Hiroshima Model

To achieve chain-reaction fission, a certain amount of fissile material, called critical mass, is necessary. The fissile material used in the Hiroshima model was uranium 235. In the bomb, the uranium was divided into two parts, both of which were below critical mass. The bomb was designed so that one part would be slammed into the other by an explosive device to achieve critical mass instantaneously.

When critical mass is achieved, continuous fission (a chain reaction) takes place in an extremely short period of time, and far more energy is released than in the case of a gun-powder explosion. The Hiroshima model is known as a gun-barrel-type atomic bomb.

Due to its long and narrow shape, the Hiroshima model was called "Thin Man" at first, but during the manufacturing process the original plans were modified, shortening the length and giving rise to the name "Little Boy". The energy released from the Hiroshima A-bomb was originally thought to be equivalent to the destructive power of 20,000 tons of TNT. Later estimates, however, put the energy equivalent to approximately 15,000 tons of TNT, based on damage done to buildings and research on the bomb's composition. Despite the release of such enormous energy, it is believed that less than one kilogram of the 10 to 30 kilograms of uranium 235 housed in the bomb achieved fission.

Vocabulary explanations
Principles of atomic bombs
Comparison and schemes of the bombs