The story of Hiroshima's 'Little Boy' bomb

Of the 64 kilograms of uranium in the Little Boy bomb dropped on Hiroshima, only about 0.6 grams, or at most 1.0 kilograms, actually underwent nuclear fission. Just 0.6 grams of uranium, about the weight of a butterfly, caused all the damage in Hiroshima - but the bomb contained 64 kilograms of uranium. 

-----------------

Matter as energy: The story of Hiroshima's 'Little Boy' bomb

The atomic explosion in Hiroshima on August 6, 1945, was a defining moment in history. The disaster caused by the bomb, known as "Little Boy," changed human views on war and the destructive power of matter.  The bomb, which had the yield of 15 kilotons of TNT, did not derive its energy from a large amount of explosives, but from the phenomenon of nuclear fission, based on Einstein's famous equation E=mc2.

At the core of the "Little Boy" bomb was a high-grade isotope of uranium-235. A total of 64 kilograms of uranium were used in it. The science behind how this small amount of matter could cause such great destruction is incredible. When the bomb explodes, two of the uranium's constituent elements combine to form a "super-critical mass." This starts an uncontrolled chain reaction.

In the process, when a neutron hits the uranium-235 nucleus, it unstable splits into two.  This splitting process is called nuclear fission. When this split occurs, a huge amount of energy is released, along with two or three new neutrons. These new neutrons then hit other uranium nuclei, causing more fissions. This spreads rapidly like a chain reaction.

The source of this tremendous energy is the relationship between matter and energy. When a uranium-235 nucleus splits, the total mass of the new nuclei and neutrons released from it is slightly less than the original nucleus. Although this mass difference is very small, according to Einstein's equation E=mc2, this small mass is converted into energy. Here, 'c' refers to the speed of light, and since its square (c²) is a very large number (about 9×1016m2/s2), even a small mass difference turns into an incredible amount of energy. 

Of the 64 kilograms of uranium in the bomb, only about 0.6 to 1.0 kilograms actually underwent fission. The force of the explosion scattered the remaining uranium, which disrupted the chain reaction. This is why the bomb's energy output was relatively low. If these 64 kilograms of uranium had completely fissed, the power of the explosion would have been over 1,000 kilotons.

From that small amount of uranium that underwent fission, the amount of matter that was converted into energy was only about 0.6 grams. This small amount of matter, about the weight of a butterfly, was enough to wipe out an entire city. This showed us one of the most fundamental truths of the universe: matter is a vast storehouse of energy. The event in Hiroshima changed our thinking about energy and fission forever.