Isotopes of uranium

Another impetus to expansion of this field was the advent of World War 11 and the development of the atomic bomb. The desired isotope of uranium, in the form of UF was prepared by a gaseous diffusion separation process of the mixed isotopes (see Fluorine). UF is extremely reactive and required contact with inert organic materials as process seals and greases. The wartime Manhattan Project successfully developed a family of stable materials for UF service. These early materials later evolved into the current fluorochemical and fluoropolymer materials industry. A detailed description of the fluorine research performed on the Manhattan Project has been pubUshed (2). 

A number of special processes have been developed for difficult separations, such as the separation of the stable isotopes of uranium and those of other elements (see Nuclear reactors Uraniumand uranium compounds). Two of these processes, gaseous diffusion and gas centrifugation, are used by several nations on a multibillion doUar scale to separate partially the uranium isotopes and to produce a much more valuable fuel for nuclear power reactors. Because separation in these special processes depends upon the different rates of diffusion of the components, the processes are often referred to collectively as diffusion separation methods. There is also a thermal diffusion process used on a modest scale for the separation of heflum-group gases (qv) and on a laboratory scale for the separation of various other materials. Thermal diffusion is not discussed herein. 

Uranium (symbol U atomic number 92) is the heaviest element to occur naturally on Earth. The most commonly occurring natural isotope of uranium, U-238, accounts for approximately 99.3 percent of the world s uranium. The isotope U-235, the second most abundant naturally occurring isotope, accounts for another 0.7 percent. A third isotope, U-234, also occurs uatiirally, but accounts for less than 0.01 percent of the total naturally occurring uranium. The isotope U-234 is actually a product of radioactive decay of U-238. 

Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split. 

Quite often, isotopes of an element are distinguished from one another by writing the mass number after the symbol of the element The isotopes of uranium are often referred to as U-235 and U-238. 

Uranium-235 is the isotope of uranium commonly used in nuclear power plants. How many... 

Thus we have returned to an isotope of uranium, 231U, but one of half-life very much shorter than that of U. This isotope begins a succession of a-decays, each moving the product upward in... 

An important selection of materials to packaging, particularly food, is based on the permeability of the materials to oxygen, water vapor, and, in the case of packaging bananas, to ethylene gas that is used to artificially ripen the bananas. Selective permeability provides chemical separations, one of the most interesting of which is the use of PTFE materials to separate the hexafluorides of the different isotopes of uranium. 

C22-0002. The most abundant isotope of uranium is U, with an isotopic molar mass of 238.0508... 

In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were previously discussed by Lally (1992), Ivanovich and Murray (1992), and Chen et al. (1992). Because ICPMS methods were not discussed in preceding reviews and have become more commonly used in the past decade, we also include some theoretical discussion of ICPMS techniques and their variants. We also primarily focus our discussion of analytical developments on the longer-lived isotopes of uranium, thorium, protactinium, and radium in the uranium and thorium decay series, as these have been more widely applied in geochemistry and geochronology. 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

Bhat SG, Krishnaswami S (1969) Isotopes of uranium and radium in Indian rivers. Proc Ind Acad Sci Earth Planet Sci 70 1-17... 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

Radioisotopes may occur in the earth naturally as primordial radioisotopes, formed when the planet was created, or be produced by natural or artificial processes. Most fast decaying primordial radioisotopes have long disappeared from the planet since the earth originated about 4.5 billion years ago, such isotopes have decayed and reached a final, stable form. The relatively few primordial radioisotopes still extant in the earth today, therefore, decay very slowly. Among these are potassium-40 and some isotopes of uranium, such as uranium-235 and uranium-238, which are of use for dating archaeologically related minerals and rocks (see Textboxes 15 and 16). 

Isotope Any of two or more species of atoms of a chemical element with the same atomic number and different atomic mass. For example, U-238 and U-235 are both isotopes of uranium. 

ISOTOPES There are total of 26 Isotopes of uranium. Three of these are considered stable because they have such long half-lives and have not all decayed Into other elements... 

All compounds as well as metallic uranium are radioactive—some more so than others. The main hazard from radioactive isotopes is radiation poisoning. Of course, another potential hazard is using fissionable isotopes of uranium and plutonium for other than peaceful purposes, but such purposes involve pohtical decisions, not science. 

Radioactive decay of the two main isotopes of uranium produces significant contents of the lead isotopes ° Pb and... 

In this brief sequence, there are 2 different isotopes of uranium and 2 of thorium. 

Gilbert GE, Casella VR, Bishop CT, et al. 1985. Isotopes of uranium and thorium, lead-210, and polonium-210 in the lungs of coal miners of Appalachia and the lungs and livers of residents of central Ohio. Energy Res Abstr 11 4090. 

Singh NP, Bennett DD, Wrenn ME, et al. 1987. Concentrations of alpha-emitting isotopes of uranium and thorium in uranium miners and millers tissues. Health Phys 53 261-266. 

High-level wastes consist of spent nuclear fuel and reprocessed wastes. Isotopes of uranium make up by far the majority of high-level wastes, accounting for about 94 percent of the mass of all such wastes. An additional 1 percent consists of plutonium isotopes, and the remaining 5 percent, of isotopes of other elements. 

But with Einstein s advocacy the Manhattan Project began, under the leadership of Oppenheimer. Named for the New York office of the Army Corps of Engineers, it was given almost a blank cheque when America entered the war after the attack on Pearl Harbor. The bomb was pursued on two fronts. One involved developing physical and chemical techniques for separating the isotopes of uranium, milligram by milligram. 

A small proportion of natural uranium consists of the isotope This decays not to lead-206 but to lead-207. By measuring the amounts of all these isotopes of uranium and lead in rocks, geologists can date all manner of minerals, and can even reconstruct the history of our planet s formation. Some meteorites are thought to be left-over remnants of the rocky material that aggregated to produce the Earth, and they show us the mixture of elements this material contained. If they contain no uranium, then... 

Uranium-238 emits an alpha particle to become an isotope of thorium. This unstable element emits a beta particle to become the element now known as Protactinium (Pa), which then emits another beta particle to become an isotope of uranium. This chain proceeds through another isotope of thorium, through radium, radon, polonium, bismuth, thallium and lead. The final product is lead-206. The series that starts with thorium-232 ends with lead-208. Soddy was able to isolate the different lead isotopes in high enough purity to demonstrate using chemical techniques that the atomic weights of two samples of lead with identical chemical and spectroscopic properties had different atomic weights. The final picture of these elements reveals that there are several isotopes for each of them. 

Natural uranium consists of different isotopes of uranium. Natural uranium is 0.7% U-235 and 99.3% U-238. Uranium-238 is nonfissionable, and therefore naturally occurring uranium must be enriched to a concentration of approximately 4% to be used as fuel for nuclear reactors or 90% for weapons-grade uranium. Yellow cake is shipped to conversion plants... 

---