The Atomic Bomb
The first atomic bomb was tested on July 16,1945 at Alamogordo, New Mexico and was developed, constructed and tested by the Manhattan Project. The new device represented a completely new type of explosion. All explosives before this time got their power for the rapid burning of a chemical compound like gunpowder. These bombs could only do a limited amount of damage.
This new group of nuclear explosives involved getting energy sources from within the nucleus of the atom. The Atomic bomb gained its power from the fission of all of the atomic nuclei in several kilos of uranium. A Ball about the size of a baseball made an explosion equal to about 20,000 tons of TNT.
An atomic bomb can also be called a fission bomb because it uses fission to release the nuclear power from the fuel. The fuel is usually either Uranium-235 or plutomium-239. Uranium-235 has an extra property that makes it useful for both nuclear power production and for nuclear bombs – it is one of the few materials that can undergo induced fission. If a neutron runs into a Uranium nucleus, the nucleus will absorb the neutron, become unstable and split immediately.
As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off 2 or 3 new neutrons. These neutrons then hit other uranium atoms and a chain reaction is started. An incredible amount of energy is released, in the form of heat and gamma radiation.
In order for these properties of U-235 to work, a sample of uranium must be enriched. Weapons-grade uranium is composed of 90% or more U-235.
In a fission bomb, the fuel must be kept in small and separate masses. These must be small enough to not support fission otherwise the bomb could explode before it is meant to. These masses are called subcritical masses. Critical mass is the minimum amount of fissionable material needed to keep a nuclear fission reaction going.
Because the masses are separate there had to be ways to bring them together to detonate the bomb. There are two ways to detonate an atomic bomb. The first is the gun-triggered device and the second is the implosion device.
In both of these types, neutrons had to be introduced to start the fission. This was done by making a little neutron generator out of a small pellet of polonium and beryllium, separated by foil. This was put in the center of the fuel core where the subcritical masses were going to meet. It was designed so that when the small masses of fuel hit each other, they also hit the generator. This breaks the foil of the generator and the polonium spontaneously emits alpha particles. These alpha particles then collide with beryllium-9 to produce beryllium-8 and some free neutrons. The neutrons then initiate fission.
The Gun Triggered device is the similar of the two methods. A sphere of Uranium-235 with the neutron generator is made and a small bullet of uranium is removed. The uranium bullet is placed at the one end of a long tube with explosives behind it, while the rest of the uranium is placed at the other end. A barometric pressure sensor determines the appropriate altitude for detonation and triggers explosives. This propels the bullet down the tube. The bullet strikes the sphere and generator, which starts the fission reaction and the bomb explodes.
Little Boy bomb that was dropped on Hiroshima was this type and had a 14.5-kiloton explosion (equivalent to 14,500 tons of dynamite) with an efficiency of about 1.5 percent. That is, 1.5 percent of the material was fissioned before the explosion blew the uranium apart.
The other detonation method is the implosion-triggered bomb. It consists of several subcritical masses of plutonium-239 surrounded by high explosives within a sphere of uranium-238 (tamper). When the bomb is detonated, the explosives go off creating a shock wave. The shock wave propels the plutonium pieces together into a sphere. The plutonium pieces strike the neutron generator, which starts the fission reaction began and the bomb explodes.
Fat Man Bomb that was dropped on Nagasaki was this type of bomb and had a 23 kiloton yield with an efficiency of 17 percent. These bombs exploded in fractions of a second.
The atomic bombs is necessary for use in fusion bombs and is now almost obsolete because the fusion bomb has a far greater efficiency.