Making a nuclear bomb

How was the A-bomb put together, and how does it work?
07 April 2020


Nuclear weapons test explosion


Now, it would be remiss of us to discuss radioactivity, and not discuss the darker side of this science, as Adam Murphy has been finding out from Alex Willerstein from the Stevens Institute of Technology in New Jersey...

Adam - One of the most infamous uses of radioactivity is to turn it into a bomb, an atom bomb. These bombs have only been deployed against humans twice, at the end of World War II, on Hiroshima and Nagasaki in Japan. Countless lives were lost both in the initial blast and due to the radiation afterwards. But how does an atom bomb work, and how were they developed? I spoke with nuclear historian Alex Willerstein from the Stevens Institute of Technology in New Jersey about the development of the A-bomb.

Alex - The United States didn't really start trying to build an atomic bomb until early fall of 1942. That's when it decided that the splitting of atoms is not just an interesting scientific phenomena, but could be in a relatively short amount of time made into a weapon that you could use in the present war. So this is the beginning of what they called the Manhattan Project; this was an army-run effort to build weapons that you could use during World War II. They got pretty much every scientist and engineer who they thought could be useful for the project; they also got the labour of hundreds of thousands of Americans doing construction and operations. They created literally hundreds of secret sites across the country. It was a huge endeavour. And one of the ways I like to contextualise it is: the total number of people who worked on the bomb project during World War II was on the order of about 500,000 people, which is about 1% of the American civilian labour force at the time.

Adam - So it was truly a massive undertaking. But how does a nuclear bomb work?

Alex - So you can think of a nuclear bomb as being an engineering device to bring about specific conditions: one where one atom of uranium or plutonium splitting will lead to the splitting of more than one other atom of uranium plutonium in a very short amount of time. If you can make it so that one split leads to two splits, leads to four splits, leads to eight splits, leads to 16 splits, leads to 32; it's exponential growth. And if you do that 80 times, 80 of those generations of splittings, you end up with on the order of a trillion, trillion splittings, which is enough to destroy a city; because each of those splittings is releasing a little bit of energy.

Adam - Eventually they had a device ready to test. When J. Robert Oppenheimer, nuclear physicist in charge of the Los Alamos site, saw the test, he was reminded of the Hindu Holy text, the Bhagavad Gita; and especially the line "I am become death, destroyer of worlds". When the bombs were set off, either by firing two pieces of uranium together within the bomb or by compressing plutonium with explosives, the blast could level cities. And the world wasn't done with nuclear weapons.

Alex - As it seemed that the world was not going to ban weapons, at least anytime soon, the US got very interested in two things. One was: what are the effects of nuclear weapons precisely, especially on military equipment? You can see some of the effects of the nuclear weapons by either setting them off in the desert, like they did at the Trinity test, or by dropping them on cities, which gave them a lot of information on the effect of nuclear weapons on a city. But how would it work on boats for example? In June and July 1946 they decided to run the first postwar nuclear testing series where they set off two Nagasaki-style bombs in the Pacific Ocean at the Bikini Atoll. As they continued through the postwar and into the cold war, the US was very interested in making new bombs.

Can we make a new design? Can we make them more efficient? What are the effects of bombs on tanks? What are the effects of bombs on food? What are the effects of bombs on livestock? These are all war scenarios they're thinking about. They ended up coming up with a rationale to do literally about a thousand nuclear tests over the course of the cold war, and the US and the Soviet Union did very similar sorts of things to learn new things. There's an element of them that's also, at times, sort of showing off, sort of sabre rattling; but there was always that technical reason that they thought that they needed to do these tests. There's a lot of reasons why the people who develop the weapons wanted to use them in war against Japan.

They were not unfeeling heartless people, but they had reasons for doing this. But one of the ones that people don't usually know about is: some of the people, including Oppenheimer, they feared that the next war would not be just fought with the weapons they had developed in World War II, but the weapons that they knew would become developed if they kept working on these weapons, things like the hydrogen bomb. And they worried that if people didn't sufficiently fear these weapons, that the next war would destroy civilization. And so one of the rationales that they had for dropping the atomic bombs on Japan was to sufficiently scare the world and wake it up, and hope that it would realise that it could not fight wars with these weapons ever again.


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