What in meant by moderator, coolant and controlling rods with reference to nuclear reactor

Moderators are used to slow down the neutrons produced from fission. This is necessary because many nuclear fuels (uranium-235, for instance) require neutrons to be slow-moving in order to absorb them. Nuclei with low mass numbers are most effective at doing so, therefore materials likewaterorgraphiteare often used.

The coolant, as its name implies, is used to remove heat from the core and move it to somewhere that it is useful.[9]This keeps the fuel from overheating and melting down, at the same time as transferring the heat to water to make steam. Light water, heavy water, and various gases are the most common coolants for nuclear reactors. Coolants may also serve as the moderator, as is the case in many water-moderated reactors.

Control rods can be inserted into the reactor core to reduce the amount of fuel which undergoes fission reactions. The rods contain neutron absorbing atoms such as gadolinium or cadmium. By absorbing neutrons within the core, it prevents those neutrons from reacting with the fuel. Control rod movement can be used to adjust the number of reactions occurring at the core, or fully inserted to shut down the reactor completely