Gas centrifuge

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Diagram of a gas centrifuge.

A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container. A prominent use of gas centrifuges is for the separation of uranium-235 from uranium-238. The gas centrifuge was developed to replace the gaseous diffusion method of uranium-235 extraction. High degrees of separation of these isotopes relies on using many individual centrifuges arranged in cascade, that successively achieve higher concentrations. This process yields higher concentrations of uranium-235 while using significantly less energy compared to the gaseous diffusion process.

Centrifugal process[edit]

The centrifuge relies on the force resulting from centripetal acceleration to separate molecules according to their mass, and can be applied to most fluids.[1] The dense (heavier) molecules move towards the wall and the lighter ones remain close to the center. The centrifuge consists of a rigid body rotor rotating at full period at high speed.[2] The Gas tube is located in the center of the rotor which is used to introduce feed gas into the rotor that removes the heavier product and waste streams in it.[2] In addition, if one creates a thermal gradient in a perpendicular direction by keeping the top of the rotating column cool and the bottom hot, the resulting convection current carries the lighter molecules to the top while the heavier ones settle at the bottom, from which they can be continuously withdrawn.[2]

In practice, several such centrifuges are connected in series. Each centrifuge receives one input and produces two output lines, corresponding to light and heavy fractions. The input of each centrifuge is the output (light) of the previous centrifuge and the output (heavy) of the following stage. This produces an almost pure light fraction from the output (light) of the last centrifuge and an almost pure heavy fraction from the output (heavy) of the first centrifuge.

Gas centrifugation process[edit]

Cascade of gas centrifuges used to produce enriched uranium. U.S. gas centrifuge testbed in Piketon, Ohio, 1984. Each centrifuge is some 40ft tall. (Conventional centrifuges in use today are much smaller, less than 5 meters high.)

The gas centrifugation process utilizes a unique design that allows gas to constantly flow in and out of the centrifuge. Unlike most centrifuges which rely on batch processing, the gas centrifuge utilizes continuous processing, allowing cascading, in which multiple identical processes occur in succession. The gas centrifuge consists of a cylindrical rotor, a casing, an electric motor, and three lines for material to travel. The gas centrifuge is designed with a casing that completely encloses the centrifuge.[3] The cylindrical rotor is located inside the casing, which is evacuated of all air to produce a near frictionless rotation when operating. The motor spins the rotor, creating the centripetal force on the components as they enter the cylindrical rotor. There are two output lines, one located at the top of the centrifuge and the other located at the bottom. The heavier molecules will segregate to the bottom of the centrifuge while the lighter molecules will segregate to the top of the centrifuge. The output lines take these separations to other centrifuges to continue to the centrifugation process.[4] The process began with the rotor is balanced in three stages.[5] Most of the technical work on gas centrifuges hardly available because most of the work is shrouded in nuclear secrecy.[5]

Separative work units[edit]

The separative work unit (SWU) is a measure of the amount of work done by the centrifuge and has units of mass (typically kilogram separative work unit). The work W_\mathrm{SWU} necessary to separate a mass F of feed of assay x_{f} into a mass P of product assay x_{p}, and tails of mass T and assay x_{t} is expressed in terms of the number of separative work units needed, given by the expression

W_\mathrm{SWU} = P \cdot V\left(x_{p}\right)+T \cdot V(x_{t})-F \cdot V(x_{f})


where V\left(x\right) is the value function, defined as


V(x) = (1 - 2x) \cdot \ln\left(\frac{1 - x}{x}\right)

Practical application of centrifugation[edit]

Separating Uranium-235 from Uranium-238[edit]

The separation of uranium requires the material in a gaseous form; uranium hexafluoride (UF6) is used for uranium enrichment. Upon entering the centrifuge cylinder, the UF6 gas is rotated at a high speed. The rotation creates a strong centrifugal force that draws more of the heavier gas molecules (containing the U-238) toward the wall of the cylinder, while the lighter gas molecules (containing the U-235) tend to collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage.

Separation of zinc isotopes[edit]

For some uses in nuclear technology, the content of zinc-64 in zinc metal has to be lowered in order to prevent formation of radioisotopes by its neutron activation. Diethyl zinc is used as the gaseous feed medium for the centrifuge cascade. An example of a resulting material is depleted zinc oxide, used as a corrosion inhibitor.

History[edit]

Suggested in 1919, the centrifugal process was first successfully performed in 1934. American scientist Jesse Beams and his team at the University of Virginia developed the process by separating two chlorine isotopes through a vacuum ultracentrifuge. It was one of the initial isotopic separation means pursued during the Manhattan Project, but research was discontinued in 1944 as it was felt that the method would not produce results by the end of the war, and that other means of uranium enrichment (gaseous diffusion and electromagnetic separation) had a better chance of success in the short term. This method was successfully used in the Soviet nuclear program, making the Soviet Union the most effective supplier of enriched uranium.

In the long term, especially with the development of the Zippe-type centrifuge, the gas centrifuge has become a very economical mode of separation, using considerably less energy than other methods and having numerous other advantages. Effective usage of gas centrifuges were discovered by Pakistan which greatly enhances its capability to produce HEU fuels for both its commercial nuclear power plants and weapons. Pioneering research in physical performance of the centrifuges were studied by the Pakistani scientist Abdul Qadeer Khan in 1970s–80s, using the meaningful vacuum methods for advancing the role of the centrifuges for the development of nuclear fuel.[3] According to one theoretical physicist involved in the program maintained that the centrifuge program was quite difficult, the most enduring, and challenging project that scientists were tackling and studying.[6] Many of the theorists working with A.Q. Khan were unsure that either gaseous and enriched uranium would be feasible on time.[6] The scientist recalled his memories: "No one in the world has used the [gas] centrifuge method to produce military-grade uranium.... This was not going to work. He [A.Q. Khan] was simply wasting time."[6] Nonetheless and in spite of skepticism, the program was made feasible by Pakistan in the shortest time possible and enrichment by centrifuge has been used in physics experiments and effective physical use, particularly by Abdul Qadeer Khan in Pakistan, and the method was smuggled to at least three different countries by the end of the 20th century.[3][6]

See also[edit]

Footnotes[edit]

  1. ^ Basics of Centrifuge - Cole Parmer
  2. ^ a b c Khan, Abdul Qadeer; Atta, M. A.; Mirza, J. A. (1 September 1986). "Flow Induced Vibrations in Gas Tube Assembly of Centrifuge". Journal of Nuclear Science and Technology 23 (9): 819–827. doi:10.1080/18811248.1986.9735059. Retrieved 10 December 2012. 
  3. ^ a b c Gas Centrifuge Uranium Enrichment
  4. ^ What is a Gas Centrifuge?
  5. ^ a b Khan, A.Q.; Suleman, M.; Ashraf, M.; Khan, M. Zubair (1 November 1987). "Some Practical Aspects of Balancing an Ultra-Centrifuge Rotor". Journal of Nuclear Science and Technology (JNS&T) 24 (11): 951–959. doi:10.1080/18811248.1987.9733526. Retrieved 10 December 2012. 
  6. ^ a b c d Brigadier-General (retired) Feroz Hassan Khan (November 7, 2012). "Mastering the Uranium Enrichment" (google book). Eating grass: the making of the Pakistani bomb (in English (Pakistan)). Stanford, California: Stanford University Press. p. 151. ISBN 978-0804776011. Retrieved 8 January 2013. 

References[edit]

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