranium having a percentage of uranium-235 smaller than the 0.7 percent found in natural uranium. It is obtained from spent (used) fuel elements or as byproduct tails, or residues, from uranium isotope separation.

[Rennhack]

Uranium containing less 0.7% uranium-235, the amount found natural uranium (See also enriched uranium.)

http://www.epa.gov/radiation/radionuclides/uranium.htm