Natural Uranium is 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) has not been changed. The Natural Uranium is commonly found at low levels (a few ppm) in all rocks, soil, water, plants, and animals that can be used as fuel in nuclear reactors that use heavy water or graphite as moderators. (Refer to the Uranium (U))