There is one element that occurs in nature that has been the raw material for nuclear bombs: uranium, chemical symbol U. Uranium occurs in nature as a mixture of three different isotopes – that is, three different atomic weights that have virtually the same chemical properties, but different nuclear properties (see Appendix 1: Uranium: Its Uses and Hazards). These isotopes are U-234, U-235, and U-238. The first is a highly radioactive trace component found in natural uranium, but it is not useful in any applications; the second isotope is the only fissile material2 that occurs in nature in significant quantities, and the third is the most plentiful isotope (99.284 percent of the weight of a sample of natural uranium is U-238), but it is not fissile. U-238 can, however, be split by high energy neutrons, releasing large amounts of energy and is therefore often used to enhance the explosive power of thermonuclear, or hydrogen, bombs.
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Report prepared for the Nuclear Policy Research Institute and presented at the October 15, 2004 NPRI conference