|Fig. 1: A simplified diagram showing the inner workings of a device for gaseous diffusion of UF6 for enriching uranium. (Courtesy of the U.S. Nuclear Regulatory Commission)|
Modern nuclear reactors operate by using the energy produced during nuclear fission reactions to heat water, 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 it absorbs, which makes the resulting fission reaction self-sustaining. However, uranium processed from ore contains far too little U-235 to be of any practical use. Thus, to make uranium metal into usable fuel, the different isotopes need to be separated from each other and redistributed so that one can make uranium with more U-235 than is found in a natural sample.
This process is called uranium enrichment, which begins in most cases with uranium metal being turned into uranium hexafluoride (UF6) through complicated chemical 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 through a process called gaseous diffusion, where the difference in partial pressures of the two isotopes is exploited, along with a semipermeable membrane, to slowly create a concentration gradient. These individual diffusion units are connected so that a higher concentration of U-235 goes one way and a lower concentration in the other (Fig. 1). In recent years, the US has tried to transition away from this technology and turn to gas centrifuges, also known as Zippe centrifuges, because it is not very cost-effective and can be quite dangerous. 
|Fig. 2: Diagram illustrating the general operation principle of a Zippe centrifuge (Courtesy of the U.S. Nuclear Regulatory Commission)|
The main operating principle for a centrifuge is that when a mixture of fluids is rotating very quickly, the rotation of the device produces, in an accelerated reference frame, a centrifugal force on the fluids inside. This force separates out the fluids by density, similar to how fresh water floats on top of salt water in an estuary because of the force of gravity. The Zippe centrifuge spins fast enough that the denser gas with U-238 tends to collect along the outside walls and the less dense gas with U-235 accretes closer to the center. Special vents going towards the center and others along the sides collect the two gas mixtures and transfer them to other centrifuge units. (See Fig. 2.)  These vents are connected so that the U-235 laden gas goes in one direction and the other gas goes in the other, similar to what is done with gaseous diffusion. Zippe centrifuges, despite their very simple operating principle, are not easy to make. To separate out the two gas mixtures, the centrifuges need to spin at around 60,000 rpm, which is fast by anyone's count.  Rotation rates this fast require very high tolerances because the slightest deformity could result in a part getting damaged and radioactive, chemically reactive gas being released. However, their basic principle is still very simple compared to gaseous diffusion plants and their operation costs are much lower. Thus, if a country decides to take on uranium enrichment for energy or nuclear weapons production, the Zippe centrifuge is a relatively inexpensive route to take. 
A case in point is Iran's recent program to develop uranium enrichment plants at Natanz, which, according to a recent announcement, will be producing 19.75% low enriched uranium (LEU).  What is interesting 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 the countries that already have one. The amount of enrichment possible from the plant at Natanz does not seem nearly enough for such a program. However, it is enough for producing a small number of nuclear weapons.  When coupled with the fact that one only needs 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 stated in a recent ISIS report, "one of the most striking lessons from reviewing Iran's accomplishments at Natanz is just how unachievable a commercial enrichment program remains, while at the same time, how comparatively little enrichment capability is required for a nuclear weapons capability." 
This result partly stems from the fact that centrifuge devices themselves do not cost very much to operate, so their usefulness in producing usable fuel must be carefully weighed against their ability to relatively easily produce weapons-grade uranium.  Especially in an economy and government as unstable as that in Iran, the weapons could pose a threat to cities in other countries if they somehow get into the wrong hands. On the contrary, the technology is still very useful for civilian purposes, shown by the recent developments in the US for getting the centrifuge program back on its feet.  Thus, it remains necessary for the world to carefully weigh the pros and cons of enrichment programs using centrifuges 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 unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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