Gaseous diffusion is a technology that was used to produceenriched uranium by forcing gaseousuranium hexafluoride (UF₆) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containinguranium-235 (²³⁵U) anduranium-238 (²³⁸U). By use of a largecascade 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of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficientgas centrifuge process (enrichment factor 1.05 to 1.2).^[1][2]

Gaseous diffusion was devised byFrancis Simon andNicholas Kurti at theClarendon Laboratory in 1940, tasked by theMAUD Committee with finding a method for separating uranium-235 from uranium-238 in order to produce a bomb for the BritishTube Alloys project. The prototype gaseous diffusion equipment itself was manufactured byMetropolitan-Vickers (MetroVick) atTrafford Park, Manchester, at a cost of £150,000 for four units, for theM. S. Factory, Valley. This work was later transferred to the United States when the Tube Alloys project became subsumed by the laterManhattan Project.^[3]

Of the 33 knownradioactive primordial nuclides, two (²³⁵U and²³⁸U) areisotopes of uranium. These twoisotopes are similar in many ways, except that only²³⁵U isfissile (capable of sustaining anuclear chain reaction ofnuclear fission withthermal neutrons). In fact,²³⁵U is the only naturally occurring fissile nucleus.^[4] Becausenatural 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.

Scientific basis

Gaseous diffusion is based onGraham's law, which states that the rate ofeffusion of a gas is inversely proportional to the square root of itsmolecular 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 adiffuser.

Uranium hexafluoride

UF₆ is the only compound of uranium sufficientlyvolatile to be used in the gaseous diffusion process. Fortunately,fluorine consists of only a single isotope¹⁹F, so that the 1% difference in molecular weights between²³⁵UF₆ and²³⁸UF₆ is due only to the difference in weights of the uranium isotopes. For these reasons, UF₆ is the only choice as afeedstock for the gaseous diffusion process.^[6] UF₆, a solid at room temperature,sublimes at 56.4 °C (133 °F) at 1 atmosphere.^[7] Thetriple point is at 64.05 °C and 1.5 bar.^[8] Applying Graham's law gives:

${\displaystyle \frac{{\text{Rate}}_1}{{\text{Rate}}_2}=\sqrt{\frac{M_2}{M_1}}=\sqrt{\frac{352.041206}{349.034348}}=\mathrm{1.004298...}}$

where:

Rate₁ is the rate of effusion of²³⁵UF₆.

Rate₂ is the rate of effusion of²³⁸UF₆.

M₁ is themolar mass of²³⁵UF₆ = 235.043930 + 6 × 18.998403  = 349.034348 g·mol⁻¹

M₂ is the molar mass of²³⁸UF₆ = 238.050788 + 6 × 18.998403  = 352.041206 g·mol⁻¹

This explains the 0.4% difference in the average velocities of²³⁵UF₆ molecules over that of²³⁸UF₆ molecules.^[9]

UF₆ is a highlycorrosive substance. It is anoxidant^[10] and aLewis acid which is able to bind tofluoride, for instance thereaction ofcopper(II) fluoride with uranium hexafluoride inacetonitrile 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 fromaustenitic stainless steel and otherheat-stabilized metals. Non-reactivefluoropolymers such asTeflon must be applied as acoating to allvalves andseals in the system.

Barrier materials

Gaseous diffusion plants typically use aggregate barriers (porous membranes) constructed ofsintered nickel oraluminum, with a pore size of 10–25nanometers (this is less than one-tenth themean 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 usinghydrogen chloride to remove thezinc from silver-zinc (Ag-Zn) or sodium hydroxide to remove aluminum from Ni-Al alloy.

Energy requirements

Because the molecular weights of²³⁵UF₆ and²³⁸UF₆ are nearly equal, very little separation of the²³⁵U and²³⁸U 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 acascade. In practice, diffusion cascades require thousands of stages, depending on the desired level of enrichment.^[6]

All components of a diffusionplant 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 tocompression heating of the gas, which then must be cooled before entering the diffuser. The requirements for pumping and cooling make diffusion plants enormous consumers ofelectric power. Because of this, gaseous diffusion was the most expensive method used until recently for producing enriched uranium.^[12]

Workers working on the Manhattan Project inOak Ridge, Tennessee, developed several different methods for theseparation of isotopes of uranium. Three of these methods were used sequentially at three different plants in Oak Ridge to produce the²³⁵U for "Little Boy" and otherearly nuclear weapons. In the first step, theS-50 uranium enrichment facility used thethermal 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 theK-25 plant, the product of which was around 23%²³⁵U. Finally, this material was fed intocalutrons at theY-12. These machines (a type ofmass spectrometer) employedelectromagnetic isotope separation to boost the final²³⁵U 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-reactivechemical compounds that could be used as coatings,lubricants andgaskets for the surfaces that would come into contact with the UF₆ gas (a highly reactive and corrosive substance). Scientists of the Manhattan Project recruitedWilliam T. Miller, a professor oforganic chemistry atCornell University, tosynthesize and develop such materials, because of his expertise inorganofluorine chemistry. Miller and his team developed several novel non-reactivechlorofluorocarbonpolymers 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, theSoviet Union (including a plant that is now inKazakhstan), theUnited Kingdom,France, andChina. 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 controlnuclear proliferation.

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'sGeorges-Besse plant) definitively closed in June 2012,^[14] and thePaducah Gaseous Diffusion Plant in Kentucky operated by theUnited States Enrichment Corporation (USEC) (the last fully functioning uranium enrichment facility in the United States to employ the gaseous diffusion process^[5][1]) ceased enrichment in 2013.^[15] The only other such facility in the United States, thePortsmouth Gaseous Diffusion Plant in Ohio, ceased enrichment activities in 2001.^[5][16][17] Since 2010, the Ohio site is now used mainly byAREVA, a Frenchconglomerate, for the conversion of depleted UF₆ touranium oxide.^[18][19]

As existing gaseous diffusion plants became obsolete, they were replaced bysecond 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.^[2]

• Capenhurst
• Fick's laws of diffusion
• K-25
• Lanzhou
• Marcoule
• Molecular diffusion
• Nuclear fuel cycle
• Thomas Graham (chemist)
• Tomsk

• Annotated references on gaseous diffusion from the Alsos Library

• Isotope separation
• Uranium
• Membrane technology

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