View source for Physics:Gas centrifuge

{{Short description|Device that performs isotope separation of gases}}
[[File:Countercurrent Gas Centrifuge.svg|thumb|Diagram of a gas centrifuge with countercurrent flow, used for separating isotopes of uranium.]]
A '''gas centrifuge''' is a device that performs [[Physics:Isotope separation|isotope separation]] of gases. A [[Medicine:Centrifuge|centrifuge]] relies on the principles of [[Physics:Centrifugal force|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 [[Physics:Uranium-235|uranium-235]] (<sup>235</sup>U) from [[Physics:Uranium-238|uranium-238]] (<sup>238</sup>U). The gas centrifuge was developed to replace the [[Physics:Gaseous diffusion|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 [[Medicine:Centrifuge|centrifuge]] relies on the force resulting from centrifugal [[Physics:Acceleration|acceleration]] to separate molecules according to their mass, and can be applied to most fluids.<ref name="first">[http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=basic-centrifugation.htm&ID=30 Basics of Centrifuge - Cole Parmer]</ref> 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.<ref name="Journal of Nuclear Science and Technology">{{cite journal |last1=Khan |first1=Abdul Qadeer |author2=Atta, M. A. |author3=Mirza, J. A. |title=Flow Induced Vibrations in Gas Tube Assembly of Centrifuge |journal=Journal of Nuclear Science and Technology |date=1 September 1986 |volume=23 |issue=9 |pages=819–827 |doi=10.1080/18811248.1986.9735059 |bibcode=1986JNST...23..819A |doi-access=free }}</ref> 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.<ref name="Journal of Nuclear Science and Technology"/> For <sup>235</sup>U production, the heavier stream is the waste stream and the lighter stream is the product stream. Modern [[Physics:Zippe-type centrifuge|Zippe-type centrifuge]]s 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 [[Physics:Countercurrent exchange|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 [[Chemistry:Fraction|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==
[[Image:Gas centrifuge cascade.jpg|thumb|Cascade of gas centrifuges used to produce enriched uranium. U.S. gas centrifuge testbed in Piketon, Ohio, 1984. Each centrifuge is some {{Convert|40|ft|m|0}} tall. (Conventional centrifuges in use today are much smaller, less than {{Convert|5|m|ft|0}} high.)]]
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 [[Software:Batch processing|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 [[Engineering:Rotor (electric)|rotor]], a casing, an [[Physics:Electric motor|electric motor]], and three lines for material to travel. The gas centrifuge is designed with a casing that completely encloses the centrifuge.<ref name="third">[http://www.globalsecurity.org/wmd/intro/u-centrifuge.htm Gas Centrifuge Uranium Enrichment]</ref> The cylindrical rotor is located inside the casing, which is [[Physics:Vacuum|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.<ref name="second">[http://www.exportcontrols.org/centrifuges.html What is a Gas Centrifuge?] {{webarchive|url=https://web.archive.org/web/20030512093801/http://www.exportcontrols.org/centrifuges.html |date=12 May 2003 }}</ref>  The process begins when the rotor is balanced in three stages.<ref name="JNSCT2">{{cite journal |last1=Khan |first1=A.Q. |author2=Suleman, M. |author3=Ashraf, M. |author4=Khan, M. Zubair |title=Some Practical Aspects of Balancing an Ultra-Centrifuge Rotor |journal=Journal of Nuclear Science and Technology |date=1 November 1987 |volume=24 |issue=11 |pages=951–959 |doi=10.1080/18811248.1987.9733526 |bibcode=1987JNST...24..951K |doi-access=free }}</ref> Most of the technical details on gas centrifuges are difficult to obtain because they are shrouded in "nuclear secrecy".<ref name="JNSCT2"/>

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.<ref>{{cite journal | last1=Bogovalov | first1=Sergey | last2=Borman | first2=Vladimir | title=Separative Power of an Optimised Concurrent Gas Centrifuge | journal=Nuclear Engineering and Technology | publisher=Elsevier BV | volume=48 | issue=3 | year=2016 | issn=1738-5733 | doi=10.1016/j.net.2016.01.024 | pages=719–726 | doi-access=free | arxiv=1506.00823 }}</ref>

Inducing a countercurrent flow uses [[Biology:Countercurrent multiplication|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<ref name="van Wissen Golombok Brouwers 2005 pp. 4397–4407">{{cite journal | last1=van Wissen | first1=Ralph | last2=Golombok | first2=Michael | last3=Brouwers | first3=J.J.H. | title=Separation of carbon dioxide and methane in continuous countercurrent gas centrifuges | journal=Chemical Engineering Science | publisher=Elsevier BV | volume=60 | issue=16 | year=2005 | issn=0009-2509 | doi=10.1016/j.ces.2005.03.010 | pages=4397–4407| bibcode=2005ChEnS..60.4397V }}</ref>).

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.<ref>{{cite web |title=Engineering Considerations for Gas Centrifuges |url=https://fas.org/issues/nonproliferation-counterproliferation/nuclear-fuel-cycle/uranium-enrichment-gas-centrifuge-technology/engineering-considerations-gas-centrifuges/ |website=Federation of American Scientists |access-date=13 January 2020}}</ref> 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 <math>W_\mathrm{SWU}</math> necessary to separate a mass <math>F</math> of feed of assay <math>x_{f}</math> into a mass <math>P</math> of product assay <math>x_{p}</math>, and tails of mass <math>T</math> and assay <math>x_{t}</math> is expressed in terms of the number of separative work units needed, given by the expression<br />
<br />
:<math>W_\mathrm{SWU} = P \cdot V\left(x_{p}\right)+T \cdot V(x_{t})-F \cdot V(x_{f})</math>
:where <math>V\left(x\right)</math> is the [[Bellman equation|value function]], defined as
:<math>V(x) = (1 - 2x) \cdot \ln\left(\frac{1 - x}{x}\right)</math>

===Practical application of centrifugation===

====Separating uranium-235 from uranium-238====
The separation of uranium requires the material in a gaseous form; [[Chemistry:Uranium hexafluoride|uranium hexafluoride]] (UF<sub>6</sub>) is used for uranium enrichment. Upon entering the centrifuge cylinder, the UF<sub>6</sub> 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 [[Physics:Zinc-64|zinc-64]] in [[Chemistry:Zinc|zinc]] metal has to be lowered in order to prevent formation of radioisotopes by its [[Physics:Neutron activation|neutron activation]]. Diethyl zinc is used as the gaseous feed medium for the centrifuge cascade. An example of a resulting material is [[Physics:Depleted zinc oxide|depleted zinc oxide]], used as a [[Chemistry:Corrosion inhibitor|corrosion inhibitor]].

==History==
Suggested in 1919, the centrifugal process was first successfully performed in 1934. American scientist [[Biography:Jesse Beams|Jesse Beams]] and his team at the [[Organization:University of Virginia|University of Virginia]] developed the process by separating two [[Physics:Isotopes of chlorine|chlorine isotopes]] through a vacuum ultracentrifuge. It was one of the initial isotopic separation means pursued during the [[Physics:Manhattan Project|Manhattan Project]], more particularly by [[Biography:Harold Urey|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 ([[Physics:Gaseous diffusion|gaseous diffusion]] and [[Physics:Calutron|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 [[Physics:Enriched uranium|enriched uranium]]. [[Biography:Francis Simon|Franz Simon]], [[Biography:Rudolf Peierls|Rudolf Peierls]], [[Biography:Klaus Fuchs|Klaus Fuchs]] and [[Biography:Nicholas Kurti|Nicholas Kurti]] made important contributions to the centrifugal process. [[Biography:Paul Dirac|Paul Dirac]] also made important theoretical contributions to the centrifugal process during [[Social:World War II|World War II]].<ref>{{Cite journal |last=Olander |first=Donald R. |date=1978 |title=The Gas Centrifuge |url=https://www.jstor.org/stable/24960352 |journal=Scientific American |volume=239 |issue=2 |pages=37–43 |issn=0036-8733}}</ref><ref>{{Cite journal |last=Kemp |first=R. Scott |date=2009-06-26 |title=Gas Centrifuge Theory and Development: A Review of U.S. Programs |url=http://www.tandfonline.com/doi/abs/10.1080/08929880802335816 |journal=Science & Global Security |language=en |volume=17 |issue=1 |pages=1–19 |doi=10.1080/08929880802335816 |issn=0892-9882|doi-access=free }}</ref> Dirac developed the fundamental theory of separation processes that underlies the design and analysis of modern uranium enrichment plants.<ref>{{Cite journal |last=Gilinsky |first=Victor |date=2010 |title=Remembrances of Dirac |url=https://pubs.aip.org/physicstoday/article/63/5/59/387409/Remembrances-of-Dirac |journal=Physics Today |volume=63 |doi=10.1063/1.3431338|doi-access=free }}</ref>

In the long term, especially with the development of the [[Physics:Zippe-type centrifuge|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 [[Biography:Abdul Qadeer Khan|Abdul Qadeer Khan]] in the 1970s–80s, using vacuum methods for advancing the role of centrifuges in the development of [[Physics:Nuclear fuel|nuclear fuel]] for Pakistan's atomic bomb.<ref name="third"/> Many of the theorists working with Khan were unsure that either gaseous and enriched uranium would be feasible on time.<ref name="Stanford University Press">{{cite book|author1=Brigadier-General (retired) Feroz Hassan Khan|title=Eating grass: the making of the Pakistani bomb|date=November 7, 2012|publisher=Stanford University Press|isbn=978-0804776011|location=Stanford, California|page=151|chapter=Mastering the Uranium Enrichment|access-date=8 January 2013|chapter-url=https://books.google.com/books?id=yGgrNAsKZjEC&q=eating+grass+the+making+of+the+pakistani+bomb|chapter-format=google book}}</ref> 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."<ref name="Stanford University Press"/> 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 [[Religion:Axis of evil|three different countries]] by the end of the 20th century.<ref name=third /><ref name="Stanford University Press"/>

==See also==
* [[Engineering:Nuclear technology|Nuclear technology]]
* [[Physics:Nuclear power|Nuclear power]]
* [[Physics:Nuclear fuel|Nuclear fuel]]

==Footnotes==
{{Reflist}}

==References==
{{Refbegin}}
* [http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=basic-centrifugation.htm&ID=30 "Basics of Centrifugation."] Cole-Parmer Technical Lab. 14 Mar. 2008
* [http://www.globalsecurity.org/wmd/intro/u-centrifuge.htm "Gas Centrifuge Uranium Enrichment."] Global Security.Org. 27 Apr. 2005. 13 Mar. 2008
* [http://exportcontrols.info/centrifuges.html "What is a Gas Centrifuge?"] 2003. Institute for Science and International Security. 10 Oct. 2013
{{Refend}}

==External links==
*[https://web.archive.org/web/20051210222620/http://alsos.wlu.edu/qsearch.aspx?browse=science%2FGas+Centrifuge Annotated bibliography on the gas centrifuge from the Alsos Digital Library]
*[https://www.npr.org/templates/story/story.php?storyId=4857123 History of the Centrifuge]
*[http://exportcontrols.info/centrifuges.html What is a Gas Centrifuge?]
*[https://purl.fdlp.gov/GPO/gpo41180 Agreement between the Government of the United States of America and the Four Governments of the French Republic, the United Kingdom of Great Britain and Northern Ireland, the Kingdom of the Netherlands, and the Federal Republic of Germany Regarding the Establishment, Construction and Operation of Uranium Enriching Installations Using Gas Centrifuge Technology in the United States of America] United States Department of State

{{DEFAULTSORT:Gas Centrifuge}}
[[Category:Isotope separation]]
[[Category:Nuclear chemistry]]
[[Category:Uranium]]
[[Category:American inventions|A]]

{{Sourceattribution|Gas centrifuge}}