The A-Bomb

Ahnaf Kabir

Image of the Enola Gay

The Enola Gay

US Army

While citizens of New York sat down for supper, an American bomber dubbed the Enola Gay had revealed a breakthrough: the Little Boy. It was the world’s first and most devastating Atomic bomb, the product of 6 years of American-British-Canadian scientific innovation, and a costly end to a second World War.

 

 

The Little Boy laid a foundation which has been built on further today by their Hydrogen bomb brethren, equipped with a powerful additional nuclear fusion stage capable of damage one thousand times that of Hiroshima1, but both involve starting with nuclear fission, relying on the same principles as the Little Boy.

 

The foundations of the Little Boy were established in a British patent (no. GB630726) by L. Szilard, a Jewish physicist who fled from Germany in the running up to World War 2. He speculated of a ‘nuclear chain reaction’, made possible by the earlier discoveries of the structure of the atom by Rutherford et al & radiation by Marie Curie.

Concept drawing of power generation from nuclear chain reactions

L. Szilard

For a nuclear chain reaction to take place, there needs to be a critical mass of a fissile material, such as uranium-235. Elements like uranium and plutonium are dense in energy as each atom contains a large, unstable nucleus, which bombs rely on ‘splitting’ for power, producing upwards of 74 terajoules each2 (about how much power 2,000 UK households use per year3). The Little Boy came in at a weight of around 4.5 tons, of which only 64kg was uranium4, yet it managed to produce a force equivalent to 20,000 tons of TNT5. Once an atom is split, it releases neutrons which then have a chance of colliding with another atom, and energy is provided once again.

 

Of the energy released, about half a terajoule is energy spent on sustaining the chain reaction and radioactive emissions, known as ‘nuclear fallout’. The nuclear fallout of uranium-235 consists of emitted alpha particles, which although short-lived, are highly radioactive, and are therefore a serious factor in increasing deaths.   

There are two different types of A-Bombs which were studied concurrently. J. Robert Oppenheimer placed his bets on the Little Boy, which used a method that was less efficient but simpler: the chain reaction started by firing two uranium cylinders into each other, earning its name as ‘the Gun Method’. The inefficiency stems from the reacting uranium cylinder expanding (becoming less dense), reducing the chance of neutrons hitting other atoms and releasing energy.  3 days after detonating the Little Boy, the USAAF dropped the Fat Man, originally worked on as a fall-back. It used plutonium-239 and was detonated by “implosion”, which used 20% of the available material; the Little Boy managed only 2%. This is a tenfold percentage increase, however consuming all available material before explosion is still a challenge for militaries today.

 

 

Uranium-235 and plutonium-239 are the only substances agreed to be plausible in a nuclear weapon, and scientists debate whether alternatives such as Neptunium will ever be, but we should hope that there is never a chance to test it. While developing the Little Boy, the isotope plutonium-240 was initially slated for use, but Emilio G. Segrè and his group found that it was so fissile it would have detonated prematurely, leading plutonium’s use to be restricted exclusively to implosion-type weapons. 

 

 

 

The Little Boy was never wholly tested – it’s full destructive potential was only fully tested in its deployment, killing 80 000 people in just a few minutes6. The innovations made by the chain of scientists from Rutherford to Slizard and those after him now sustainably power up to 800 million homes today, but the bombing of Hiroshima makes it abundantly clear that progress and technology – whether as revolutionary as an atomic bomb or as commonplace as a hammer, is fundamentally neutral, and its impact is decided by proprietors.

 

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