A nuclear reactor has several main components: 1. The reactor core contains fuel elements, control rods, coolant, and moderator. It is typically a right circular cylinder 0.5-15 meters in diameter. 2. A reflector surrounds the core to reflect neutrons back into the core. 3. Control rods made of heavy elements like cadmium or lead are used to maintain or stop the nuclear reaction by absorbing neutrons. 4. A moderator like water, heavy water, or graphite is used to slow neutrons to increase the chance of fission.