* Fission involved breaking apart the nuclei of heavy elements like uranium or plutonium.
* Fusion involves forcing the nuclei of lighter elements, like hydrogen or deuterium, together.
* And deuterium, which is basically heavy hydrogen, is far easier to get your hands on than uranium.
* But there’s still not a ton of it.
* There is one D atom in 6420 of H.
* D accounts for approximately 0.0156% of all the naturally occurring hydrogen in the oceans, while protium, the other isotope of hydrogen, accounts for more than 99.98%.
* But a fusion bomb is also a lot more powerful than a fission bomb.
* That’s why all of the nuclear weapons today operate by fusion instead of fission.
* BTW, a fusion bomb is also known as a hydrogen bomb or thermonuclear bomb, becaue a fusion bomb actually contains a fission bomb which creates the heat, thermo, required to initiate the fusion reaction, the nuclear part.
* In late November, there was a scare.
* Neither Groves nor the S-1 Executive had been told that Compton was building the experimental pile at Stagg Field.
* They were faced with the vision of a chain reaction possibly running wild in heavily populated Chicago.
* However, Fermi’s calculations provided reasonable assurance that this was not going to happen.
* But for a few days there, everyone was panicking.
* So let’s talk about k.
* And I’m not talking about Tommy Lee Jones from Men In Black.
* Here’s the situation.
* Remember that to get a fission reaction to happen, you had to get just the right number of neutrons to hit the right number of uranium nuclei, causing them to fission, which would give off more neutrons, which would hit more nuclei, etc.
* Some of the neutrons would be lost, they might bounce in a direction where there wasn’t a uranium nuclei.
* So you have to put up a kind of shielding that would make the neutrons bounce back into the chamber.
* To quantify this, the physicists came up with a number - k.
* If the number of neutrons in the chamber was less than k, there was no chain reaction, the process would just fizzle out.
* If it was exactly k, when k = 1, you had a sustainable reaction.
* But if it was larger than k, it could go supercritical, and you might have a bomb go off in the middle of Chicago.
* But of course at this stage nobody knew if achieving k was even possible.
* To try and achieve k, they put the uranium in the middle of the pile and surrounded it with cubes of graphite, which would act as a moderator, slowing down the neutrons.
* The first pile that Fermi built on the campus at Columbia in September 1941 comprised cans of uranium oxide surrounded by graphite bricks.
* Its k was 0.87.
* Which he said sucked but at least it was a starting point.
* By July 1942, at Stagg Field, they had edged k up to 0.918, then 0.94.
* To get closer to k they realised they were going to need purer graphite and uranium metal, instead of uranium oxide, which had too many impurities.
* The problem was - uranium metal of that purity didn’t exist.
* It wasn’t until November that they could get enough manufactured, from a range of companies who were all working without knowing exactly why, to their specifications.
* So in November, Fermi started to build the main pile in Chicago.
* interestingly, some of the physicists working on the project were pacifists.
* They believed that the existence of atom bombs would prevent future wars.
* But Fermi still didn’t know if the pile would go critical.
* So they had the idea to cover the entire thing in a huge rubber balloon so they could pump all of the air out of it.
* Gases absorb neutrons and they wanted to negate that factor.
* The balloon was made by Goodyear, who of course weren’t allow to know WHY they were building this huge rubber balloon.
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