Editing Isotope separation (section)

==Methodology==

===Diffusion===
[[File:Gaseous Diffusion (44021367082) (cropped).jpg|thumb|upright=1.2|Gaseous diffusion uses microporous membranes to enrich uranium]]
Often done with gases, but also with liquids, the [[diffusion]] method relies on the fact that in thermal equilibrium, two isotopes with the same energy will have different average velocities. The lighter atoms (or the molecules containing them) will travel more quickly through a membrane, whose pore diameters are not larger than the mean free path length ([[Free molecular flow|Knudsen flow]]). The speed ratio is equal to the inverse square root of the mass ratio, so the amount of separation is small. For example for <sup>235</sup>UF<sub>6</sub> versus <sup>238</sup>UF<sub>6</sub> it is 1.0043. Hence many cascaded stages are needed to obtain high purity. This method is expensive due to the work needed to push gas through a membrane and the many stages necessary, each requiring recompression of the gas.

The first large-scale separation of uranium isotopes was achieved by the United States in large [[gaseous diffusion]] separation plants at [[Clinton Engineering Works]], which were established as part of the [[Manhattan Project]]. These used [[uranium hexafluoride]] gas as the process fluid. Nickel powder and electro-deposited nickel mesh diffusion barriers were pioneered by Edward Adler and Edward Norris.<ref name="Rhodes1986">{{cite book |author=Richard Rhodes |title=The Making of the Atomic Bomb |url=https://archive.org/details/makingofatomicbo00rhod |url-access=registration |access-date=January 17, 2014 |year=1986 |publisher=Simon & Schuster |isbn=978-0-684-81378-3 |page=[https://archive.org/details/makingofatomicbo00rhod/page/494 494]}}</ref> Due to the high energy consumption, enrichment of uranium by diffusion was gradually replaced by more efficient methods. 

The last diffusion plant closed in 2013.<ref>World Nuclear Association, US Nuclear Fuel Cycle, (2015), http://www.world- 

nuclear.org/info/Country-Profiles/Countries-T-Z/USA--Nuclear-Fuel-Cycle/</ref> The [[Paducah Gaseous Diffusion Plant]] was a US government effort to generate highly enriched uranium to power military reactors and create nuclear bombs which led to the establishment of the facility in 1952. Paducah's enrichment was initially kept to low levels, and the facility operated as a "feed facility" for other defence facilities that processed the enriched uranium at [[Oak Ridge National Laboratory]] in [[Oak Ridge, Tennessee]], and [[Portsmouth Gaseous Diffusion Plant]] in [[Piketon, Ohio]]. The goal of Paducah and its sister facility in Piketon was adjusted in the 1960s when they started to enrich uranium for use in commercial nuclear reactors to produce energy.<ref>{{Cite web |title=Paducah |url=https://www.centrusenergy.com/who-we-are/history/gaseous-diffusion-plants/paducah/ |access-date=2023-04-30 |website=Centrus Energy Corp |language=en-US}}</ref>

===Centrifugal===
[[File:Gas centrifuge cascade.jpg|right|150px|thumb|A cascade of gas centrifuges at a US uranium enrichment plant.]]

[[Centrifugal force|Centrifugal]] schemes rapidly rotate the material allowing the heavier isotopes to go closer to an outer radial wall. This is often done in gaseous form using a [[Zippe-type centrifuge]]. Centrifuging [[plasma (physics)|plasma]] can separate isotopes as well as separating ranges of elements for radioactive waste reduction, nuclear reprocessing, and other purposes. The process is called "plasma mass separation"; the devices are called "plasma mass filter" or "plasma centrifuge" (not to be confused with [[Laboratory centrifuge|medical centrifuges]]).<ref>{{cite journal |last1=Zweben |first1=Stewart J. |last2=Gueroult |first2=Renaud |last3=Fisch |first3=Nathaniel J. |date=12 September 2018 |title=Plasma mass separation |journal=[[Physics of Plasmas]] |volume=25 |issue=9 |page=090901 |doi=10.1063/1.5042845 |bibcode=2018PhPl...25i0901Z |osti=1472074 |s2cid=226888946 |issn=1070-664X|url=https://www.osti.gov/biblio/1472074 }}</ref>

The centrifugal separation of isotopes was first suggested by Aston and Lindemann<ref>{{cite journal |last1=Lindemann |first1=F. A |author-link=Frederick Lindemann, 1st Viscount Cherwell |last2=Aston |first2=F. W. |author-link2=Francis William Aston |title=The possibility of separating isotopes |journal=Philosophical Magazine |series=Series 6 |volume=37 |issue=221 |pages=523–534 |date=1919 |doi=10.1080/14786440508635912 |url=https://zenodo.org/record/1430756}}</ref> in 1919 and the first successful experiments were reported by Beams and Haynes<ref>{{cite journal |last1=Beams |first1=J. W. |last2=Haynes |first2=F. B. |title=The Separation of Isotopes by Centrifuging |journal=Physical Review |publisher=American Physical Society (APS) |volume=50 |issue=5 |date=1936-09-01 |issn=0031-899X |doi=10.1103/physrev.50.491 |pages=491–492|bibcode=1936PhRv...50..491B }}</ref> on isotopes of chlorine in 1936. However attempts to use the technology during the Manhattan Project were unproductive. In modern times it is the main method used throughout the world to enrich uranium and as a result remains a fairly secretive process, hindering a more widespread uptake of the technology. In general a feed of UF<sub>6</sub> gas is connected to a cylinder that is rotated at high speed. Near the outer edge of the cylinder heavier gas molecules containing U-238 collect, while molecules containing U-235 concentrate at the centre and are then fed to another cascade stage.<ref>{{cite journal |last=Whitley |first=Stanley |title=Review of the gas centrifuge until 1962. Part I: Principles of separation physics |journal=Reviews of Modern Physics |publisher=American Physical Society (APS) |volume=56 |issue=1 |date=1984-01-01 |issn=0034-6861 |doi=10.1103/revmodphys.56.41 |pages=41–66|bibcode=1984RvMP...56...41W }}</ref> 

Use of gaseous centrifugal technology to enrich isotopes is desirable as power consumption is greatly reduced when compared to more conventional techniques such as diffusion plants since fewer cascade steps are required to reach similar degrees of separation. As well as requiring less energy to achieve the same separation, far smaller scale plants are possible, making them an economic possibility for a small nation attempting to produce a nuclear weapon. Pakistan is believed to have used this method in developing its nuclear weapons.

[[Vortex tube]]s were used by [[South Africa]] in their [[Helikon vortex separation process]]. The gas is injected tangentially into a chamber with special geometry that further increases its rotation to a very high rate, causing the isotopes to separate.<ref>{{cite web | url=https://inis.iaea.org/search/search.aspx?orig_q=RN:8303315 | title=The Helikon technique for isotope enrichment | date=1976 | last1=p. c. | first1=Haarhoff }}</ref> The method is simple because vortex tubes have no moving parts, but energy intensive, about 50 times greater than gas centrifuges. A similar process, known as ''jet nozzle'' was created in Germany, with a demonstration plant built in Brazil, and they went as far as developing a site to fuel the country's nuclear plants.<ref>{{cite web | url=https://inis.iaea.org/search/search.aspx?orig_q=RN:22063382 | title=Uranium enrichment by jet nozzle separation process in the German-Brazil cooperation program | last1=e. w. | first1=Becker }}</ref>

===Electromagnetic===
[[File:Diagram of uranium isotope separation in the calutron.png|thumb|upright=1.3|Schematic diagram of uranium isotope separation in a [[calutron]]]]
Electromagnetic separation is [[mass spectrometry]] on a large scale, so it is sometimes referred to as mass spectrometry. It uses the fact that charged particles are deflected in a [[magnetic field]] and the amount of deflection depends upon the particle's mass. It is very expensive for the quantity produced, as it has an extremely low throughput, but it can allow very high purities to be achieved. This method is often used for processing small amounts of pure isotopes for research or specific use (such as [[isotopic tracer]]s) but is impractical for industrial use.

At Oak Ridge National Laboratory and at the [[University of California, Berkeley]], [[Ernest O. Lawrence]] developed electromagnetic separation for much of the uranium used in the [[Manhattan Project|first atomic bombs]]. Devices using his principle are named [[calutron]]s. After the war the method was largely abandoned as impractical. It had only been undertaken (along with diffusion and other technologies) to guarantee there would be enough material for use, whatever the cost. Its main eventual contribution to the war effort was to further concentrate material from the gaseous diffusion plants to higher levels of purity.

===Laser===
In this method a [[laser]] is tuned to a wavelength which excites only one isotope of the material and ionizes those atoms preferentially. For atoms, the resonant absorption of light for an isotope depends on<ref>{{Cite journal |last1=Stern |first1=R. C. |last2=Snavely |first2=B. B. |date=January 1976 |title=The Laser Isotope Separation Program at Lawrence Livermore Laboratory.: Laser Isotope Separation |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.1976.tb41598.x |journal=Annals of the New York Academy of Sciences |language=en |volume=267 |issue=1 Third Confere |pages=71–80 |doi=10.1111/j.1749-6632.1976.tb41598.x |s2cid=97058155 |issn=0077-8923}}</ref>

* the nuclear mass (noticeable mainly with light elements)
* the nuclear volume (causing a deviation from the [[Electric potential energy|Coulomb potential]], noticeable for heavier elements) 
* [[hyperfine structure|hyperfine]] splitting of electronic transitions, if the nucleus has a spin,

allowing finely tuned lasers to interact with only one isotope. After the atom is ionized it can be removed from the sample by applying an [[electric field]]. This method is often abbreviated as AVLIS ([[atomic vapor laser isotope separation]]). This method has only been developed as laser technology has improved in the 1970s to 1980s. Attempts to develop it to an industrial scale for uranium enrichment were successively given up in the 1990s "due to never ending technical difficulties" and because centrifuges have reached technical maturity in the meantime.<ref>Werner Fuß: ''Laser isotope separation and proliferation risks''. (PDF) Max-Planck-Institut für Quantenoptik, 2015, https://www.mpq.mpg.de/5178012/MPQ346.pdf</ref><ref>Schneider, K. R., LIS: the view from Urenco (1995). (https://inis.iaea.org/search/search.aspx?orig_q=rn:27014297)</ref> However, it is a major concern to those in the field of [[nuclear proliferation]], because it may be cheaper and more easily hidden than other methods of isotope separation. [[Tunable laser]]s used in AVLIS include the [[dye laser]]<ref>[[F. J. Duarte]] and L.W. Hillman (Eds.), Dye Laser Principles (Academic, New York, 1990) Chapter 9.</ref> and more recently [[diode laser]]s.<ref>F. J. Duarte (Ed.), Tunable Laser Applications, 2nd Ed. (CRC, 2008) Chapter 11</ref>

A second method of laser separation is known as [[molecular laser isotope separation]] (MLIS). In this method, an infrared laser is directed at [[uranium hexafluoride]] gas (if enrichment of uranium is desired), exciting molecules that contain a [[uranium-235|U-235]] atom. A second laser, either also in the IR ([[infrared multiphoton dissociation]]) or in the UV, frees a [[fluorine]] atom, leaving [[uranium pentafluoride]] which then precipitates out of the gas. Cascading the MLIS stages is more difficult than with other methods because the UF<sub>5</sub> must be fluorinated back to UF<sub>6</sub> before being introduced into the next MLIS stage. But with light elements, the isotope selectivity is usually good enough that cascading is not required.

Several alternative MLIS schemes have been developed. For example, one uses a first laser in the near-infrared or visible region, where a selectivity of over 20:1 can be obtained in a single stage. This method is called OP-IRMPD (Overtone Pre-excitation—[[infrared multiphoton dissociation|IR Multiple Photon Dissociation]]). But due to the small absorption probability in the overtones, too many photons remain unused, so that the method did not reach industrial feasibility. Also some other MLIS methods suffer from wasting of the expensive photons.

Finally, the '[[Separation of isotopes by laser excitation]]' (SILEX) process, developed by [[Silex Systems]] in Australia, has been licensed to General Electric for the development of a pilot enrichment plant. For uranium, it uses a cold molecular beam with UF<sub>6</sub> in a carrier gas, in which the <sup>235</sup>UF<sub>6</sub> is selectively excited by an infrared laser near 16&nbsp;μm. In contrast to the excited molecules, the nonexcited heavier isotopic molecules tends to form clusters with the carrier gas, and these clusters stay closer to the axis of the molecular beam, so that they can pass a skimmer and are thus separated from the excited lighter isotope.

Quite recently{{when|date=January 2018}} yet another scheme has been proposed for the [[deuterium]] separation using Trojan wavepackets in circularly polarized electromagnetic field. The process of [[Trojan wave packet]] formation by the adiabatic-rapid passage depends in ultra-sensitive way on the [[reduced mass|reduced]] electron and nucleus mass which with the same field frequency further leads to excitation of Trojan or anti-Trojan wavepacket depending on the kind of the isotope. Those and their giant, rotating [[electric dipole moment]]s are then <math>\pi</math>-shifted in phase and the beam of such atoms splits in the gradient of the electric field in the analogy to [[Stern–Gerlach experiment]].{{Citation needed|date=January 2014}}

===Chemical methods===
Although isotopes of a single element are normally described as having the same chemical properties, this is not strictly true. In particular, [[reaction rate]]s are very slightly affected by atomic mass.

Techniques using this are most effective for light atoms such as hydrogen. Lighter isotopes tend to react or [[evaporation|evaporate]] more quickly than heavy isotopes, allowing them to be separated. This is how [[heavy water]] is produced commercially, see [[Girdler sulfide process]] for details. Lighter isotopes also disassociate more rapidly under an electric field. This process in a large [[cascade (chemical engineering)|cascade]] was used at the heavy water production plant at [[Rjukan]].

One candidate for the largest [[kinetic isotope effect|kinetic isotopic effect]] ever measured at room temperature, 305, may eventually be used for the separation of [[tritium]] (T). The effects for the oxidation of tritiated [[formate]] anions to HTO were measured as:

: {|
|-
| k(HCO<sub>2</sub><sup>−</sup>) = 9.54 M<sup>−1</sup>s<sup>−1</sup>
| k(H)/k(D) = 38
|-
| k(DCO<sub>2</sub><sup>−</sup>) = 9.54 M<sup>−1</sup>s<sup>−1</sup>
| k(D)/k(T) = 8.1
|-
| k(TCO<sub>2</sub><sup>−</sup>) = 9.54 M<sup>−1</sup>s<sup>−1</sup>
| k(H)/k(T) = 305
|}

===Distillation===

Isotopes of hydrogen, carbon, oxygen, and nitrogen can be enriched by distilling suitable light compounds over long [[Fractionating column|columns]]. The separation factor is the ratio of vapor pressures of two isotopic molecules. In equilibrium such a separation results at each [[theoretical plate]] of the column and is multiplied by the same factor in the next step (at the next plate). Because the elementary separation factor is small, a large number of such plates is needed. This requires total column heights of 20 to 300 m.

The lower vapor pressure of the heavier molecule is due to its higher [[energy of vaporization]], which in turn results from its lower energy of zero-point vibration in the intermolecular potential. As expected from formulas for vapor pressure, the ratio becomes more favorable at lower temperatures (lower pressures). The vapor pressure ratio for H<sub>2</sub>O to D<sub>2</sub>O is 1.055 at 50&nbsp;°C (123 mbar) and 1.026 at 100&nbsp;°C (1013 mbar). For <sup>12</sup>CO to <sup>13</sup>CO it is 1.007 near the normal boiling point (81.6&nbsp;K), and 1.003 for <sup>12</sup>CH<sub>4</sub> to <sup>13</sup>CH<sub>4</sub> near 111.7&nbsp;K (boiling point).<ref name=":0">{{Cite book |title=Separation of isotopes of biogenic elements |date=2007 |publisher=Elsevier |author=B.M. Andreev |author2=E.P.Magomedbekov |author3=A.A. Raitman |author4=M.B.Pozenkevich |author5=Yu.A. Sakharovsky |author6=A.V. Khoroshilov |isbn=978-0-444-52981-7 |location=Amsterdam |oclc=162588020}}</ref>

The <sup>13</sup>C enrichment by ([[Air separation#Cryogenic distillation process|cryogenic]]) distillation was developed in the late 1960s by scientists at Los Alamos National Laboratory.<ref>{{cite web |url=http://www.lanl.gov/quarterly/q_w03/spotlight.shtml |title=Spotlight Los Alamos in the News |date=Winter 2003 |publisher=Los Alamos National Laboratory |access-date=2014-02-18 |archive-url=https://web.archive.org/web/20160421202634/http://www.lanl.gov/quarterly/q_w03/spotlight.shtml |archive-date=2016-04-21}}</ref><ref>{{cite web |url=http://www.lanl.gov/orgs/pa/News/080801.html |title=Laboratory alliance to put "Made in America" stamp on stable isotopes |access-date=2007-09-01 |archive-url=https://web.archive.org/web/20061012173925/http://www.lanl.gov/orgs/pa/News/080801.html |archive-date=2006-10-12}}</ref> It is still the preferred method for<sup>13</sup>C enrichment. Deuterium enrichment by water distillation is only done, if it was preenriched by a process (chemical exchange) with lower energy demand.<ref>{{Cite journal |last=Miller |first=Alistair I. |date=2001 |title=Heavy Water: A Manufacturers' Guide for the Hydrogen Century |journal=Canadian Nuclear Society Bulletin |volume=22 |issue=1 |pages=1–14}}</ref> Beginning with the low natural abundance (0.015% D) would require evaporation of too large quantities of water.