Role of a moderator in nuclear power plants: identify the primary function. Context: A moderator is mixed with the fuel mass in thermal reactors. Choose the correct purpose it serves.

Given data

• Component: Moderator in a nuclear reactor

Concept/Approach
A moderator (e.g., light/heavy water, graphite) reduces the kinetic energy of fast neutrons via elastic scattering, increasing the probability of thermal fission in fuels like U-235.

Why other options are incorrect

• Control the reaction: That is primarily the job of control rods (absorbers), not the moderator.
• Reduce temperature / Extract heat: Heat removal is by the coolant circuit, not by the moderator per se (though some moderators are also coolants).

Final Answer
Slow down the speed of fast moving neutrons.