Gaseous diffusion

[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, for the M. S. Factory, Valley.

[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.

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.

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).

[12] 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.

These machines (a type of mass spectrometer) employed electromagnetic isotope separation to boost the final 235U concentration to about 84%.

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).

In 2008, gaseous diffusion plants in the United States and France still generated 33% of the world's enriched uranium.

[15] The only other such facility in the United States, the Portsmouth Gaseous Diffusion Plant in Ohio, ceased enrichment activities in 2001.

[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.