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Fig. 1:A simplified diagram showing theinner workings of a device for gaseousdiffusion of UF6for enriching uranium.

(Courtesy of the U.S. Nuclear RegulatoryCommission)

An Overview of Gas Centrifuges and their

Modern Implications

Fedja Kadribasic

March 26, 2013Submitted as coursework for PH241, Stanford University, Winter 2013

Modern nuclear reactors operate by using the energyproduced 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 otherand redistributed so that one can make uranium withmore U-235 than is found in a natural sample.

This process is called uranium enrichment, whichbegins in most cases with uranium metal being turnedinto uranium hexafluoride (UF6) through complicated

chemical reactions. This gas has slightly different properties depending on which uranium isotope isbonded 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 thawhen a mixture of fluids is rotating very quickly, the

rotation of the device produces, in an acceleratedreference frame, a centrifugal force on the fluidsinside. This force separates out the fluids by densitysimilar 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 centerSpecial vents going towards the center and othersalong the sides collect the two gas mixtures and

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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.