A nuclear reactor is essentially a device to initiate and control a sustained nuclear chain reaction. They’re used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines. Some reactors are used to produce isotopes for medical and industrial use, or for production of plutonium for weapons. Some are run only for research. Today there are about 450 nuclear power reactors that are used to generate electricity in about 30 countries around the world. Continue reading for more.