Uranium isotope percentage

If you know the half-lives of uranium isotopes and the percentage of lead isotopes in some uranium-bearing rock, you can calculate the date the rock was formed. Rocks dated in this way have been found to be as much as 3.7 billion years old. Samples from the moon have been dated at 4.2 billion years, which is close to the estimated age of our solar system 4.6 billion years. 

TABLE 29.1. Forms of uranium and their respective isotope percentages... 

Other is possible using the conversion factor of 0.68 pCi/g (EPA 1991b). Likewise, 1 ppm is equivalent to 1 pg/g and, therefore, to 0.68 pCi/g. Other conversion values have been used, such as 0.72 pCi/pg (EPA 1985b) and 0.67 pCi/pg (NCRP 1984a). These different values are largely accounted for by the periodic refinement of values for uranium isotopic half-lives and relative percentages in crustal rock. 

The table below shows the percentage of natural abundance of each natural uranium isotope, and their respective half-lives. 

Natural uranium shall mean chemically separated uranium containing the naturally occurring distribution of uranium isotopes (approximately 99.28% uranium-238, and 0.72% uranium-235 by mass). Depleted uranium shall mean uranium containing a lesser mass percentage of ura-nium-235 than in natural uranium. Enriched uranium shall mean uranium containing a greater... 

Notice tiiat the initial uranium atom is the U-235 isotope, which makes up less than 1% of all naturally occurring uranium.. The most abundant uranium isotope, U-238 does not undergo fission. Therefore, the uranium used for fuel in nuclear reactions must be enriched in U-235 (it must contain more than the naturally occurring percentage of U-235). Notice also that the process produces three neutrons, which have the potential to initiate fission in three other U-235 atoms. 

TABLE 33.1 Forms of Uranium and Their Respective Isotope Percentages... 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

Naturally occurring uranium contains a very small percentage (0.7%) of the fissile isotope to be used either in nuclear weapons, which require a very... 

Inductively coupled plasma mass spectrometry is a relatively new technique for elemental analysis and has superior limits of detection over optical methods. Also, this technique has an order of magnitude better detection limit than that obtained by the conventional fluorometric method. Uranium has many stable and unstable isotopes but 238U has the largest percentage abundance (99.274%). 

The element uranium is a mixture of two isotopes, uranium-235 and uranium-238. Both isotopes have 92 protons in the nucleus, but uranium-238 has three additional neutrons. Both isotopes have 92 orbital electrons to balance the 92 protons, so their chemical properties are identical. When uranium is bombarded with neutrons, the two isotopes have differing nuclear reactions. A high percentage of the uranium-235 nuclei undergo fission, as described previously. The uranium-238, on the other hand, simply absorbs a neutron and is converted to the next heavier isotope, uranium-239. It is not possible to build a bomb out of natural uranium. The reason is that the chain reaction would be halted by uranium-238 because it removes neutrons without reproducing any new ones. 

The uranium(IV) oxide, UO2, used as fuel in nuclear power plants has a higher percentage of the fissionable isotope uranium-235 than is present in the UO2 found in nature. To make fuel grade UO2, chemists first convert uranium oxides to uranium hexafluoride, UFg, whose concentration of uranium-235 can be increased by a process called gas diffusion. The enriched UFg is then converted back to UO2 in a series of reactions, beginning with... 

The logical explanation for the low percentages of U-235 was that a nuclear fission reaction at the mine must have consumed some of the U-235 isotopes. But how did this happen There are several conditions under which such a nuclear fission reaction could take place. In the presence of heavy water, for example, a chain reaction is possible with unenriched uranium. Without heavy water, such a fission reaction could still occur if the uranium ore and the moderator were arranged according to some specific geometric constraints at the site of the reaction. Both of the possibilities seem rather farfetched. The most plausible explanation is that the uranium ore originally present in the mine was enriched with U-235 and that a nuclear fission reaction took place with light water, as in a conventional nuclear reactor. 

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