THE PRINCIPLES

FISSION/CHAIN REACTION

The atomic bomb gets it's energy from fission (splitting) of the nulcei (core) of uranium or plutonium atoms. Albert Einstein explained how the fission of heavy atoms can produce energy released as dangerously high levels of heat and radiation. He published his theory in 1905 which is the well-known equation E = m c-squared.
This states that a given mass (m),
is associated with an amount of energy (E),
equal to this mass multiplied by the square of the speed of light (c).

A very small amount of matter is equivalent to a vast amount of energy. For example, 1 kg of matter converted completely into energy would be equivalent to the energy released by exploding 22 megatons of TNThe neutron is the most effective particle to cause uranium fission. Only one nuetron is needed to split an atom. When the atom fissions (splits), it splits into two smaller atoms which are most always radioactive and releases an enormous amount of energy and two or three nuetrons. The nuetrons released could then possibly hit other nuclei of uranium which causes them to split in the same fashion. This is a chain reaction (a series of fissions). A baseball made of plutonium produced an explosion equal to 20,000 tons of TNT.

CRITICAL MASS

If you had a small sphere of pure fissile material, such as uranium-235, about the size of a golf ball, it would not sustain a chain reaction. Too many neutrons escape through the surface area, and in turn are lost to the chain reaction. This is called a subcritical amount.
In a mass of uranium-235 about the size of a baseball, there are more neutrons hitting the atoms of the fissile material than are escaping through the surface area, thus sustaining the chain reaction.

The minimum amount of fissile material required to maintain the chain reaction is known as the critical mass.
Increasing the size of the sphere produces a supercritical assembly, in which the successive generations of fissions increase very rapidly, leading to a possible explosion as a result of the extremely rapid release of a large amount of energy.

A heavy material, called a tamper, surrounds the fissile mass and prevents its premature disruption. The tamper also reduces the number of neutrons that escape.

PRODUCING AN EXPLOSION

When the scientist assemble the bomb, they cannot just create a supercritical mass of fissile material because it would explode.
We get around this problem by creating two subcritical amounts of fissile material then assembling them in the bomb apart from each other.
They do not become critical until an explosion is set off to fire one of the subcritical masses at the other one. The force of the impact welds the two pieces together. Together, these create a critical mass.
It takes about 1 millionth of a second for the nuclear explosion to occur.

THE MANHATTON PROJECT

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. Many prominent American scientists including the physicists Enrico Fermi and J. Robert Oppenheimer, and the chemist Harold Urey, were associated with the project, which was headed by a U.S. Army engineer, Major General Leslie Groves.
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, the "Fat Man" test, codename Trinity. he bomb used in the Trinity test was called the "Fat Man".

When the bomb exploded and the fireball continued to consume the desert, General Thomas F. Farrell, Groves' assistant cried out, "the longhaors have let it get away from them!"

The next day he described the blast a bit more accurately.

"For the first time in history there was a nuclear explosion; and what an explosion! The lighting effects beggarded description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, grey and blue. It lighted every peak.....of the nearby mountain range with a clarity and beauty that......the great poets dream about but describe most poorly and inadequately. Thirty seconds after the explosion came, first, the air blast pressing hard against people and things, to be followed almost immediately by the strong, sustained, awesome roar which warned of doomsday and made us feel that we puny things were blasphemous to dare tamper with the forces heretofore reserved to the Almighty."