The Chemistry of the Atomic Bomb (Level 2 and 3)

Level 2: Isotopes in Atomic Bombs

For atoms heavier than Nickel-62, the stability of the nucleus decreases as atomic mass (nucleon number) increases. Energy given off by atomic bombs arise from nuclear fission in which heavier isotopes such as uranium or plutonium split into more tightly bound stable elements.

The atomic bombs ‘Little Boy’ and ‘Fat Man’ were used by the USA in WWII. Little Boy used Uranium-235 and Fat Man used Plutonium-239. 

Level 3: Comparison between Little Boy and Fat Man

Little Boy

Little Boy used the gun-type assembly method. In the diagram on the left, the red rings represent 80% Uranium-235. When the explosive (orange section) is detonated, the uranium ‘bullet’ in front of the explosive is accelerated towards the other uranium section. Each section of uranium is sub-critical, that is of an insufficient mass for an explosion to occur. (For more information of critical mass, refer to Level 3 of Physics) When the uranium sections are 25cm from each other, free neutrons may hit the uranium, resulting in the formation of the highly unstable Uranium-236 which is deformed elastically. It then splits into the highly radioactive fission products of Barium-144, Krypton-89 and 3 neutrons which collide with more uranium to cause a chain reaction. This could cause a pre-donation.

To prevent pre-detonation, the speed of the ‘bullet’ would have to be very high which requires a long and heavy barrel. The gun-type method is unsuitable for Plutonium-239 because it contains about 20% Plutonium-240 which makes pre-donation inevitable. 

In contrast, Fat Man used the more complicated implosion assembly method. As seen from the figure of the left, the center contains a sub-critical mass of plutonium-239. When the explosion lenses are detonated, the density of the plutonium increases until it becomes super-critical (sufficient to start and sustain a chain reaction). This method is much safer as it prevents accidental pre-detonations. However, the complexity of the design leads to higher costs and allows for the creation of a smaller bomb. 

The table below summarises the comparisons

	Little Boy	Fat Man
Method	Gun-type	Implosion: higher complexity, higher costs, safer
Explosiveness	Less explosive: energy generated equivalent to 15 kilotons of TNT (a common explosive)	More explosive: energy generated equivalent to 20 kilotons of TNT
Fissile material	60kg of 80% Uranium-235	8kg of 80% Plutonium-239
Size	Larger and heavier	Smaller and lighter