Gaseous diffusion

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Gaseous diffusion uses microporous membranes to enrich uranium Gaseous Diffusion (44021367082) (cropped).jpg
Gaseous diffusion uses microporous membranes to enrich uranium

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of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficient gas centrifuge process (enrichment factor 1.05 to 1.2). [1] [2]

Contents

Gaseous diffusion was devised by Francis Simon and Nicholas Kurti at the Clarendon Laboratory in 1940, tasked by the MAUD Committee with finding a method for separating uranium-235 from uranium-238 in order to produce a bomb for the British Tube Alloys project. The prototype gaseous diffusion equipment itself was manufactured by Metropolitan-Vickers (MetroVick) at Trafford Park, Manchester, at a cost of £150,000 for four units, for the M. S. Factory, Valley. This work was later transferred to the United States when the Tube Alloys project became subsumed by the later Manhattan Project. [3]

Background

Of the 33 known radioactive primordial nuclides, two (235U and 238U) are isotopes of uranium. These two isotopes are similar in many ways, except that only 235U is fissile (capable of sustaining a nuclear chain reaction of nuclear fission with thermal neutrons). In fact, 235U is the only naturally occurring fissile nucleus. [4] Because natural uranium is only about 0.72% 235U by mass, it must be enriched to a concentration of 2–5% to be able to support a continuous nuclear chain reaction [5] when normal water is used as the moderator. The product of this enrichment process is called enriched uranium.

Technology

Scientific basis

Gaseous diffusion is based on Graham's law, which states that the rate of effusion of a gas is inversely proportional to the square root of its molecular mass. For example, in a box with a microporous membrane containing a mixture of two gases, the lighter molecules will pass out of the container more rapidly than the heavier molecules, if the pore diameter is smaller than the mean free path length (molecular flow). The gas leaving the container is somewhat enriched in the lighter molecules, while the residual gas is somewhat depleted. A single container wherein the enrichment process takes place through gaseous diffusion is called a diffuser.

Uranium hexafluoride

UF6 is the only compound of uranium sufficiently volatile to be used in the gaseous diffusion process. Fortunately, fluorine consists of only a single isotope 19F, so that the 1% difference in molecular weights between 235UF6 and 238UF6 is due only to the difference in weights of the uranium isotopes. For these reasons, UF6 is the only choice as a feedstock for the gaseous diffusion process. [6] UF6, a solid at room temperature, sublimes at 56.4 °C (133 °F) at 1 atmosphere. [7] The triple point is at 64.05 °C and 1.5 bar. [8] Applying Graham's law gives:

where:

Rate1 is the rate of effusion of 235UF6.
Rate2 is the rate of effusion of 238UF6.
M1 is the molar mass of 235UF6 = 235.043930 + 6 × 18.998403  = 349.034348 g·mol−1
M2 is the molar mass of 238UF6 = 238.050788 + 6 × 18.998403  = 352.041206 g·mol−1

This explains the 0.4% difference in the average velocities of 235UF6 molecules over that of 238UF6 molecules. [9]

UF6 is a highly corrosive substance. It is an oxidant [10] and a Lewis acid which is able to bind to fluoride, for instance the reaction of copper(II) fluoride with uranium hexafluoride in acetonitrile is reported to form copper(II) heptafluorouranate(VI), Cu(UF7)2. [11] It reacts with water to form a solid compound, and is very difficult to handle on an industrial scale. [6] As a consequence, internal gaseous pathways must be fabricated from austenitic stainless steel and other heat-stabilized metals. Non-reactive fluoropolymers such as Teflon must be applied as a coating to all valves and seals in the system.

Barrier materials

Gaseous diffusion plants typically use aggregate barriers (porous membranes) constructed of sintered nickel or aluminum, with a pore size of 10–25 nanometers (this is less than one-tenth the mean free path of the UF6 molecule). [4] [6] They may also use film-type barriers, which are made by boring pores through an initially nonporous medium. One way this can be done is by removing one constituent in an alloy, for instance using hydrogen chloride to remove the zinc from silver-zinc (Ag-Zn) or sodium hydroxide to remove aluminum from Ni-Al alloy.

Energy requirements

Because the molecular weights of 235UF6 and 238UF6 are nearly equal, very little separation of the 235U and 238U occurs in a single pass through a barrier, that is, in one diffuser. It is therefore necessary to connect a great many diffusers together in a sequence of stages, using the outputs of the preceding stage as the inputs for the next stage. Such a sequence of stages is called a cascade. In practice, diffusion cascades require thousands of stages, depending on the desired level of enrichment. [6]

All components of a diffusion plant must be maintained at an appropriate temperature and pressure to assure that the UF6 remains in the gaseous phase. The gas must be compressed at each stage to make up for a loss in pressure across the diffuser. This leads to compression heating of the gas, which then must be cooled before entering the diffuser. The requirements for pumping and cooling make diffusion plants enormous consumers of electric power. Because of this, gaseous diffusion was the most expensive method used until recently for producing enriched uranium. [12]

History

Workers working on the Manhattan Project in Oak Ridge, Tennessee, developed several different methods for the separation of isotopes of uranium. Three of these methods were used sequentially at three different plants in Oak Ridge to produce the 235U for "Little Boy" and other early nuclear weapons. In the first step, the S-50 uranium enrichment facility used the thermal diffusion process to enrich the uranium from 0.7% up to nearly 2% 235U. This product was then fed into the gaseous diffusion process at the K-25 plant, the product of which was around 23% 235U. Finally, this material was fed into calutrons at the Y-12. These machines (a type of mass spectrometer) employed electromagnetic isotope separation to boost the final 235U concentration to about 84%.

The preparation of UF6 feedstock for the K-25 gaseous diffusion plant was the first ever application for commercially produced fluorine, and significant obstacles were encountered in the handling of both fluorine and UF6. For example, before the K-25 gaseous diffusion plant could be built, it was first necessary to develop non-reactive chemical compounds that could be used as coatings, lubricants and gaskets for the surfaces that would come into contact with the UF6 gas (a highly reactive and corrosive substance). Scientists of the Manhattan Project recruited William T. Miller, a professor of organic chemistry at Cornell University, to synthesize and develop such materials, because of his expertise in organofluorine chemistry. Miller and his team developed several novel non-reactive chlorofluorocarbon polymers that were used in this application. [13]

Calutrons were inefficient and expensive to build and operate. As soon as the engineering obstacles posed by the gaseous diffusion process had been overcome and the gaseous diffusion cascades began operating at Oak Ridge in 1945, all of the calutrons were shut down. The gaseous diffusion technique then became the preferred technique for producing enriched uranium. [4]

At the time of their construction in the early 1940s, the gaseous diffusion plants were some of the largest buildings ever constructed.[ citation needed ] Large gaseous diffusion plants were constructed by the United States, the Soviet Union (including a plant that is now in Kazakhstan), the United Kingdom, France, and China. Most of these have now closed or are expected to close, unable to compete economically with newer enrichment techniques. Some of the technology used in pumps and membranes remains top secret. Some of the materials that were used remain subject to export controls, as a part of the continuing effort to control nuclear proliferation.

Current status

In 2008, gaseous diffusion plants in the United States and France still generated 33% of the world's enriched uranium. [12] However, the French plant (Eurodif's Georges-Besse plant) definitively closed in June 2012, [14] and the Paducah Gaseous Diffusion Plant in Kentucky operated by the United States Enrichment Corporation (USEC) (the last fully functioning uranium enrichment facility in the United States to employ the gaseous diffusion process [5] ) ceased enrichment in 2013. [15] The only other such facility in the United States, the Portsmouth Gaseous Diffusion Plant in Ohio, ceased enrichment activities in 2001. [5] [16] [17] Since 2010, the Ohio site is now used mainly by AREVA, a French conglomerate, for the conversion of depleted UF6 to uranium oxide. [18] [19]

As existing gaseous diffusion plants became obsolete, they were replaced by second generation gas centrifuge technology, which requires far less electric power to produce equivalent amounts of separated uranium. AREVA replaced its Georges Besse gaseous diffusion plant with the Georges Besse II centrifuge plant.

See also

Related Research Articles

<span class="mw-page-title-main">Uranium</span> Chemical element with atomic number 92 (U)

Uranium is a chemical element; it has symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium radioactively decays, usually by emitting an alpha particle. The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.

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.

<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 and toxic 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">Yellowcake</span> Uranium concentrate powder

Yellowcake is a type of powdered uranium concentrate 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.

<span class="mw-page-title-main">Gas centrifuge</span> Device that performs isotope separation of gases

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 235U 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 235U while using significantly less energy compared to the gaseous diffusion process.

<span class="mw-page-title-main">Calutron</span> Mass spectrometer

A calutron is a mass spectrometer originally designed and used for separating the isotopes of uranium. It was developed by Ernest Lawrence during the Manhattan Project and was based on his earlier invention, the cyclotron. Its name was derived from California University Cyclotron, in tribute to Lawrence's institution, the University of California, where it was invented. Calutrons were used in the industrial-scale Y-12 uranium enrichment plant at the Clinton Engineer Works in Oak Ridge, Tennessee. The enriched uranium produced was used in the Little Boy atomic bomb that was detonated over Hiroshima on 6 August 1945.

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<span class="mw-page-title-main">Atomic vapor laser isotope separation</span>

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<span class="mw-page-title-main">K-25</span> Manhattan Project codename for a program to produce enriched uranium

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).

<span class="mw-page-title-main">Eurodif</span> Nuclear enrichment company in France

Eurodif, which means European Gaseous Diffusion Uranium Enrichment Consortium, is a subsidiary of the French company Orano, which operates a uranium enrichment plant, called the Georges-Besse plant, established at the Tricastin Nuclear Power Center in Pierrelatte in Drôme. The nuclear site of Pierrelatte includes many nuclear installations, of which the largest are the Eurodif fuel factory and the Tricastin nuclear power station.

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. MLIS was conceived in 1971 at the Los Alamos National Laboratory.

Separation of isotopes by laser excitation (SILEX) is a process for enriching uranium to fuel nuclear reactors that may also present a growing nuclear weapons proliferation risk. It is strongly suspected that SILEX utilizes laser condensation repression to excite a vibrational mode of the uranium-235 isotope in uranium hexaflouride (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.

<span class="mw-page-title-main">Portsmouth Gaseous Diffusion Plant</span> Former uranium enrichment plant in Pike County, Ohio, USA

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.

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<span class="mw-page-title-main">Paducah Gaseous Diffusion Plant</span> Decommissioned uranium enrichment facility

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William Taylor Miller was an American professor of organic chemistry at Cornell University. His experimental research included investigations into the mechanism of addition of halogens, especially fluorine, to hydrocarbons. His work focused primarily on the physical and chemical properties of fluorocarbons and chlorofluorocarbons, and the synthesis of novel electrophilic reagents.

ConverDyn is a general partnership between American multinational firms General Atomics and Honeywell that provides uranium hexafluoride (UF6) conversion and related services to utilities operating nuclear power plants in North America, Europe, and Asia. The company is the sole marketing agent of UF6 produced at the Honeywell Uranium Hexafluoride Processing Facility in Metropolis, Illinois.

Depleted uranium hexafluoride (DUHF; also referred to as depleted uranium tails, depleted uranium tailings or DUF6) is a byproduct of the processing of uranium hexafluoride into enriched uranium. It is one of the chemical forms of depleted uranium (up to 73-75%), along with depleted triuranium octoxide (up to 25%) and depleted uranium metal (up to 2%). DUHF is 1.7 times less radioactive than uranium hexafluoride and natural uranium.

<span class="mw-page-title-main">Georges-Besse plant</span> French nuclear facility (1979–2012)

The Georges-Besse plant, known as the Eurodif plant from 1978 to 1988, was a nuclear facility specializing in uranium enrichment by gaseous diffusion. The plant, now in the nuclear dismantling phase, is located on the Tricastin nuclear site at Pierrelatte in the Drôme region.

References

  1. ↑ "Module 4.0: Gas Centrifuge" (PDF). USNRC Technical Training Center. Retrieved September 20, 2024.
  2. ↑ "Uranium Enrichment". US Nuclear Regulatory Commission . Retrieved July 17, 2020.
  3. ↑ Colin Barber. "The Tube Alloys Project". Rhydymwyn Valley History Society.
  4. 1 2 3 Cotton S (2006). "Uranium hexafluoride and isotope separation". Lanthanide and actinide chemistry (1st ed.). Chichester, West Sussex, England: John Wiley and Sons, Ltd. pp. 163–5. ISBN   978-0-470-01006-8 . Retrieved November 20, 2010.
  5. 1 2 3 U.S. Nuclear Regulatory Commission (2009). "Fact Sheet on Gaseous Diffusion". Washington, DC: U.S. Nuclear Regulatory Commission. Retrieved November 20, 2010.
  6. 1 2 3 4 Beaton L (1962). "The slow-down in nuclear explosive production". New Scientist . 16 (309): 141–3. Retrieved November 20, 2010.[ permanent dead link ]
  7. ↑ DeWitt, R. (1960). Uranium Hexafluoride: A Survey Of The Physicochemical Properties. p. 102. doi:10.2172/4025868.
  8. ↑ "Uranium Hexafluoride: Source: Appendix A of the PEIS (DOE/EIS-0269): Physical Properties". Archived from the original on March 29, 2016. Retrieved November 18, 2010.
  9. ↑ "Gaseous Diffusion Uranium Enrichment". GlobalSecurity.org. April 27, 2005. Retrieved November 21, 2010.
  10. ↑ Olah GH, Welch J (1978). "Synthetic methods and reactions. 46. Oxidation of organic compounds with uranium hexafluoride in haloalkane solutions". Journal of the American Chemical Society . 100 (17): 5396–402. doi:10.1021/ja00485a024.
  11. ↑ Berry JA, Poole RT, Prescott A, Sharp DW, Winfield JM (1976). "The oxidising and fluoride ion acceptor properties of uranium hexafluoride in acetonitrile". Journal of the Chemical Society, Dalton Transactions (3): 272–4. doi:10.1039/DT9760000272.
  12. 1 2 Michael Goldsworthy (2008). "Lodge Partners Mid-Cap Conference" (PDF). Lucas Heights, New South Wales, Australia: Silex Ltd. Retrieved November 20, 2010.
  13. ↑ Blaine P. Friedlander Jr. (December 3, 1998). "William T. Miller, Manhattan Project scientist and Cornell professor of chemistry, dies at 87". Cornell News . Ithaca, New York: Cornell University. Retrieved November 20, 2010.
  14. ↑ "Georges Besse finally depleted". World Nuclear News. June 8, 2012.
  15. ↑ "PADUCAH GASEOUS DIFFUSION PLANT (USDOE)". Uranium enrichment activities ceased in 2013
  16. ↑ United States Enrichment Corporation (2009). "Overview: Portsmouth Gaseous Diffusion Plant". Gaseous Diffusion Plants. Bethesda, Maryland: USEC, Inc. Archived from the original on November 24, 2010. Retrieved November 20, 2010.
  17. ↑ United States Enrichment Corporation (2009). "History: Paducah Gaseous Diffusion Plant". Gaseous Diffusion Plants. Bethesda, Maryland: USEC, Inc. Archived from the original on January 2, 2011. Retrieved November 20, 2010.
  18. ↑ Tom Lamar (September 10, 2010). "AREVA Starts Operations at the Portsmouth Facility". Nuclear Power Industry News. Waynesboro, Virginia: Nuclear Street. Retrieved November 20, 2010.
  19. ↑ AREVA, Inc. (2010). "DOE Gives AREVA Joint Venture Permission to Begin Operational Testing of New Ohio Facility" (PDF). Press Release. Bethesda, Maryland: AREVA, Inc. Retrieved November 20, 2010.[ permanent dead link ]
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