A nuclear reactor is a complex device that is designed to start and sustain a controlled nuclear chain reaction. They are usually utilized to generate electrical power or to provide propulsion for ships and submarines. Controlled nuclear fission is used to heat either water or a gas which is then passed through a turbine. The turbine in turn either spins the propellers of a ship or the generators of an electrical power station.
Uranium-235 and plutonium-239 are fuels that are used in nuclear reactors. When a neutron hits a nucleus of either, it may split into two smaller fragments that are nuclei of lighter elements. It also releases kinetic energy, gamma radiation and free neutrons. Some of the released neutrons go on to hit other uranium or plutonium nuclei and cause them to fission releasing still more neutrons. This is known as a nuclear chain reaction.
If the number of free neutrons continues to rise, a meltdown of the nuclear fuel can result. If the number of free neutrons falls off, the chain reaction will stop. In order to use nuclear materials for fuel in a reactor, it is necessary to keep the neutron flux in a certain stable range. This is accomplished by using something called a moderator to soak up free neutrons. Ordinary water is used to moderate about three quarters of the world's reactors. Solid graphite is used in another fifth. And heavy water which contains extra neutrons in the nuclei of its hydrogen atoms accounts for most of the rest.
Heat is generated in the reactor by several processes. The fast moving fragments from the fissioning of the uranium or plutonium collide with other atoms and heat them. Some of the gamma radiation is absorbed by the material of the reactor core and heats it. Some of the fission products are radioactive and will decay, release more heat.
The heat generated in the reactor core is captured by water, a gas or a liquid metal. These substances are referred to as coolants. The heat absorbed by the coolant is used to generate steam which turns the turbines. Most reactors keep the coolant separate from the water that is heated to produce steam. However, the boiling water reactor directly heats the water that is used to power the turbines.
There have been three generations of nuclear reactor put into operation so far. The fourth generation is currently being designed. Over the years, the reactors have been made more sophisticated with more automation, more backup controls, greater safety and greater reliability. These reactors have functioned well for the most part but every now and then extraordinary circumstances such as the tsunami in Fukushima have revealed short-comings which in turn lead to new designs.