When the atomic nuclei in the center of an atomic bomb, which is composed of fissile materials, are split, an enormous amount of energy is released as dangerously high levels of heat and radiation. Atomic bombs use this energy as a weapon for killing.
The splitting of atomic nuclei is called "fission." When a single neutron strikes the nucleus of a fissile material such as uranium 235 (or plutonium 239), two or three more neutrons are released. When those neutrons are ejected, enormous energy is released. The flying neutrons then hit other nuclei of the uranium and cause them to split in a similar manner, releasing more energy and neutrons. When this fission spreads, a huge amount of energy is generated instantaneously.
Research on atomic bombs was begun around the same time in several countries, including Germany, but in the United States, the actual building of an atomic bomb was already underway by 1942 under the code name "Manhattan Project." The project was carried out in extreme secrecy using a large amount of the national budget and outstanding scientists. In September 1944 it was determined that an A-bomb would be used against Japan. On July 16, 1945 in the desert near Alamogordo, New Mexico, the United States successfully conducted the world's first nuclear test.
On July 25 immediately after the test, an order was issued to drop an atomic bomb on Japan. That order was written as follows.
A field operation order dated August 2 stated that the day of attack would be August 6 and the primary target would be Hiroshima.
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.
The fissile material used in the Nagasaki bomb was plutonium 239. The plutonium
239 was divided into below-critical-mass units and packed into a spherical
case. At the time of detonation, the units were compressed to the center
with a gun-powder explosion to achieve fission. The
Nagasaki model is known as an implosion-type atomic bomb.
Compared to the Hiroshima A-bomb, the one used in Nagasaki was larger in diameter and round so it was called "Fat Man."
Only slightly more than one kilogram of the plutonium 239 is thought to have achieved fission, but the energy released is estimated to be equivalent to the destructive power of about 22,000 tons of TNT.
Hiroshima Nagasaki Height of explosion 580+-15 m 500+-10 m Radius in which ceramic roof tiles melted 600 m 1,000 m Radius in which granite stone melted 1,000 m 1,600 m
