Depleted uranium

Depleted uranium (DU) is uranium primarily composed of the isotope uranium-238 (U-238). Natural uranium is about 99.27 percent U-238, 0.72 percent U-235, and 0.0055 percent U-234. Because U-235 is used for fission in nuclear reactors and nuclear weapons, natural uranium is enriched in U-235 by separating the isotopes by mass. The byproduct of enrichment, called depleted uranium or DU, contains less than one third as much U-235 and U-234 as natural uranium, making it less radioactive due to the longer 4.5 billion year half-life of U-238. The external radiation dose from depleted uranium is about 60 percent of that from the same mass of natural uranium. Another less common source of depleted uranium is reprocessed spent nuclear reactor fuel, which can be distinguished from DU produced as a byproduct of uranium enrichment by the presence of U-236, produced in reactors. In the past, depleted uranium has been called Q-metal, depletalloy, and D-38, but those names are no longer used.

Usage
Depleted uranium is used for its very high density of 19.1 g/cm3. Civilian uses include counterweights in aircrafts, radiation shielding in medical radiation therapy and industrial radiography equipment, and containers used to transport radioactive materials. Military uses include defensive armor plates and armor-piercing projectiles.

In the Halo Universe
Depleted uranium is used exclusively by the UNSC, in Magnetic Accelerator Cannon rounds, Armor Piercing Rounds, the 14.5 x 114mm APFSDS rounds of the SRS99C-S2 AM and SRS99D-S2 AM sniper rifles, the rounds shot by the M68 Gauss Cannon, the 110mm Rotary Cannons on Longswords, and the rounds shot by the chain-guns mounted on the D77-TC and D77H-TCI Pelican dropships.