The far more common uranium-238, however, can't be used in most nuclear reactors and cannot be used in nuclear bombs. Uranium-238 is heavier than uranium-235 and much more common, making up more than 99% of natural uranium. Uranium found in the Earth's crust is primarily made up of two different isotopes, or forms of uranium, that differ based on their atomic weight. Uranium is a radioactive element that occurs naturally in the environment.
