Question

The graphite rods used in a nuclear reactor:

• A. React with uranium to release energy
• B. Produce neutrons
• C. Undergo combustion that triggers fission
• D. Convert fast neutrons into thermal neutrons

Hint:

Graphite acts as a moderator in a nuclear reactor, not a fuel.

Answer 1

The Correct Option is D

Fission of a $U-235$ nucleus releases 2 or 3 new neutrons, each moving very fast. Fast neutrons are actually poor at causing further fission because they tend to fly past other nuclei without being captured.

• For the chain reaction to continue smoothly, these fast neutrons need to be slowed to thermal energies where capture probability is high.
• Graphite has light carbon nuclei of a similar mass order to a neutron, so a neutron loses a good fraction of its kinetic energy in each collision with a carbon atom, much like a moving ball losing speed after hitting an object of comparable mass.
• After many such elastic collisions inside the graphite rods, the fast neutrons are reduced to slow thermal neutrons.
• This moderating action, not any chemical reaction or combustion, is the real role of graphite rods in a reactor core.

Hence graphite rods convert fast neutrons into thermal neutrons, so option D is correct.