Fission describes the splitting of an atom's nucleus into two or more smaller nuclei. Most atoms will not fission because a binding energy that holds its protons and neutrons together prevents it. However, some atoms with big, unstable nuclei, like U-235, are possible to break apart. Under certain conditions, when U-235 is struck with a neutron it divides and produces two lighter atoms. The mass of these two lighter atoms added together is less than the original U-235 atom. In the process of fission the mass that seems to have disappeared has been converted into energy.
According to Einstein's formula E = mc2, even a small amount of mass (m) inside the atom can be magnified by a huge number (c2, the speed of light squared) to create enormous amounts of energy (E). The fissioning of one U-235 nucleus releases 50 million times more energy than the combustion of a single carbon atom. Nuclear fission produces far more heat than burning a comparable volume of hydrocarbon fuel such as oil, natural gas or coal.
Chain reaction
In addition to the creation of two new smaller nuclei, fission frees some neutrons to make other atoms divide. They strike other U-235 atoms and release more neutrons. As long as there are uranium atoms present, the fission process continues. This is called a chain reaction. It is this chain reaction that makes a sustained nuclear reaction possible. It creates an ongoing release of energy from one atom to the next and therefore provides a continuous source of energy.
If uncontrolled the fission reaction multiplies rapidly and can produce an explosion. However, in a nuclear reactor, fission is controlled. Only one neutron is allowed to produce another fission. Control rods prevent the number of neutrons in a nuclear reactor from growing too large by absorbing excess neutrons. To do this, control rods are inserted into the core of the reactor. Pushed in, they soak up neutrons and slow down the reaction; pulled out they allow it to speed up again. In this way the chain reaction is controlled.
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