Uranium with a percentage of uranium-235 lower than the 0.7 percent (by mass) contained in natural uranium. (The normal residual U-235 content in depleted uranium is 0.2–0.3 percent, with U-238 comprising the remaining 98.7–98.8 percent.) Depleted uranium is produced during uranium isotope separation and is typically found in spent fuel elements or byproduct tailings or residues. Depleted uranium can be blended with highly-enriched uranium, such as that from weapons, to make reactor fuel. For further detail, see Background Information on Depleted Uranium and Fact Sheet on Updating Disposal Rules for New Types of Low-Level Waste.