Deplete 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 DU is about 60 percent of that from the same mass of natural uranium. Another less common source of DU 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, DU has been called Q-metal, depletalloy, and D-38, but those names are no longer used.