On August 6th and August 9th 1945, the nature of warfare was irrevocably changed and the world had become a far more dangerous place. Two atomic bombs, named ‘Little Boy’ and ‘Fat Man’, were dropped on the Japanese cities of Hiroshima and Nagasaki killing at least 129,000 people. An estimated half of all the deaths occurred on the first day with large numbers dying from radiation sickness, burns, illness and malnutrition in the following months. This remains the only use of nuclear weaponry to date, but the threat of another nuclear war has remained ever since. Many people are aware of the impact of these weapons, but may not know how they work. This is the physics behind the atomic bombs.
The atomic bombs derive their immense destructive power from the sudden release of energy that occurs from splitting the nuclei of fissile elements in their core. The two bombs that were dropped on Japan used different cores and a different assembly method. Little Boy was dropped on Hiroshima and used a uranium core in a gun-type assembly method, while Fat Man used a plutonium core in an implosion assembly method. I will explain the difference in these methods a little later, but I think a good place to start the deconstruction of these bombs is the process of fission.
The nuclei of atoms consist of protons and neutrons. The number of protons determine the element, (Uranium has 92 and Plutonium has 94) while the number of neutrons determine the isotope. The isotopes chosen for the atomic bombs were uranium-235 and plutonium-239. These were chosen because they readily undergo fission. U-235 is a valuable isotope for nuclear weaponry because it can function as the primary fuel for a weapon. When U-235 absorbs a neutron it breaks into two new atoms plus three new neutrons and some binding energy. Two of those neutrons become absorbed by a uranium-238 atom. However, the remaining neutron does collide with another U-235; it then fissions and releases two neutrons. Both of those collide with U-235 atoms which then releases between one and three neutrons. This begins the chain reaction that grows exponentially. To ensure that enough neutrons are produced to cause the chain reaction requires a critical mass of fissionable material. The more fissionable material you have, the better the chance of a chain reaction event occurring.
The difference between the two bombs is in the process of detonation. Little boy which used the uranium isotope 235 was constructed using a gun type design. This design involves firing one amount of U-235 at another to combine the two masses. This creates a critical mass that then sets off the chain reaction that will eventually detonate the bomb. The collision of the two masses has to happen quick enough to avoid spontaneous fission; which would cause the bomb to fizzle and fail to detonate. Fat Man, powered by plutonium, could not use the same gun-type design. This was due to the plutonium extracted from the nuclear reactors not being pure plutonium-239, but contained traces of the plutonium-240 isotope. The higher fission rate of 240 meant that in a gun type design it would cause spontaneous fission before the two masses collide which would lower the energy. Instead they used a central mass of plutonium, known as a plutonium pit, placed in a shell of conventional explosives, know as an explosive lens. These explosions detonate at precise times to cause a spherical shock wave, squeezing the plutonium and increasing the pressure and density of the substance. This increase in density allows the plutonium to reach its critical mass, firing neutrons and initiating the fission chain reaction.
Following the success of the atomic bombs, physicists went on to evolve this weaponry creating fusion or hydrogen bombs. In these bombs, the energy is not produced by fission but fusion. Fusion bombs contain a fission bomb inside them that creates the high temperature and pressure needed for fusion to take place. In these conditions the hydrogen isotopes deuterium and tritium can readily fuse and release enormous amounts of energy.
In many ways the construction of the nuclear bomb was an accomplishment of the practical uses of physics, and an example of the power of nuclear physics, but the fact that this accomplishment of human endeavour was used to create weaponry of such great devastation will forever remain a scar on humanity.