Some Details Behind the Manhattan Project

The Manhattan Project at first seems to be an arduous, burdensome, and herculean task. Being told to design, refine, build, and launch a nuclear warhead in only seven years with computers thousands of times weaker than a single iPhone is a tremendous challenge. But the hundreds of shrewd scientists, tens of thousands of workers, and many billions of dollars thrown into the project quickly made it a bitter-sweet reality. In this post I would like to scrape the surface of some of the organization and science behind not just the explosives, but also the project itself.

However, let us not forget that much of the project, including designs of the two WW2 warheads, is still kept secret. Most of the information the government released about the project came right after the Bombings of Japan in the Smyth Report (sadly the report only contained theoretical information, which makes sense because quantitate data from experiments would be too valuable to the Soviets). It is important to note that Fat Man and Little Boy both had different fuels (plutonium and uranium respectively) and different ignitions (implosion-type, where the fuel is compressed by explosions then set off and gun-type, where two blocks of fuel are shot at each other, respectively). The gun-type ignition used on Little Boy is terribly inefficient also. Of the 60 kg of uranium it is believed only about 1 kg reacted. Compare this to Fat Man, where of the 3 kg of Plutonium about 1 kg of it reacted.

Compared to many other everyday mechanical devices, an atomic bomb is actually a surprisingly simple design. For example, it is believed that Fat Man (the bomb dropped on Nagasaki) had only twenty major parts and a relatively straightforward solid-state spherical geometry. Compare this to a rocket engine, which has tens of thousands of components involving pumps and valves. Why then do more nations not have atomic weapons? The greatest difficulty in creating a warhead is amassing enough of the radioactive isotope (either Uranium-235 or Plutonium-239).

Instead of focusing on one primary way of processing the already abundant nuclear isotopes, General Leslie Groves, who was the head of the project, chose to simultaneously study four methods of extraction (gaseous diffusion, centrifuge, electromagnetic separation and liquid thermal diffusion). Although today the centrifuge is the predominant form of isotope separation, the researchers at Los Alamos failed to create a working centrifuge. Instead they settled on the gaseous diffusion technique, which works on a similar basis as a TLC that you might take in a chemistry class or the electrophoresis you might do in a biology class.

Those taking physics might know that power is the rate-of-change of work. By this definition, a nuclear weapon is the most powerful entity man has ever made. The conventional units of measure for the yield of a nuclear device is a ton of TNT (which is about equivalent to 4.184 gigajoules of energy release). Little Boy and Fat Man had 13 and 22 kilotons of TNT respectively. Both bombs were single stage fission reactions though. Modern arsenals consist almost entirely of two stage fusion bombs (where the first step is a regular fission reaction which then forces a fusion reaction). To compare this to fission bombs, the Tsar Bomba, a fusion device had 50 megatons of TNT. The chemical reaction for a two step fusion bomb is shown below.

63Li + 10n → 42He + 31H + 4.7 MeV

21H + 31H → 42He + 10n + 17.6 MeV

Also here is an interesting statistic.


https://physics.info/weapons/catalog.svg