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Uranium Geochemistry <br />Uranium has three natural isotopes U-234 (0.005% of all natural U by weight), U-235 • <br />(0.72%), and U-238 (99.27%). U-234 is a daughter product from the radioactive decay of <br />U-238, while U-235 is a parent isotope and has its own decay series. The U-238 decay <br />series is shown in Table 1. <br />Table 1 <br />Uramium-238 Decay Series <br />Symbol Name Radiation Half Life Decay Product <br />28sU Uranium-238 alpha 4,460,000,000 ears 2UTh <br />2mTh Thorium-234 beta 24.1 days 234Pa <br />2mPa Protactinium-234 beta 1.17 minutes 2MU <br />2MU Uranium-234 alpha 247,000 ears 230Th <br />230Th Thorium-230 alpha 80,000 ears 226Ra <br />226Ra Radium-226 alpha 1,602 ears 222Rn <br />222Rn Radon--222 alpha 3.82 days 218po <br />218po Polonium-218 alpha 3.05 minutes 214Pb <br />214Pb Lead-214 beta 27 minutes 214Bi <br />214Bi Bismuth-214 beta 19.7 minutes 214Po <br />214Po Polonium-214 alpha 1 microsecond 210Pb <br />210pb Lead-210 beta 22.3 ears 21OBi <br />21OBi Bismuth-210 beta 5.01 days 210po <br />210Po Polonium-210 alpha 138.4 days 206pb <br />206pb Lead-206 beta Stable None <br />The isotopes with very :short half lives decay almost immediately after being formed <br />and are considered intermediaries that do not exist for long enough to be measured in <br />significant quantities. Other species, with longer half lives such as Ra-226 persist for <br />longer periods of time. <br />• <br />As U-238 represents the vast majority of the uranium present on earth, the following <br />discussion will be limited to U-238. In addition, because the decay of U-238 is extremely <br />slow (the half life is 4,500 million years) the daughter products of uranium decay will <br />not be considered and radioactive decay will not be evaluated as a fate and transport <br />process. Therefore, when uranium (U) is discussed in the following sections it refers to <br />U-238. 0