The substance that slows down the neutrons to have a controlled chain reaction during nuclear energy production is called a

A moderator is the substance that slows down neutrons to sustain a controlled chain reaction.

Slower neutrons are more readily captured by the fuel nuclei to keep fission going steadily.

A controller is not the standard term for this neutron slowing material.

An inhibitor suggests something that stops a reaction rather than moderating neutron speed.

A reducer refers to chemical reduction and not to slowing neutrons in a reactor.

Common moderators include ordinary water, heavy water and graphite.