diffusion appeared more promising. Based on the well-known
principle that molecules of a lighter isotope would pass through a
porous barrier more readily than those of a heavier isotope, this
approach proposed to separate U235 by utilizing a series of
repetitions through many "cascades".
Theoretically, this method
could achieve high concentrations of uranium 235 but, like the
electromagnetic method, would be extremely costly.
British researchers led the
way on gaseous diffusion, with John Dunning at Columbia joining the
effort in late 1940.
The biggest obstacle was
finding a suitable material for the "porous barrier" that
was able to withstand the corrosive properties of the uranium
gas. The research done at Columbia University eventually led to
the design and construction of the K-25 plant at Oak Ridge, Tennessee.