The word fission means "a splitting or breaking up into parts" (Merriam-Webster Online, www.m-w.com). Nuclear fission releases heat energy by splitting atoms. The surprising discovery that it was possible to make a nucleus divide was based on Albert Einstein’s prediction that mass could be changed into energy. In 1939, scientist began experiments, and one year later Enrico Fermi built the first nuclear reactor.
Nuclear fission takes place when a large, somewhat unstable isotope (atoms with the same number of protons but different number of neutrons) is bombarded by high-speed particles, usually neutrons. These neutrons are accelerated and then slammed into the unstable isotope, causing it to fission, or break into smaller particles. During the process, a neutron is accelerated and strikes the target nucleus, which in the majority of nuclear power reactors today is Uranium-235. This splits the target nucleus and breaks it down into two smaller isotopes (the fission products), three high-speed neutrons, gamma rays.
This resulting energy is then used to heat water in nuclear reactors and ultimately produces electricity. The high-speed neutrons that are ejected become projectiles that initiate other fission reactions, or chain reactions.
Fission products, by their very nature are usually unstable and therefore radioactive; due to being relatively neutron-rich for their atomic number, many of them quickly undergo beta decay. This releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in beta and gamma radiation, even though this radiation is not produced directly by the fission event itself.
The process produces radionuclides with varying half-lives. These vary in radioactivity are poisonous to humans and our planet.
Fission is also dangerous. Some of the fission products, such as xenon-135 and samarium-149, have a high neutron absorption cross section. Since a nuclear reactor depends on a balance in the neutron production and absorption rates, those fission products that remove neutrons from the reaction will tend to shut the reactor down or "poison" the reactor. Nuclear fuels and reactors are designed to address this phenomenon through such features as burnable poisons and control rods. Build-up of xenon-135 during shutdown or low-power operation may poison the reactor enough to impede restart or to interfere with normal control of the reaction during restart or restoration of full power, possibly causing or contributing to an accident scenario.
There are safety concerns with fission nuclear power. These include radiation to the environment and the management of the radioactive waste.
Fission energy is expensive and dangerous.