A normal chemical reaction involves only some of the outermost electrons of the reacting atoms-the atoms themselves remains unchanged but combine with other kinds of atoms to produce molecules. During chemical reactions some of the chemical energy that lies in the arrangement of the atoms may be released as heat, for example, in the burning of fuels such as wood, coal, and oil.
A nuclear reaction involves the nucleus, the very heart of the atom composed of protons and neutrons, and some of the mass of the atom is converted into energy. A nuclear reaction releases several milion times more energy than a chemical reaction and, unlike a chemical reaction, involves a change in which one element is turned into another. When an atomic nucleus is split into two or more smaller nuclei, a nuclear fission is said to have occured as opposed to nuclear fusion.
Each neutron released during a fission reaction is capable of starting another fission reaction by splitting another uranium atom. The neutrons released by each of these reactions then split several more atoms. One neutron striking one uranium atom initiates a chain of nuclear fission reactions. The process in which the splitting of one atom causes the splitting of additional atoms is called a nuclear chain reaction.
If a nuclear chain reaction is uncontrolled, the resulting energy that is released will create an atomic explosion. That is just what happens in an atomic bomb. However, if the chain reaction is carefully controlled, the energy that is released can be a valuable energy resource. Controlled chain reactions take place in nuclear power plants.
Fission reactions in a nuclear power plant are produced and controlled in nuclear reactors. The three main parts of a nuclear reactor are the core, moderator, and control rods.
The core is the central part of the reactor. It is within the core that nuclear fission takes place. To begin a fission reaction, nuclear fuel rods are placed in the core. When neutrons strike the rods, nuclear fission begins. If the fuel rods are placed at just the right distance from one another, a chain reaction occurs.
The neutrons released during fission are too fast to be absorbed by other uranium nuclei, so, the neutrons must be slowed down. A material that slows down neutrons is called a moderator. Water is often used as a moderator.
In order for a nuclear reaction to produce useful energy, the overall speed of the reaction must be carefully controlled. This task is accomplished with control rods. In most reactors, the control rods are made of the element cadmium. Cadmium rods have the ability to "soak up" neutrons. By placing control rods deep in the reactor core, extra neutrons are absorbed and the fission process is slowed down. By pulling the control rods out of the reactor a bit, the fission reaction is speeded up as more neutrons become available to split uranium nuclei.
Most radioactive wastes are stored as liquids in multi-walled tank. They can also be "fixed" in a kind of glassy substance, which can be burried underground. Because of their extremely slow decay, many waste products remain dangerously radioactive for thousands of years and thus have to be stored under guard against terrorists or criminals for all that time.