So I’ve watched Oppenheimer and a recurring theme in the movie was how tedious task it was to get fission material (demonstrated as filling a large fish bowl by marbles). All they manage to collect in two year period was just enough for two bombs. but fast forward few years US have a complete arsenal of bombs to flat the earth. I understand they must’ve innovated a method and the exact method maybe classified, what i’m interested in is knowing what was the obstacle(s) for this and rough idea of how they might’ve overcome it.
In: 450
So the movie is a little misleading with the marble sequence. One gets why they did it that way: to visually give the audience a “counter” for progress. But it absolutely did not reflect the pace of the fissile material production. A realistic “counter” would have them with literally microscopic quantities for most of that time, then suddenly some bigger ones, and then all of the material coming in at the very last minute, more or less.
[Here’s the actual graph of how much enriched uranium they had at Los Alamos from 1944-1946](https://blog.nuclearsecrecy.com/wp-content/uploads/2023/08/Reel-9-920-Cumulative-Shipments-HEU-to-Los-Alamos.jpg). You can see that it is very slow to get going. This is because the plants to produce it didn’t go online until towards the end of the process (the project timeline was mostly about getting these plants working), and then they had to get them truly working correctly. They didn’t actually even get them really working as planned until _after_ the war: they had to chain three different methods together to get the right level of enrichment in the right quantities ready for the summer of 1945, but after the war they figured out how to get it all working with just one process, gaseous diffusion.
The captions indicated on that graph are also sort of useful. The graph itself is shipments from Y-12 (one of the enrichment plants, which used the electromagnetic method) to Los Alamos. During the war, the gaseous diffusion plant (K-25) didn’t work right and could only be used to “feed” slightly enriched uranium to Y-12, which would finish the job (after the war, again, they made it so K-25 could do all the work by itself). At Y-12 they had two different plants, Alpha and Beta. So the graph is indicating that they got K-25 hooked to Alpha in March 1945, then hooked to Beta in June 1945. There is a little line (looks like it was drawn by hand) at the end of July/beginning of August 1945, which is when the final pieces were shipped to Tinian (they sent about half of the material on the USS Indianapolis while they waited for the final pieces to be finished, and then sent the final pieces by airplane — the bomb had a total of 64 kg of 80% enriched uranium in it). Then Hiroshima on August 6, 1945. So you can see that almost all of that work took place between March 1945 and July 1945, and that they would have had another Little Boy bomb’s worth of uranium by October 1945. (If the war had continued, they would likely have not made more Little Boy bombs, but instead made composite uranium+plutonium Fat Man bombs, which would have been a much better use of material.)
At Hanford, with plutonium, it was a similar story. Interestingly the plutonium production decreased after the war ended because they had technical problems that came up in running the first industrial-sized nuclear reactors, and that took some time to solve.
In 1947, the US Atomic Energy Commission was created and took over the Manhattan Project facilities. Over the 1940s and early 1950s, it not only worked out the bugs in the existing production pipeline, it was given money by Congress to increase production capacity dramatically. So they expanded Hanford from three to nine reactors, built _another_ Hanford (Savannah River, construction began 1950, five reactors went online 1953-1955), and built _two more_ Oak Ridge equivalents for uranium enrichment (Portsmouth, construction began 1952, completed 1956; Paducah, construction began 1953, completed 1954).
So this means that by the late 1950s the US had _significantly_ more fissile production capability than they had during the Manhattan Project. The Manhattan Project itself was less of a “slow accumulation of materials” than it was “building several massive factories, getting the working, and suddenly getting a bunch of fissile material right at the end.”
The Manhattan Project production facilities were built to have the capabilities of producing, once online, about 30 kg of U-235 per month, and about 21 kg of plutonium per month. That works out to one Little Boy bomb core every two months, and 3 Fat Man plutonium cores every month. It took about all of 1945 for this capacity to get online, so they ended up with ~60 kg after about six months, but from that point onward, they were at 30 kg per month. But as you can see from that graph, by 1946 they were producing more like 90 kg per month, so a significant upgrade even just by then. For the plutonium, they didn’t really start receiving kilogram quantities of plutonium until March 1945, they had enough by July 1945 to make a test, then enough by the end of the month for the Nagasaki bomb, then enough to cast another core by August 10th (this became the “Demon core” after the war ended). But from that point onward, they were supposed to have 3 cores per month (but again, had some production difficulties that slowed this down).
(What makes the movie sequence even more misleading is that it implies they know exactly how much fissile material they need from the start. They didn’t. That target changed as the project work evolved, especially when they reorganized the laboratory in the summer of 1944 around the implosion problem, something the movie totally skips over for whatever reason. I’m not criticizing the movie — it’s art, it’s not history — but I just want to highlight that you should not take its representation of this particular aspect very literally. It is a “device” so the audience gets a sense of pacing and progress, and has no relationship with the actual history of it.)
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