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Uranium 235
Uranium is a dense, metallic radioactive element. This naturally occurring element is rare, but more common than gold, silver or mercury. Uranium is the primary source of heat within the earth. There are several isotopes, or varieties, of uranium. Each of these has a slightly different atomic weight. Pure uranium contains about 99 percent U-238, 0.07 U-235 and trace amounts of U-234. Uranium 235 is called fissile because its nucleus can be easily split in a process known as nuclear fission. It is comprised of 92 protons and 143 protons, resulting in an atomic weight of 235.
Radioactivity Versus Fission
Radioactivity represents a spontaneous release of energy from unstable atoms, such as U-235. When this occurs, the atoms disintegrate and break down into lighter atoms. This natural decay is slow and releases tiny amounts of energy. Scientists are unable to control this process, such as speeding up, slowing down, starting or stopping the decay. On the other hand, fission is a process whereby unstable atoms are split by bombarding them with neutrons. Scientists can control this type of reaction, which produces approximately 400 times the energy of natural decay.
The Fission Process
When an atom of U-235 is bombarded by a neutron, the atom absorbs it. This creates the new isotope U-236, which is highly likely to spit. When it splits, the two resulting atoms are called fission products. Common fission products are krypton and barium, but other possibilities include strontium, xenon, caesium and iodine. Some of these are radioactive and form part of the nuclear waste from a fission reaction. In addition to the two new atoms, two or three neutrons are also released. These neutrons are then capable of splitting more uranium atoms, continuing the fission process and creating a chain reaction.
Chain Reaction
Left unchecked, this fission chain reaction can split forty quintrillion uranium atoms within a thousandth of a second. This is what occurs in a nuclear weapon. As each atom splits, a tiny amount of mass is lost. However, this tiny mass represents a large amount of energy. This is explained by Einstein's formula, where energy equals mass times the speed of light squared. In a nuclear reactor, the chain reaction is kept in check by control rods that absorb some of the neutrons. This reduces the number available to split other uranium atoms, thereby limiting the chain reaction. The energy released in a fission reaction creates a lot of heat. In a nuclear power plant, this heat is used create steam, which spins a generator that produces electricity.