The Manhattan Project and refining uranium

Okay, I know that the Manhattan Project set about getting enough uranium-238 for a bomb by two different paths: the gaseous-diffusion process (they built an enormous plant in record time, dealing with an impossibly awful gas at horrendous pressures and flows–a marvel of engineering) and some kind of crude brute-force centrifugal process (?)

Which of those processes produced the uranium used for the first bomb? I have half a recollection that it wasn’t the state-of-the-art engineering wonder, the gaseous-diffusion process, but rather the other (centrifugal?) process.

And what happened to the Oak Ridge plant built to hold the gaseous-diffusion process? Did they get it running properly, or just mothball the whole thing, once they knew how to make plutonium?

How does one separate U-238 today? More generally, how do you separate isotopes of, say, carbon?

And the second bomb was a plutonium bomb, if I remember correctly. How does one make plutonium industrially? Is it a batch process (insert a container of uranium in a reactor, count to five hundred, remove, repeat)?

The fissile isotope of uranium is U-235, not U-238.

Check out this book:

The Making of the Atomic Bomb

This USAF website also answers many of your questions.

More info:

Thank you; very informative links.

Oak Ridge is still there. Here’s a history page including current and future plans:

Oak Ridge
And here’s a historical overview of how they got started, that specifically discusses the history of the two separation methods:

General index of “first 50 years” pages

This sublink cuts through the fluff and has relevant content


Here’s the history of Hanford, where they made the plutonium. Unfortunately it’s a government document, so it’s full of digressions about enabling statutes and the purpose, objectives, scope, and strategy of the document itself, that exist largely to cover various bureaucratic arses, and not to inform you about Hanford. There is some good stuff in there, however.


Each chapter is a separate .pdf, linked together. Use the bookmarks to navigate, and if you go back up to the root you can see the whole document, but a lot of tiresome bureaucratic text is involved.

Fun fact: during World War II, 1/7 of all electricity produced in the United States was used by Oak Ridge (I’m guessing mainly by the electromagnetic separators in the Y-12 plant).


What none of the links already provided cover is perhaps the most crucial aspect of the wartime uranium enrichment process: none of the three methods developed was by itself anywhere near sufficient to deliver enough material on the required timescale. It was only by serially combining the methods that success could be achieved during 1945. Realising this in May-June 1944 shaped the way the whole uranium project was organised during the year prior to Hiroshima.
The method that hasn’t been mentioned was thermal diffusion, which was used in a set-up in Philadelphia and in the S-50 plant on the site at Oak Ridge. This could only enrich by a few percentage points, but doing this first made the gaseous diffusion process significantly more efficient. The uranium was thus passed through S-50 first. K-25, the gaseous diffusion plant, then took this as input. It would enrich to about 23%. That was then passed to the Y-12 elecromagnetic plant. With this headstart, that could get to 84%, which was sufficient.
Somewhat crudely, the situation was that electromagnetic separation could get the high levels of enrichment, but was difficult to run using large quantities. The above solution let them minimise the amount of uranium involved in that stage.

At Hanford, the X-10 reactors were more like a conveyor belt system. Uranium billets were inserted on one side of a channel through the reactor, in which they were irradiated as they passed through and then removed at the far side. Amongst the reasons for doing it this way was that that removal was the most dangerous part of the entire process and had to be done remotely. Organising it this way helped isolate the hazardous part.

Also note that the first bomb, the one detonated in the Trinity test, was a plutonium design. The uranium design was so simple that they didn’t feel the need to test it beforehand; Little Boy was the first detonation.

For things like carbon, it’s a lot easier. The interesting isotopes of uranium are 235 and 238, which means a difference of about 1.26% in mass. The interesting isotopes of carbon, on the other hand, are 16 and 14, for a difference of 12.5%, and the isotopes of, say, hydrogen differ by 100% or 200%. So any mechanical method (centrifuge, gas diffusion, etc.) will work much more easily for carbon or hydrogen than for uranium. There are also chemical methods which can be used for the lighter atoms: The usual model of the atom assumes that the nucleus stays stationary, while the electron(s) orbit around it. This is a pretty good approximation, since a proton is about 1800 times heavier than an electron. But it’s not quite exact, and that does make a small but measurable difference in the spectrum, reactivity, and other chemical effects. For deuterium or tritium, assuming that the nucleus doesn’t move is an even better approximation, since a deuterium nucleus is about 3600 times more massive than the electron, so those effects show up less. So, for instance, by looking in enough detail at the spectrum of a bunch of hydrogen, you can tell how much of it is deuterium. But for uranium, with either 235 or 238, either way, “nucleus much more massive than electron” is an excellent approximation, so the effects are too small to be measured.

Almost but not quite that simple. Round slugs of uranium were welded is aluminum cladding ‘cans’ for their trip through the reactors is batches. As new slugs were inserted the irradiated ones dropped out the far side into a water filled canal for their transfer to the chemical plant for separation and recovery processing…

I’m not quite sure where you see the simplification.

That was too prickly. Rephrase as “I’m not quite sure where you see the unwarranted simplification.”

It is a bit off topic, but this looks like a thread with the enthusiasm to answer a question I have had: What is holding up the Iranians?

Oak Ridge started producing U-235 cores **sixty years ** ago. Why are the Iranians still years away from bombs? I know the mass difference between the isotopes is small, but come on.

It’s a hard problem. The principles of isotope separation are easy to understand, but turning that into an industrial process that can produce large quantities of HEU (highly enriched uranium) is much more difficult. Uranium hexafluoride is nasty stuff. It takes many stages of processing to get from natural uranium to HEU.

A bunch of reasons, I’m sure. The US started before the Iranians did; it’s not like they’ve been at it since 1940. The US is a lot bigger than Iran, and we probably have a healthier economy on a per capita basis as well, so we had more resources available for the project. Beyond that, there’s probably a significant amount of “brain drain” going on, too: Any Iranian physicists could probably get a better offer for working on some project in the US or other Western countries, so they’d have even less trained folks working on it in proportion to their economic resources. And the rest of the world is watching Iran like a hawk, and doing what they can short of outright war to hinder their progress.

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.

Wow. Thank you all for your replies. Fascinating stuff.

Groves did a heck of a job.