The term 'Parts of a Nuclear Reactor' can be summarized by four letter solutions: RODS. Nuclear reactor rods are cylindrical pieces of metal that are used to control the rate of fission in a nuclear reactor. They are typically made of materials such as boron, cadmium, or hafnium, which absorb neutrons and slow down the reaction. The rods can be raised or lowered to adjust the rate of fission, allowing for greater control over the reactor's output.