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An Overview of Gas Centrifuges and Their Modern Implications

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An Overview of Gas Centrifuges and Their Modern Implicatio
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  10/31/2014An Overview of Gas Centrifuges and their Modern Implicationshttp://large.stanford.edu/courses/2013/ph241/kadribasic1/1/3 Fig. 1:  A simplified diagram showing theinner workings of a device for gaseousdiffusion of UF 6  for enriching uranium.(Courtesy of the U.S. Nuclear RegulatoryCommission) An Overview of Gas Centrifuges and theirModern Implications Fedja KadribasicMarch 26, 2013 Submitted as coursework for PH241, Stanford University, Winter 2013 Modern nuclear reactors operate by using the energy produced during nuclear fission reactions to heatwater, which drives a turbine that produces electricity.Uranium is most commonly used for this process because one of its isotopes, uranium-235 (U-235),releases three neutrons for every one neutron itabsorbs, which makes the resulting fission reactionself-sustaining. However, uranium processed from orecontains far too little U-235 to be of any practical use.Thus, to make uranium metal into usable fuel, thedifferent isotopes need to be separated from each other and redistributed so that one can make uranium withmore U-235 than is found in a natural sample.This process is called uranium enrichment, which begins in most cases with uranium metal being turnedinto uranium hexafluoride (UF 6 ) through complicatedchemical reactions. This gas has slightly different properties depending on which uranium isotope is bonded to the fluorine. In facilities in the United States, the main way to separate the isotopes is througha process called gaseous diffusion, where the difference in partial pressures of the two isotopes isexploited, along with a semipermeable membrane, to slowly create a concentration gradient. Theseindividual diffusion units are connected so that a higher concentration of U-235 goes one way and alower concentration in the other (Fig. 1). In recent years, the US has tried to transition away from thistechnology and turn to gas centrifuges, also known as Zippe centrifuges, because it is not very cost-effective and can be quite dangerous. [1]The main operating principle for a centrifuge is thatwhen a mixture of fluids is rotating very quickly, therotation of the device produces, in an acceleratedreference frame, a centrifugal force on the fluidsinside. This force separates out the fluids by density,similar to how fresh water floats on top of salt water inan estuary because of the force of gravity. The Zippecentrifuge spins fast enough that the denser gas withU-238 tends to collect along the outside walls and theless dense gas with U-235 accretes closer to the center.Special vents going towards the center and othersalong the sides collect the two gas mixtures and  10/31/2014An Overview of Gas Centrifuges and their Modern Implicationshttp://large.stanford.edu/courses/2013/ph241/kadribasic1/2/3 Fig. 2:  Diagram illustrating the generaloperation principle of a Zippe centrifuge(Courtesy of the U.S. Nuclear RegulatoryCommission)transfer them to other centrifuge units. (See Fig. 2.) [2]These vents are connected so that the U-235 laden gasgoes in one direction and the other gas goes in theother, similar to what is done with gaseous diffusion.Zippe centrifuges, despite their very simple operating principle, are not easy to make. To separate out thetwo gas mixtures, the centrifuges need to spin ataround 60,000 rpm, which is fast by anyone's count.[3] Rotation rates this fast require very high tolerances because the slightest deformity could result in a partgetting damaged and radioactive, chemically reactivegas being released. However, their basic principle isstill very simple compared to gaseous diffusion plantsand their operation costs are much lower. Thus, if acountry decides to take on uranium enrichment for energy or nuclear weapons production, the Zippecentrifuge is a relatively inexpensive route to take. [3]A case in point is Iran's recent program to developuranium enrichment plants at Natanz, which,according to a recent announcement, will be producing19.75% low enriched uranium (LEU). [4] What isinteresting about this situation is that it takes a lot of resources to start a full-scale nuclear energy or weapons production comparable to that in some of thecountries that already have one. The amount of enrichment possible from the plant at Natanz does notseem nearly enough for such a program. However, it isenough for producing a small number of nuclear weapons. [4] When coupled with the fact that one onlyneeds about 5% enrichment for nuclear power plants,it means that Iran could already be on the path for making a small number of nuclear weapons. As statedin a recent ISIS report, one of the most strikinglessons from reviewing Iran's accomplishments at Natanz is just how unachievable a commercialenrichment program remains, while at the same time, how comparatively little enrichment capability isrequired for a nuclear weapons capability. [4]This result partly stems from the fact that centrifuge devices themselves do not cost very much tooperate, so their usefulness in producing usable fuel must be carefully weighed against their ability torelatively easily produce weapons-grade uranium. [1] Especially in an economy and government asunstable as that in Iran, the weapons could pose a threat to cities in other countries if they somehow getinto the wrong hands. On the contrary, the technology is still very useful for civilian purposes, shown bythe recent developments in the US for getting the centrifuge program back on its feet. [1] Thus, itremains necessary for the world to carefully weigh the pros and cons of enrichment programs usingcentrifuges as the world's energy budget keeps increasing in the future.© Fedja Kadribasic. The author grants permission to copy, distribute and display this work in unalteredform, with attribution to the author, for noncommercial purposes only. All other rights, includingcommercial rights, are reserved to the author.  10/31/2014An Overview of Gas Centrifuges and their Modern Implicationshttp://large.stanford.edu/courses/2013/ph241/kadribasic1/3/3 References [1] H. G. Wood, A. Glaser and R. S. Kemp. The Gas Centrifuge and Nuclear Weapons Proliferation, Physics Today 61 , No. 9, 40 (September 2008).[2] G. Zippe et al.   Centrifugal Separators, U.S. Patent 3289925, 6 Dec 66.[3] A. Glaser, Characteristics of the Gas Centrifuge for Uranium Enrichment and Their Relevance for  Nuclear Weapon Proliferation, Science and Global Security 16 , 1 (2008).[4] D. Albright and C. Walford, Supplement to Iran's Gas Centrifuge Program: Taking Stock , Institutefor Science and International Security, 3 Mar 10.
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