Gas centrifuge

Last updated January 03, 2024

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 (²³⁵U) from uranium-238 (²³⁸U). 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 series, that achieve successively higher concentrations. This process yields higher concentrations of uranium-235 while using significantly less energy compared to the gaseous diffusion process.

Centrifugal process

The centrifuge relies on the force resulting from centrifugal 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] Concentric gas tubes located on the axis of the rotor are used to introduce feed gas into the rotor and extract the heavier and lighter separated streams.^[2] For ²³⁵U production, the heavier stream is the waste stream and the lighter stream is the product stream. Modern Zippe-type centrifuges are tall cylinders spinning on a vertical axis. A vertical temperature gradient can be applied to create a convective circulation rising in the center and descending at the periphery of the centrifuge. Such a countercurrent flow can also be stimulated mechanically by the scoops that take out the enriched and depleted fractions. Diffusion between these opposing flows increases the separation by the principle of countercurrent multiplication.

In practice, since there are limits to how tall a single centrifuge can be made, 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

The gas centrifugation process uses 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 uses 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 centrifugal force on the components as they enter the cylindrical rotor. This force acts to separate the molecules of the gas, with heavier molecules moving towards the wall of the rotor, and the lighter molecules towards the central axis. There are two output lines, one for the fraction enriched in the desired isotope (in uranium separation, this is U-235), and one depleted in it. The output lines take these separations to other centrifuges to continue the centrifugation process.^[4] The process begins when the rotor is balanced in three stages.^[5] Most of the technical details on gas centrifuges are difficult to obtain because they are shrouded in "nuclear secrecy".^[5]

The early centrifuges used in the UK used an alloy body wrapped in epoxy impregnated glass fibre. Dynamic balancing of the assembly was accomplished by adding small traces of epoxy at the locations indicated by the balancing test unit. The motor was usually a pancake type located at the bottom of the cylinder. The early units were typically around 2 metres long (approx.), but subsequent developments gradually increased the length. The present generation are over 4 metres in length. The bearings are gas-based devices, as mechanical bearings would not survive at the normal operating speeds of these centrifuges.

A section of centrifuges would be fed with variable-frequency AC from an electronic (bulk) inverter, which would slowly ramp them up to the required speed, generally in excess of 50,000 rpm. One precaution was to quickly get past frequencies at which the cylinder was known to suffer resonance problems. The inverter is a high frequency unit capable of operating at frequencies around 1 kilohertz. The whole process is normally silent; if a noise is heard coming from a centrifuge, it is a warning of failure (which normally occurs very quickly). The design of the cascade normally allows for the failure of at least one centrifuge unit without compromising the operation of the cascade. The units are normally very reliable, with early models having operated continuously for over 30 years.

Later models have steadily increased the rotation speed of the centrifuges, as it is the velocity of the centrifuge wall that has the most effect on the separation efficiency.

A feature of the cascade system of centrifuges is that it is possible to increase plant throughput incrementally, by adding cascade "blocks" to the existing installation at suitable locations, rather than having to install a completely new line of centrifuges.

Concurrent and countercurrent centrifuges

The simplest gas centrifuge is the concurrent centrifuge, where separative effect is produced by the centrifugal effects of the rotor's rotation. In these centrifuges, the heavy fraction is collected at the periphery of the rotor, and the light fraction from nearer the axis of rotation.^[6]

Inducing a countercurrent flow uses countercurrent multiplication to enhance the separative effect. A vertical circulating current is set up, with the gas flowing axially along the rotor walls in one direction, and a return flow closer to the center of the rotor. The centrifugal separation continues as before (heavier molecules preferentially moving outwards), which means that the heavier molecules are collected by the wall flow, and the lighter fraction collects at the other end. In a centrifuge with downward wall flow, the heavier molecules collect at the bottom. The outlet scoops are then placed at the ends of the rotor cavity, with the feed mixture injected along the axis of the cavity (ideally, the injection point is at the point where the mixture in the rotor is equal to the feed^[7]).

This countercurrent flow can be induced mechanically or thermally, or a combination. In mechanically induced countercurrent flow, the arrangement of the (stationary) scoops and internal rotor structures are used to generate the flow.^[8] A scoop interacts with the gas by slowing it, which tends to draw it into the centre of the rotor. The scoops at each end induce opposing currents, so one scoop is protected from the flow by a "baffle": a perforated disc within the rotor which rotates along with the gas—at this end of the rotor, the flow will be outwards, towards the rotor wall. Thus, in a centrifuge with a baffled top scoop, the wall flow is downwards, and heavier molecules are collected at the bottom.

Thermally induced convection currents can be created by heating the bottom of the centrifuge and/or cooling the top end.

Separative work units

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

Separating uranium-235 from uranium-238

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

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

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, more particularly by Harold Urey and Karl P. Cohen, 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. Franz Simon, Rudolf Peierls, Klaus Fuchs and Nicholas Kurti made important contributions to the centrifugal process. Paul Dirac also made important theoretical contributions to the centrifugal process during World War II.^[9]^[10] Dirac developed the fundamental theory of separation processes that underlies the design and analysis of modern uranium enrichment plants.^[11]

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.

Research in the physical performance of centrifuges was carried out by the Pakistani scientist Abdul Qadeer Khan in the 1970s–80s, using vacuum methods for advancing the role of centrifuges in the development of nuclear fuel for Pakistan's atomic bomb.^[3] Many of the theorists working with Khan were unsure that either gaseous and enriched uranium would be feasible on time.^[12] One scientist recalled: "No one in the world has used the [gas] centrifuge method to produce military-grade uranium.... This was not going to work. He was simply wasting time."^[12] In spite of skepticism, the program was quickly proven to be feasible. Enrichment via centrifuge has been used in experimental physics, and the method was smuggled to at least three different countries by the end of the 20th century.^[3]^[12]

Footnotes

↑ Basics of Centrifuge - Cole Parmer
1 2 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. Bibcode:1986JNST...23..819A. doi: 10.1080/18811248.1986.9735059 .
1 2 3 Gas Centrifuge Uranium Enrichment
↑ What is a Gas Centrifuge? Archived 12 May 2003 at the Wayback Machine
1 2 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. 24 (11): 951–959. Bibcode:1987JNST...24..951K. doi: 10.1080/18811248.1987.9733526 .
↑ Bogovalov, Sergey; Borman, Vladimir (2016). "Separative Power of an Optimised Concurrent Gas Centrifuge". Nuclear Engineering and Technology. Elsevier BV. 48 (3): 719–726. arXiv: 1506.00823 . doi: 10.1016/j.net.2016.01.024 . ISSN 1738-5733.
↑ van Wissen, Ralph; Golombok, Michael; Brouwers, J.J.H. (2005). "Separation of carbon dioxide and methane in continuous countercurrent gas centrifuges". Chemical Engineering Science. Elsevier BV. 60 (16): 4397–4407. Bibcode:2005ChEnS..60.4397V. doi:10.1016/j.ces.2005.03.010. ISSN 0009-2509.
↑ "Engineering Considerations for Gas Centrifuges". Federation of American Scientists. Retrieved 13 January 2020.
↑ Olander, Donald R. (1978). "The Gas Centrifuge". Scientific American. 239 (2): 37–43. ISSN 0036-8733.
↑ Kemp, R. Scott (26 June 2009). "Gas Centrifuge Theory and Development: A Review of U.S. Programs". Science & Global Security. 17 (1): 1–19. doi: 10.1080/08929880802335816 . ISSN 0892-9882.
↑ Gilinsky, Victor (2010). "Remembrances of Dirac". Physics Today. 63. doi: 10.1063/1.3431338 .
1 2 3 Brigadier-General (retired) Feroz Hassan Khan (7 November 2012). "Mastering the Uranium Enrichment" (google book). Eating grass: the making of the Pakistani bomb. Stanford, California: Stanford University Press. p. 151. ISBN 978-0804776011 . Retrieved 8 January 2013.

Related Research Articles

Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research. By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. In the following text, mainly uranium enrichment is considered. This process is crucial in the manufacture of uranium fuel for nuclear power plants, and is also required for the creation of uranium-based nuclear weapons. Plutonium-based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade.

Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238, uranium-235, and uranium-234. ²³⁵U is the only nuclide existing in nature that is fissile with thermal neutrons.

A centrifuge is a device that uses centrifugal force to subject a specimen to a specified constant force, for example to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby separating fluids of different densities or liquids from solids. It works by causing denser substances and particles to move outward in the radial direction. At the same time, objects that are less dense are displaced and moved to the centre. In a laboratory centrifuge that uses sample tubes, the radial acceleration causes denser particles to settle to the bottom of the tube, while low-density substances rise to the top. A centrifuge can be a very effective filter that separates contaminants from the main body of fluid.

Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate away from the axis of the centrifuge, while the less dense components of the mixture migrate towards the axis. Chemists and biologists may increase the effective gravitational force of the test tube so that the precipitate (pellet) will travel quickly and fully to the bottom of the tube. The remaining liquid that lies above the precipitate is called a supernatant or supernate.

<span class="mw-page-title-main">Uranium hexafluoride</span> Chemical compound

Uranium hexafluoride, sometimes called hex, is an inorganic compound with the formula UF₆. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with aluminium, forming a thin surface layer of AlF₃ that resists any further reaction from the compound. UF₆ is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.

<span class="mw-page-title-main">Harold Urey</span> American physical chemist (1893–1981)

Harold Clayton Urey was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 for the discovery of deuterium. He played a significant role in the development of the atom bomb, as well as contributing to theories on the development of organic life from non-living matter.

<span class="mw-page-title-main">Yellowcake</span> Uranium concentrate powder

Yellowcake is a type of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. It is a step in the processing of uranium after it has been mined but before fuel fabrication or uranium enrichment. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores. Typically, yellowcakes are obtained through the milling and chemical processing of uranium ore, forming a coarse powder that has a pungent odor, is insoluble in water, and contains about 80% uranium oxide, which melts at approximately 2880 °C.

Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF₆) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containing uranium-235 (²³⁵U) and uranium-238 (²³⁸U). By use of a large cascade of many stages, high separations can be achieved. It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficient gas centrifuge process (enrichment factor 1.05 to 1.2).

Erich Rudolf Bagge was a German scientist. Bagge, a student of Werner Heisenberg for his doctorate and Habilitation, was engaged in German Atomic Energy research and the German nuclear energy project during the Second World War. He worked as an Assistant at the Kaiser-Wilhelm-Institut für Physik in Berlin. Bagge, who became associated professor at the University of Hamburg in 1948, was in particular involved in the usage of nuclear power for trading vessels, and he was one of the founders of the Society for the Usage of Nuclear Energy in Ship-Building and Seafare.

Atomic vapor laser isotope separation, or AVLIS, is a method by which specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions. A similar technology, using molecules instead of atoms, is molecular laser isotope separation (MLIS).

K-25 was the codename given by the Manhattan Project to the program to produce enriched uranium for atomic bombs using the gaseous diffusion method. Originally the codename for the product, over time it came to refer to the project, the production facility located at the Clinton Engineer Works in Oak Ridge, Tennessee, the main gaseous diffusion building, and ultimately the site. When it was built in 1944, the four-story K-25 gaseous diffusion plant was the world's largest building, comprising over 5,264,000 square feet (489,000 m²) of floor space and a volume of 97,500,000 cubic feet (2,760,000 m³).

Molecular laser isotope separation (MLIS) is a method of isotope separation, where specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions of uranium hexafluoride molecules. It is similar to AVLIS. Its main advantage over AVLIS is low energy consumption and use of uranium hexafluoride instead of vaporized uranium.

The Helikon vortex separation process is an aerodynamic uranium enrichment process designed around a device called a vortex tube. Paul Dirac thought of the idea for isotope separation and tried creating such a device in 1934 in the lab of Peter Kapitza at Cambridge. Other methods of separation were more practical at that time, but this method was designed and used in South Africa for producing reactor fuel with a uranium-235 content of around 3–5%, and 80–93% enriched uranium for use in nuclear weapons. The Uranium Enrichment Corporation of South Africa, Ltd. (UCOR) developed the process, operating a facility at Pelindaba to produce hundreds of kilograms of HEU. Aerodynamic enrichment processes require large amounts of electricity and are not generally considered economically competitive because of high energy consumption and substantial requirements for removal of waste heat. There are other ways in which it is advantageous, e.g. In simplicity, lack of precision required, even if more expensive. The South African enrichment plant was closed on 1 February 1990.

Separation of isotopes by laser excitation (SILEX) is a process under development to enrich uranium on an industrial scale for nuclear reactors. It is strongly suspected that it utilizes laser condensation repression to excite the uranium-235 isotope in uranium hexafluoride (UF₆), allowing this lighter molecule to move more rapidly to the outer rim of a gaseous jet and resist condensing compared to the heavier, unexcited ²³⁸UF₆. This differs greatly from previous methods of laser enrichment explored for their commercial prospects: one using atomic uranium (Atomic Vapor Laser Isotope Separation (AVLIS)) and another molecular method that uses lasers to dissociate a fluorine atom from ²³⁵UF₆ (Molecular Laser Isotope Separation (MLIS)), allowing the enriched product to precipitate out as a solid.

The Portsmouth Gaseous Diffusion Plant is a facility located in Scioto Township, Pike County, Ohio, just south of Piketon, Ohio, that previously produced enriched uranium, including highly enriched weapons-grade uranium, for the United States Atomic Energy Commission (AEC), the U.S. nuclear weapons program and Navy nuclear propulsion; in later years, it produced low-enriched uranium for fuel for commercial nuclear power reactors. The site never hosted an operating nuclear reactor.

The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile isotope uranium-235 (²³⁵U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in mass of the isotopes. The Zippe design was originally developed in the Soviet Union by a team led by 60 Austrian and German scientists and engineers captured after World War II, working in detention. In the West the type is known by the name of the man who recreated the technology after his return to the West in 1956, based on his recollection of his work in the Soviet program, Gernot Zippe. To the extent that it might be referred to in Soviet/Russian usage by any one person's name, it was known as a Kamenev centrifuge.

A separation process is a method that converts a mixture or a solution of chemical substances into two or more distinct product mixtures, a scientific process of separating two or more substance in order to obtain purity. At least one product mixture from the separation is enriched in one or more of the source mixture's constituents. In some cases, a separation may fully divide the mixture into pure constituents. Separations exploit differences in chemical properties or physical properties between the constituents of a mixture.

Separative work – the amount of separation done by an enrichment process – is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units which are so calculated as to be proportional to the total input and to the mass processed.

Centrifugal partition chromatography is a special chromatographic technique where both stationary and mobile phase are liquid, and the stationary phase is immobilized by a strong centrifugal force. Centrifugal partition chromatography consists of a series-connected network of extraction cells, which operates as elemental extractors, and the efficiency is guaranteed by the cascade.

A centrifuge is a device that uses centrifugal force to separate its components.

References

"Basics of Centrifugation." Cole-Parmer Technical Lab. 14 Mar. 2008
"Gas Centrifuge Uranium Enrichment." Global Security.Org. 27 Apr. 2005. 13 Mar. 2008
"What is a Gas Centrifuge?" 2003. Institute for Science and International Security. 10 Oct. 2013

External links

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[first-1] Basics of Centrifuge - Cole Parmer

[Journal_of_Nuclear_Science_and_Technology-2] 1 2 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. Bibcode:1986JNST...23..819A. doi: 10.1080/18811248.1986.9735059 .

[third-3] 1 2 3 Gas Centrifuge Uranium Enrichment

[second-4] What is a Gas Centrifuge? Archived 12 May 2003 at the Wayback Machine

[JNSCT2-5] 1 2 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. 24 (11): 951–959. Bibcode:1987JNST...24..951K. doi: 10.1080/18811248.1987.9733526 .

[6] Bogovalov, Sergey; Borman, Vladimir (2016). "Separative Power of an Optimised Concurrent Gas Centrifuge". Nuclear Engineering and Technology. Elsevier BV. 48 (3): 719–726. arXiv: 1506.00823 . doi: 10.1016/j.net.2016.01.024 . ISSN 1738-5733.

[van_Wissen_Golombok_Brouwers_2005_pp._4397–4407-7] van Wissen, Ralph; Golombok, Michael; Brouwers, J.J.H. (2005). "Separation of carbon dioxide and methane in continuous countercurrent gas centrifuges". Chemical Engineering Science. Elsevier BV. 60 (16): 4397–4407. Bibcode:2005ChEnS..60.4397V. doi:10.1016/j.ces.2005.03.010. ISSN 0009-2509.

[8] "Engineering Considerations for Gas Centrifuges". Federation of American Scientists. Retrieved 13 January 2020.

[9] Olander, Donald R. (1978). "The Gas Centrifuge". Scientific American. 239 (2): 37–43. ISSN 0036-8733.

[10] Kemp, R. Scott (26 June 2009). "Gas Centrifuge Theory and Development: A Review of U.S. Programs". Science & Global Security. 17 (1): 1–19. doi: 10.1080/08929880802335816 . ISSN 0892-9882.

[11] Gilinsky, Victor (2010). "Remembrances of Dirac". Physics Today. 63. doi: 10.1063/1.3431338 .

[Stanford_University_Press-12] 1 2 3 Brigadier-General (retired) Feroz Hassan Khan (7 November 2012). "Mastering the Uranium Enrichment" (google book). Eating grass: the making of the Pakistani bomb. Stanford, California: Stanford University Press. p. 151. ISBN 978-0804776011 . Retrieved 8 January 2013.

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