Why uranium enrichment was slow/near impossible in the beginning of Manhattan project but few years later it became trivial?


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


The problem is that U-235 (fissile uranium) and U-238 (non-fissile uranium) have the same chemical properties. The U-235 is just a bit lighter. So to enrich uranium to increase the U-235 content, you need to use the very small difference of physical properties that U-235 has because it is a tiny bit lighter.

The Manhattan Project built three different plants for enrichment using different techniques – the Y-12 Electromagnetic Isotope Separation Plant, the K-25 Gaseous Diffusion Plant, and the S-50 Liquid Thermal Diffusion Plant. These plants were expanded during the war, so by the time the war ended, they had significant capacity to enrich more Uranium. So they had more capacity, and the processes had improved to increase efficiency.

The enriched Uranium went into breeder reactors that use uranium fussion to make fissile plutonium for weapons. Plutonium is a different element, and can be isolated from the raw uranium fuel by chemical processes.

Just some trivia:

1) General Nichols said Oak Ridge was using 1/7th of all the electricity produced in the US from 1943-1945. Source: [https://www.nps.gov/places/oak-ridge-wayside-powering-the-manhattan-project.htm](https://www.nps.gov/places/oak-ridge-wayside-powering-the-manhattan-project.htm)

2) STUXNET is the state sponsored malware that specifically targeted Iranian enrichment (above mentioned) centrifuges, destroying them

They had to set up the initial infrastructure to ramp up production. At the beginning of the project there were experiments that could produce the materials but they weren’t designed to be highly scalable or to really produce any large quantity. They had to create entirely new facilities that were dedicated to the production of the quantities in fast enough time. It’s also important to note that the quantity produced was still tiny even at the time little boy and fat man were produced.

The type of weapon little boy was never had a test so most of the uranium stockpile went into little boy whereas the plutonium went into Gadget and Fat Man. Making plutonium was also far more easier than enriching uranium. It could also be made out of the isotope of uranium that is considered undesirable for nuclear weapons.

After the war further knowledge was developed for enrichment of uranium which made plants more efficient and scalable. Most critically was the development of the gas centrifuge process.

There were some improvements on the processes but it’s mostly that they’d finished building a significant quantity of infrastructure to produce the fissile materials.

Also I think you’re assuming the US had a much larger nuclear arsenal sooner than it did, post Hiroshima/Nagasaki the US had the capability of producing 3 Fatman equivalents a month but had 300 by 1950, ~2k in 1955 and then it starts to get silly in the ’60’s.