Uranium enrichment
Federal Agency of Higher EducationTomsk Polytechnic UniversityApplied Physics and Engineering Faculty“Uranium enrichment”Made by:Student of group 0850Kiselyova U.V..Tomsk, 20081. Uranium Enrichment – IntroductionThere 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. 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 material 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. Because of the presence of small quantities of U-235, natural uranium can sustain a chain reaction under certain conditions, and therefore can be used as a fuel in certain kinds of reactors (graphite-moderated reactors and heavy water3 reactors, the latter being sold commercially by Canada). For the most common reactor type in use around the world today, which uses ordinary water as a coolant and moderator, the percentage of U-235 in the fuel must be higher than the 0.7 percent found in natural uranium. The set of industrial processes that are used to increase the percentage of U-235 in a given quantity of uranium go under the general rubric of “uranium enrichment” – with the term “enrichment” referring to the increase in the percentage of the fissile isotope U-235. Light water reactors typically use 3 to 5 percent enriched uranium – that is, the proportion of U-235 in the fuel is 3 to 5 percent, with almost all the rest being U-238. Material with this level of U-235 is called “low enriched uranium” or LEU. Nuclear bombs cannot be made from natural or low enriched uranium. The proportion of U-235, which is the only one of the three isotopes that can sustain a chain reaction in uranium, is just too small to enable a growing “super-critical” chain reaction to be sustained. Uranium must have a minimum of 20 percent U-235 in it in order to be useful in making a nuclear bomb. However, a bomb made with uranium at this minimum level of enrichment would be too huge to deliver, requiring huge amounts of uranium and even larger amounts of conventional explosives in order to compress it into a supercritical mass. In practice, uranium containing at least 90 percent U-235 has been used to make nuclear weapons. Material with this level of enrichment is called highly enriched uranium or HEU. The bomb that destroyed Hiroshima on August 6, 1945, was made with approximately 60 kilograms of HEU. Highly enriched uranium is also used in research reactors and naval reactors, such as those that power aircraft carriers and submarines. The HEU fuel meant for research reactors is considered particularly vulnerable to diversion for use in nuclear weapons. Thorium-232, which is also naturally occurring, can be used to make bombs by first converting it into U-233 in a nuclear reactor. However, uranium fuel for the reactor, or fuel derived from uranium (such as plutonium) is needed for this conversion if U-233 is to be produced in quantity from thorium-232. A fissile material is one that can be split (or fissioned) by low energy neutrons and is also capable of sustaining a chain reaction. Only fissile materials may be used as fuel for nuclear reactors or nuclear weapons. Examples of other fissile materials, besides uranium-235, are uranium-233 and plutonium-239. “Heavy water” is water that contains deuterium in place of the ordinary hydrogen in regular water (also called light water). Deuterium has one proton and one neutron in its nucleus as opposed to hydrogen, which has only a single proton. The same process and facilities can be used to enrich uranium to fuel commercial light water reactors – that is to make LEU – as well as to make HEU for nuclear bombs. Therefore all uranium enrichment technologies are potential sources of nuclear weapons proliferation. In addition, some approaches to uranium enrichment are more difficult to detect than others, adding to concerns over possible clandestine programs.2.Uranium Enrichment technologiesOnly four technologies have been used on a large scale for enriching uranium. Three of these, gaseous diffusion, gas centrifuges, and jet nozzle / aerodynamic separation, are based on converting uranium into uranium hexafluoride (UF6) gas. The fourth technique, electromagnetic separation, is based on using ionized uranium gas produced from solid uranium tetrachloride (Ucl4). 2.1 Gaseous DiffusionThe gaseous diffusion process has been used to enrich nearly all of the low and highly enriched uranium that has been produced in the United States. It was first developed in the 1940s as part of the Manhattan Project and was used to enrich a portion of the uranium used in the bomb that was dropped on Hiroshima. All five acknowledged nuclear weapons states within the nuclear non-proliferation treaty (NPT) regime have operated gaseous diffusion plants at one time or another, but currently only the United States and France continue to operate such facilities. The diffusion process requires pumping uranium in a gaseous form through a large number of porous barriers and, as noted above, is very energy intensive. In order to make the uranium into a gaseous form that can be used in the diffusion process, the natural uranium is first converted into uranium hexafluoride (UF6). The uranium hexafluoride molecules containing U-235 atoms, being slightly lighter, will diffuse through each barrier with a slightly higher rate than those containing U-238 atoms. A simple analogy to help visualize this process is to imagine blowing sand through a series of sieves. The smaller grains of sand will preferentially pass through each sieve, and thus after each stage they would represent a slightly higher percentage of the total than they did before passing through the stage. A schematic representation of one such stage from a gaseous diffusion plant is shown in Figure 1.
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