* 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.
* Maybe they thought it was a huge condom. For a giant.
* The pile had graphite bricks in the center.
* Surrounded by a wooden frame.
* Then uranium was placed on the next layer.
* More wooden frames.
* Then alternating graphite and uranium for each layer.
* Into a roughly spherical shape.
* And these guys needed to carve the graphite into the shapes they needed.
* One of them said: ‘We found out how coal miners feel. After eight hours of machining graphite, we looked as if we were made up for a minstrel. One shower would remove only the surface graphite dust. About a half-hour after the first shower the dust in the pores of your skin would start oozing. Walking around the room where we cut the graphite was like walking on a dance floor. Graphite is a dry lubricant, you know, and the cement floor covered with graphite dust was slippery.’
* Imagine you’re one of the world’s leading physicists, and you’re spending your days carving graphite, covered in dust, trying to build the world’s biggest bomb.
* That’s a hard day’s work.
* Fermi was described by his associates as ‘completely self-confident but wholly without conceit’.
* He’d made his calculations and he was certain of them.
* In Chicago in the early afternoon of 1 December, tests indicated that the pile was reaching critical size.
* At that point Fermi’s massive lattice pile contained some 400 tons of graphite, 6 tons of uranium metal and 50 tons of uranium oxide.
* The average African elephant weighs between 2.5 and 7 tons, to 400 tons of graphite is about 80 elephants worth.
* Imagine spending your days carving 80 elephants into nice little cubes.
* About 8.30am on the morning of Wednesday, 2 December, the scientists began to assemble at the squash court where the pile had been built.
* The pile was built in the doubles squash court in the west stands of Stagg Field.
* Fermi and other scientists watch from the balcony of the squash court.
* The giant balloon was fitted, and the air was pumped out.
* What if it went supercritical?
* They had a an emergency cadmium rod, which would absorb neutrons, suspended above the pile, attached to a rope, which could be cut with an axe if needed.
* But that wasn’t enough security.
* Remember – this had never been done before.
* So there was a suicide squad on stand-by.
* If things went pear shaped, these three guys would rush in and cover the entire pile with cadmium-sulphate solution.
* What a job description.
* The experiment started at 10am.
* First the emergency rod was slowly pulled out a bit.
* The neutron counter in the pile started ticking, showing Fermi the number of neutrons being released.
* 37 minutes later, they removed the rod a bit further.
* The ticking went higher.
* Fermi is doing manual calculations on a slide rule to make sure the counter matches his calculations.
* They pull the rod out a little more.
* Another half an hour goes by. They pull it out another foot.
* Fermi does his calculations.
* Then suddenly – there is a loud crash.
* Everyone freezes.
* It turns out that the rod had just slipped back down.
* “Let’s go to lunch, I’m starving” said Fermi.
* They spent a couple of hours at lunch, talking about anything BUT the experiment.
* Then at 2pm they go back to try again.
* They repeat the process.
* Slowly removing the rod up a bit.
* Then Fermi watches the needle on the counter.
* Does his math.
* Gives the order to pull it up a little more.
* And a little more.
* Finally Fermi says “This is it. The reaction will now be self-sustaining.”
* He watches the needle.
* They all watch him watching the needle, doing his math, calm as a cucumber on a cold day in the Antarctic.
* Then Fermi broke into a huge smile and closed his slide rule.
* ‘The reaction is self-sustaining,’ he announced quietly, happily. ‘The curve is exponential.’
* The world’s first nuclear chain reactor operated for 28 minutes.
* At 3.53pm, the control rods were replaced.
* The counters slowed and the pen headed downwards across the paper.
* The test was over.
* The team had succeeded in initiating a self-sustaining nuclear reaction – and then stopping it.
* They had released the energy of the atom’s nucleus and controlled it.
* One of the team presented Fermi with a bottle of Chianti.
* He’d kept it hidden behind his desk the entire time.
* The U.S. was at war with Italy and it was illegal to import Italian wine.
* Fermi pulled out some paper cups and they all drank in silence.
* No toast.
* They were all aware of what they had accomplished.
* They just weren’t sure if they were the first.
* No way of telling if the Nazis hadn’t beaten them to it.
* Fermi got all of the guys to sign the straw wrapping around the bottle.
* Compton then phoned Conant at Harvard and delivered a message in the pre-arranged code.
* ‘Jim,’ said Compton. ‘The Italian navigator has landed in the New World.’
* ‘Is that so,’ said Conant. ‘How were the natives?’
* ‘Everyone landed safe and happy,’ Compton replied.
* As the crew filed out of the West Stands, one of the guards asked one of the scientists: ‘What’s going on, Doctor, something happen in there?’
* On this momentous day in scientific history, they had generated half a watt of energy.
* That would be increased 10 days later to 200 watts – enough to power two light bulbs.
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