Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller (lighter) nuclei. This reaction often produces free neutrons and photons (in the form of gamma rays) and releases a very large amount of energy, even by the standards of radioactive decay . The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile nuclides.
Nuclear Fission Reaction
An induced nuclear fission event. A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons
For example, when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei (the fission products), releasing kinetic energy, gamma radiation, and free neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.
Just as conventional power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, the thermal energy released from nuclear fission can be converted in electricity by nuclear reactors. A nuclear chain reaction can be controlled by using neutron poisons and neutron moderators to change the percentage of neutrons that will go on to cause more fissions. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if unsafe conditions are detected.
The reactor core generates heat in a number of ways. The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms. Some of the gamma rays produced during fission are absorbed by the reactor, and their energy is converted to heat. Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shut down.
A nuclear reactor coolant -- usually water, but sometimes a gas, liquid metal, or molten salt -- is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam.
The power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions. Control rods that are made of a neutron poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission, so pushing the control rod deeper into the reactor will reduce the reactor's power output, and extracting the control rod will increase it.
Control Rod Assembly
Control rod assembly, above fuel element
Some serious nuclear and radiation accidents have occurred. Nuclear power plant accidents include the Chernobyl disaster (1986), the Fukushima Daiichi nuclear disaster (2011), the Three Mile Island accident (1979), and the SL-1 accident (1961).
Nuclear safety involves the actions taken to prevent nuclear and radiation accidents or to limit their consequences. The nuclear power industry has improved the safety and performance of reactors and has proposed new safer (but generally untested) reactor designs. However, there is no guarantee that these reactors will be designed, built, and operated correctly.
Fukushima Daiichi Nuclear Disaster
Satellite image taken March 16, 2011 of the four damaged reactor buildings