A reactor is a device used to initiate and control a nuclear reaction. In nuclear power plants, reactors convert nuclear energy into heat, which is then used to produce steam that drives turbines for electricity generation. The core of a reactor contains fuel, typically made of uranium or plutonium, and is surrounded by materials that help manage the reaction and absorb radiation. There are different types of reactors, including pressurized water reactors and boiling water reactors, each with unique designs and operational methods. Safety systems are crucial in reactors to prevent accidents and manage any potential release of radioactive materials.