Working principle of an atomic bomb
     The raw material used in an atomic bomb is the radioactive uranium-235. Since naturally
     occurring uranium contains only 0.3% of uranium-235, the uranium sample used in the 
     bomb must be enriched so that it contains 90% or more of uranium-235.

     The atomic bomb make use of the principle of the chain reaction, where a large amount of
     energy is released in the form of heat and gamma ray. When the uranium is above a 
     certain [critical mass], the chain reaction will take place very rapidly. The atomic bomb can
     be made by separating two slightly sub-critical masses and bringing them together when 
     detonation is desired. When those two masses are brought together, an uncontrolled 
     [chain reaction] of uranium occurs and an enormous amount of energy will be released 
     instantaneously, with a vast amount of gamma rays and neutrons.


     A brief history of atomic bombs

The first atomic bomb was invented in 1945 and detonated at a test site in New 
Mexico, U.S.A., during the period of the Second World War (1939-45). On the 
6th of August 1945, the US army dropped an atomic bomb on the city of 
Hiroshima in Japan, and dropped another one on Nagasaki on the 9th, causing 
massive destruction of two cities and injuring more than 200,000 people. Many 
others suffered radiation sickness and died of cancer years later. The dropping
of two atomic bombs prompted the surrendering of Japan on August 15 and 
ended the Second World War.

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