According to Hewitt (1997), within all known nuclei exist both an attractive force
known as the nuclear strong force and a repulsive force created by electrical forces.
In every known nucleus, with the exception of the uranium atom, the nuclear strong
forces dominate, holding the atom together. In the uranium atom, the nuclear strong
forces may become compromised if the nucleus becomes elongated or stretched. This
elongation is initiated when the uranium atom is hit with a neutron. Stretching of
the uranium nucleus beyond a critical point will result in the domination of electrical
forces and the eventual split of the nucleus. The process of splitting of atomic nuclei
is referred to as nuclear fission (Hewitt, 1997). Figure 1 is a graphic of nuclear fission
depicting the bombardment of a Uranium-235 nucleus by a neutron and the resultant products.
Uranium-235 is the naturally occurring fissionable isotope, the most unstable, and therefore
the most likely atom to be fissioned.
It is important to note that fission is not exact. Although the graphic in Figure 1 shows the formation of two daughter nuclei, this is not always the case. In some rare cases, three daughter nuclei result. In any case, the daughter nuclei are rarely identical in mass since the velocity of the bombarding neutron influences the masses of the resulting daughter nuclei. Also created, and as shown in Figure 1, are 2 or 3 neutrons, which can also bombard other original nuclei (Clark, 1980).
As these next nuclei undergo fisson, they release additional free neutrons which cause more fissioning. This ongoing process is known as chain reaction. Figure 2 is a graphic of a chain reaction of Uranium-235.
To see an animation of the fission process provided by Kansas State University click on this link: Fission Animation (requires ActiveX).
As a uranium atom splits, energy is released. The amount of energy released depends on the size of uranium targeted. If a chain reaction were to be initiated in a very large chunk of U-235 (say the size of a baseball), the resulting explosion would be massive. But a chain reaction taking place in a smaller piece of U-235 would possibly not produce an explosion. This is because in a smaller piece of uranium, a bombarding neutron could travel to the surface of the material and escape before coming in contact with another uranium nucleus. In a large piece of U-235 a bombarding neutron is much more likely to hit another uranium nucleus, since it can travel farther through the material before surfacing (Hewitt, 1997). If a chain reaction occurs like the one depicted in Figure 2, the number of fissions doubles after each generation (atomicarchive.com). It is important to note however, that not every neutron produced in a chain reaction is used to continue the reaction. Some are lost. If the reaction produces enough usable neutrons then it has reached critical mass, or has become self-sustaining (Clark, 1980).