Gaseous diffusion

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of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficient gas centrifuge process (enrichment factor 1.05 to 1.2).

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 (est. £10–11 million today), 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.

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. The triple point is at 64.05 °C and 1.5 bar. Applying Graham's law gives:

:<math>{\mbox{Rate}_1 \over \mbox{Rate}_2}=\sqrt{M_2 \over M_1}=\sqrt{352.041206 \over 349.034348}=1.004298...</math>

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 &times; 18.998403 = 349.034348&nbsp;g·mol^&minus;1

:M₂ is the molar mass of ²³⁸UF₆ = 238.050788 + 6 &times; 18.998403 = 352.041206&nbsp;g·mol^&minus;1

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

UF₆ is a highly corrosive substance. It is an oxidant 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) ceased enrichment in 2013. The only other such facility in the United States, the Portsmouth Gaseous Diffusion Plant in Ohio, ceased enrichment activities in 2001.

See also

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

References

External links

• Annotated references on gaseous diffusion from the Alsos Library