Uranium, isotopes

Nuclear fission is also involved in nuclear weapons. To create a bomb, the concentration of the isotope uranium-235 must be increased to at least 85 percent from its natural concenti ation of only 0.7 percent. This increase ot concentration is difficult and expensive. In a typical nuclear reactor the uranium-235 concentration in the fuel is only 3 to 4 percent, and hence a nuclear reactor cannot explode like a bomb. In a nuclear bomb... 

Half-lives can be interpreted in terms of the level of radiation of the corresponding isotopes. Uranium has a very long half-life (4.5 X 109 yr), so it gives off radiation very slowly. At the opposite extreme is fermium-258, which decays with a half-life of 3.8 X 10-4 s. You would expect the rate of decay to be quite high. Within a second virtually all the radiation from fermium-258 is gone. Species such as this produce very high radiation during their brief existence. 

Our discussion concentrates on the uranium-235 isotope. It makes up only about 0.7% of naturally occurring uranium. The more abundant isotope, uranium-238, does not undergo fission. The first process used to separate these isotopes, and until recently the only one available, was that of gaseous effusion (Chapter 5). The volatile compound uranium hexafluoride, UF6, which sublimes at 56°C, is used for this purpose. 

Once uranium is incorporated into buried bone, shell, coral, or speleothems, the isotope uranium-235 decays, initially into the short-lived isotope (thorium-231) and then into long-lived protoactinium-231. Uranium-238, on the other hand, decays first into two successive short-lived isotopes (thorium-234 and protoactinium-234) and only then into a long-lived isotope, uranium-234 (see Fig. 12). The decay of uranium-235 to long-lived protoactinium-231 is used to date events up to 150,000 years in age that of uranium-234 (derived from uranium-238) to thorium-230 is of use for dating events within the time range 1000-500,000 years. 

Isotope One of several radionuclides of the same element (i.e., with the same number of protons in their nuclei) with different numbers of neutrons and different energy contents. A single element may have many isotopes. Uranium, for example, may appear naturally as 234U (142 neutrons), 235U (143 neutrons), or 238U (146 neutrons) however, each uranium isotope has 92 protons. 

The half-lives of some radioisotopes are measured in billions of years for others, the half-life is measured in fractions of seconds. Following are some examples of the half-lives of a few isotopes uranium-238 = 4.6 billion years carbon-14 = 5730 years strontium-90 = 38 years phosphorus-32 = 14.3 days radon-222 = 3.8 days uranium-239 = 23.5 minutes. 

Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fission chain reaction that results in a rapid explosion with tremendous energy and becomes a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these fuels are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass... 

The most stable isotope of plutonium is Pu-244, with a half-life of S.OOxlO+ years (about 82,000,000 years). Being radioactive, Pu-244 can decay in two different ways. One way involves alpha decay, resulting in the formation of the isotope uranium-240, and the other is through spontaneous fission. 

The essential ingredients for producing heat in a thermal fission nuclear reactor are the fuel and a moderator. A heat transport system with its coolant is necessary to convey the heat from the reactor to boilers where steam is produced to drive the turbogenerator. The natural materials available for fuel and moderator are uranium ore and water natural uranium extracted from the ore comprises the fissionable isotope uranium-235 and water contains hydrogen which is a good moderator. (Table I)... 

Scientists use radioactive minerals to date very old nonliving things. The naturally occurring mineral isotopes uranium-238 and uranium-235 decay very slowly and ultimately become lead—but not the common isotope lead-208. Instead, as... 

Chain reactions do not occur to any great extent in naturally occurring uranium ore because not all uranium atoms fission so easily. Fission occurs mainly in the isotope uranium-235, which is rare and makes up only 0.7 percent of the uranium in pure uranium metal (Figure 4.23)- When the more abundant isotope uranium-238 absorbs neutrons created by fission of a uranium-235 atom,... 

Uranium hexafluoride, UF6, shown here, is a relatively volatile molecular compound that is used in the separation of the naturally occurring isotopes uranium-235 and uranium-236. There is only one naturally occurring isotope of fluorine, so any difference in the molar mass of the molecule is due to the uranium. 

Of all the elements, fluorine is the most reactive and the most electronegative (a measure of tendency to acquire electrons). In its chemically combined form, it always has an oxidation number of -1. Fluorine has numerous industrial uses, such as the manufacture of UF6, a gas used to enrich uranium in its fissionable isotope, uranium-235. Fluorine is used to manufacture uranium hexafluoride, SF6, a dielectric material contained in some electrical and electronic apparatus. A number of organic compounds contain fluorine, particularly the chlorofluorocarbons used as refrigerants and organofluorine polymers, such as DuPont s Teflon. 

Step 2. Prepare the set of samples in dilute (2%) nitric acid for measurement. Also prepare duplicates, the uranium standards, and blanks of the nitric acid solution and deionized water. Note that a trial measurement of the uranium concentration is needed to select the appropriate isotopic uranium concentration ranges and prepare uranium standards in these ranges. 

When nuclear fission was first discovered, only two isotopes, uranium-233 and uranium-235, were known of being capable of undergoing this nuclear change. 

Uranium-233. A second fissionable isotope uranium-233, can be produced from naturally occurring thorium. It does not present an economically attractive option at present because of its dependence on highly enriched U-235 to bring the thorium cycle into operation and the large R D expenditures required... 

Isotopes of Uranium.—Ordinary uranium is a mixture of Ua o isotopes, uranium I and uranium II, whose atomic weights differ by 4, the weight of one a-particle which is ejected by an atom of the former according to the following scheme (see p. 342) ... 

Lead isotopes Uranium isotopes Neodymium isotopes Osmium and hafnium isotopes... 

Isotopes— Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties. Uranium has three naturally occurring isotopes, uranium-238, uranium-235, and uranium-234. 

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. 

Table 12.5 shows expected isotopic uranium contents which might be expected as product from cascade type (gaseous diffusion or gas centrifuge) enrichment facilities. While the highly enriched U is actually the target of these efforts one should also be able to verify the production of low enriched uranium. Columns 5 and 6 of Table 12.5 show the isotopic enrichments which would result from a 1000-1 mixture of namral U and low or high enriched U. Table 12.6 shows the ratios of the isotopes U and U relative to the in each mixture. The 0.6% isotopic shift in the ratio in the case of LEU-MIX should be detectable by today s technology. The 0.13% in ratio... 

But there are two probiems with using uranium for nuciear fission. First, of uranium s three isotopes (uranium-234, uranium-235, and uranium-238) oniy one of these isotopes—uranium-235— undergoes fission. The second probiem is that this isotope of uranium is quite rare. Out of every 1,000 atoms of uranium, oniy seven are uranium-235. Tons of uranium ore must be processed and enriched to make tiny amounts of this criticai isotope, it is difficuit and extremeiy expensive. 

The answer to all these questions is no. Only one isotope of uranium undergoes nuclear fission, uranium-235. The most common isotope, uranium-238, does not undergo fission. There is no way to make a bomb or a nuclear power plant with a chunk of natural uranium metal. 

CAS 7440-61-1. U. Metallic element number 92 a member ofthe actinide series aw 238.029 valences of 3, 4, 6 three natural radioactive isotopes uranium-234 (0.006%), uranium-235 (0.7%), and uranium-238 (99%). 

Use Source of fissionable isotope uranium-235, source of plutonium by neutron capture, electric power generation. 

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... 

Monoenergetic heavy ions necessary for energy calibration can be provided only by accelerators. Fission fragments, which are heavy ions, cover a wide spectrum of energies (Fig. 13.19). The isotope Cf is a very convenient source of fission fragments produced by the spontaneous fission of that isotope. Uranium, plutonium, or thorium fission fragments can only be produced after fission is induced by neutrons therefore, a reactor or some other intense neutron source is needed. 

Table 1 Average isotopic uranium composition of sea water with salinity 35...

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