Uranium containing the relative concentrations of isotopes found in nature (0. 7 percent uranium-235, 99. 3 percent uranium-238, and a trace amount of uranium-234 by mass). In terms of radioactivity, however, natural uranium contains approximately 2. 2 percent uranium-235, 48. 6 percent uranium-238, and 49. 2 percent uranium-234. Natural uranium can be used as fuel in nuclear reactors.