Nuclear Fuel
It consists of the fissionable material namely U233 or U235 or Pu239 Generally U235 is piled inside a sealed aluminum cylinder. Thermal neutrons hit these fuel atoms, fission starts and energy is released.
Moderators
They slow down the fast moving secondary neutrons. Usually heavy water, graphite, deuterium, paraffin etc., act as moderators. Control rods have the ability to capture the slow neutrons. Rods of boron or cadmium are inserted in the holes of the reactor core up to a desirable length for absorbing the neutrons.
Coolant
Coolants are substances used to remove and transfer the heat produced to the surrounding. It passes this heat over to water in a heat exchanger.
This results in superheated steam, which drives a turbine coupled to an electric generator. The whole reactor is protected with concrete walls so that radiations may not produce harmful effects to its surroundings.