Uranium-235 is a radioactive isotope of uranium. This isotope has a half-life of about 700 million years. Uranium-235 makes up about 0.72% of natural uranium. When it is separated from the other uranium isotopes, it is called enriched uranium. Enriched uranium is best known for its use in the bombing of Hiroshima and Nagasaki on August 6, 1945, and August 9, 1945, respectively. The bomb called Little Boy used Uranium-235. Uranium-235 can be used for nuclear fission.