Nuclear fission is the act of splitting the nucleus of a radioactive element in two. Fission was first discovered in 1938, when a sample of uranium-235, after being bombarded with neutrons, was found to contain traces of barium. The question was, why did barium simply appear in the uranium sample? The answer was simple: a uranium-235 nucleus had captured a neutron, forming the unstable isotope of uranium-236. This isotope had undergone nuclear fission, splitting the nucleus in two and forming smaller samples of more stable elements. The reaction was as follows:
with 2 x 1010 kJ/mol of energy produced.
As you can see from this reaction, a single neutron is all that is needed to push the uranium-235 (now 236) over the edge, releasing not only energy, but three more neutrons as well. And when those neutrons hit three other atoms of uranium-235, those atoms will split, releasing a total of nine neutrons, and then twenty-seven neutrons, and so forth. A chain reaction occurs... Shown below is an animation of the reaction. This is nuclear fission.
If the masses of the uranium-235 atom and the fission products were compared, one would find that a small amount of mass disappeared and a huge amount of energy was released. As explained in Einstein's famous equation E = mc2, in which E is energy, m is mass, and c is the speed of light, the missing mass has been converted into energy. In Einstein's equation, E is often expressed in joules, m is kilograms, and c is meters per second. For example, if only one gram of matter is converted to energy, the amount released is .001 kg * (3 x 108 m/sec)2 = 9 x 1010 kilojoules, or ninety terajoules!
The applications of nuclear fission are varied. The first that comes to mind is the one suggested by the mushroom cloud on this page. That is the result of a test fire code-named Ivy Mike. It was the first thermonuclear detonation in the history of this planet, with a yield of about 5 megatons (5 million tons of TNT, trinitrotoluene). But there are other uses for nuclear power. Power plants have been constructed, utilizing controlled nuclear fission to generate heat. The heat boils water to produce steam, and this steam turns turbines which generate electricity. Nuclear-powered ships, such as submarines and aircraft carriers, also utilize fission reactions for power. This is achieved by tightly controlling the number of neutrons that are allowed to react within the uranium piles. If the reactions get out of control, disaster results, as it did with Chernobyl in the 1980's: reactors can overheat, or "melt down," possibly breaching the containment system and releasing radiation into the environment. However, no nuclear explosion is possible with standard nuclear reactor fuel. More applications of nuclear power are being found as time goes on, and as our understanding of the phenomenon grows.
Nuclear fission is generally a dangerous process, both due to the amounts of energy released and the radioactivity generated. Radiation is released directly by the fission reaction, in the form of neutrons, and also through the decay of fission products. Depleted uranium nuclear fuel and other components of nuclear reactors are highly radioactive, and can remain "hot" for hundreds of years.