Almost 20 years ago, I worked on a Smithsonian research project on the history of the Manhattan Project. I’ve actually been to all the places mentioned in this thread: the Y-12, K-25, X-10 sites in Oak Ridge, as well as the first plutonium production reactor (now decommissioned) at Hanford. At that time, all of those facilities were still extant, but either abandoned in place or only partly used.
The decisions faced by Gen. Leslie Groves, the military commander of the Manhattan Project, were immense. The stakes couldn’t have been higher, and the information available on which to base them was often slim to none. In the case of enriching uranium, he was presented with three possible methods: gaseous diffusion, electromagnetic separation, and gaseous centrifuges. Each had been done in the lab, but now had to be scaled up to industrial levels. It was far from clear what would be necessary to accomplish this, or even if it could be done.
So rather than gamble on one, Groves decided to use all of them. In so doing he created a complex of plants that was roughly equivalent to the entire U.S. automobile industry of the time. In about 18 months.
The scale of these plants is simply astounding. For instance, take a look at this picture of the K-25 gaseous diffusion plant. Each of the legs of that U-shaped building is a quarter of a mile long. At the time it was built, it was the largest building under one roof in the world. It housed thousands of stages consisting of diffusion chambers, pumps that ran at thousands of RPM, and valves, all connected by hundreds of miles of pipes to transport the highly toxic and corrosive uranium hexafluoride gas.
Everything the gas came in contact with had to be plated in nickel, which is one of the only substances that resists the stuff. And the gas explodes in the presence of grease, so they had to come up with a completely new way to lubricate the valves. The solution was Teflon, which had been invented only a short time before and was a top-secret classified material until after the war.
An interesting point about K-25 was that it was almost completely automatic in its operation. The huge plant could be manned by a relatively small staff.
Unlike Y-12, the electromagnetic separation plant. EM separation works by vaporizing a quantity of uranium in the presence of a strong magnetic field in a device called a Calutron. Because the U-235 atoms are slightly less massive than the U-238, they follow an arc with a slightly smaller radius than U-238 atoms. The beam hits a carbon plate and after the Calutron has been running for a while, they remove the carbon plate and dissolve it to capture the tiny quantities of U-235.
Each of the Calutrons had to be operated by someone who would monitor the status of the beam and make minor adjustments to keep it on target. It was very boring work, done mostly by women, thousands of them, who worked around the clock. Obviously, the vast majority of these people had no idea what they were working on, and the purpose of this big plant was a big mystery. All this equipment and power being used, but nothing (apparently) coming out because the quantities of U-235 being produced were so minuscule. One of the jokes of the place was that it must be making toilet paper, because the rolls that the workers sneaked out in their lunch boxes were the only thing that ever came out of the plant. A wag commented, “I don’t know what they’re making in there, but whatever it is, it would be easier to just go out and buy it.”
At Hanford, the processing of plutonium was much more productive because the quantities created in the reactor were relatively high, and it could be separated from uranium (and other radioactive by-products) by chemical means, which isn’t possible with the two isotopes of uranium. So although it’s a complex process, because the stuff is highly radioactive and has to be handled with remote-control arms in giant thick-walled concrete processing plants known as canyons, it’s “simpler” to work with chemical separation processes than the mechanical isotope separation methods. BTW, these remote control systems were among the first industrial applications of television cameras and monitors, long before broadcast TV became common.
The gaseous centrifuge process was the least effective, and IIRC it was barely used during the war.
For further reading on this fascinating chapter in history, I highly recommend Rhodes’ book mentioned above, as well as Stephane Groueff’s The Manhattan Project. It’s out of print now, but you can probably find a copy at an online used book seller.