A nuclear reactor is a piece of scientific equipment which is designed to generate controlled chain reactions of fissionable elements like uranium. Nuclear reactors can be used for a number of purposes, including the generation of electrical power, the production of nuclear fuels, and scientific research. Nuclear reactors of various designs can be found in many regions of the world, from scientific labs in the United States to the French countryside, with the French relying on nuclear power for around 75% of their power needs as of 2009.
Nuclear reactors rely on a process known as fission, which occurs when a heavy element absorbs a neutron and breaks apart into two lighter elements. In the process, the element releases energy. In a controlled fission reaction, this energy can be harnessed for various activities, and coaxed into establishing a chain reaction which encourages the rest of the fissionable element to break apart as well. An uncontrolled reaction creates a nuclear bomb.
Inside a nuclear reactor, fuel rods made up of a fissionable element are inserted into a moderator, a material which will slow down neutrons to encourage fission. The operator of the nuclear reactor can control the reaction with the use of control rods, rods made from materials which absorb neutrons. When the rods are lowered into the reactor core with the fuel, they absorb the neutrons, slowing or stopping the fission process, and when they are raised, they allow the fuel rods to absorb the neutrons and undergo fission. Periodically, the fuel rods become spent and need to be replaced with fresh fuel, while the spent fuel must be carefully disposed of to avoid generating pollution.
The nuclear reactor also requires a coolant to transfer heat away from the reactor core. Heat, a byproduct of fission, may in fact be the goal of the reactor's operators, because it can be used to generate electricity by generating steam which will push turbines. When a nuclear reactor is being used for scientific research, the heat may be an undesirable byproduct, although some reactors are designed for both research and electrical generation for maximum efficiency.
A huge number of systems are involved in the control and containment of a nuclear reactor. The inherent risk of creating a controlled reaction is that the reaction may cascade out of control. This concern is addressed with numerous failsafes and safety technologies which are designed to keep the reactor safely operating. Relatively few major accidents have occurred with nuclear reactors, and those which do tend to occur in older reactors with insufficient support infrastructure in place.