Incoming links: Isotope separation
This isotope separation method was the first big successful method, having been used in the Manhattan Project, notably in the K-25 reactor.
This method was superseded by the more efficient gas centrifuges.
Gaseous diffusion diagram
. Source. 
Located in Tennessee in the East of the United States.
The precursor organization to ORNL was called Clinton Engineer Works, where groundbreaking Manhattan Project experiments and nuclear production took place during World War II
Some key experiments carried out there include:
- 1943: X-10 Graphite Reactor: prototype Plutonium breeder reactor
- isotope separation to purify Uranium-235:
For nuclear weapons you need a certain level of isotope purity of either plutonium-239 or uranium-235.
And the easiest way by far to achieve this purity is to produce plutonium-239 in a breeder reactor, which allows you to get it out with much cheaper chemical processes rather than costly isotope separation methods.
fissilematerials.org/ summarizes stockpiles and production status. 20224 Archive.