A nuclear reactor is a device that initiates and controls a sustained nuclear chain reaction. It is primarily used for generating electricity in power plants, where the heat produced from nuclear fission is used to produce steam that drives turbines. The most common type of reactor is the Pressurized Water Reactor, which uses water as both a coolant and a neutron moderator. Nuclear reactors also have applications in research and medical fields, such as producing isotopes for radiotherapy and conducting experiments in nuclear physics. Safety measures and regulations are critical in reactor design to prevent accidents and ensure the protection of the environment and public health.