Question

A moderator in a nuclear reactor is used to

• A. Increase neutrons
• B. Decrease neutrons
• C. Slow down the speed of neutrons
• D. None of these

Hint:

A moderator changes neutron speed through collisions with light nuclei, it does not add or remove neutrons.

Answer 1

The Correct Option is C

Use a simple collision picture to see why a moderator must be made of light atoms, not heavy ones.

1. Imagine a fast moving ball hitting another ball. If it hits a ball of similar mass, it transfers a lot of its kinetic energy in the collision, like one billiard ball hitting another of the same size.
2. If it hits a much heavier ball, like a bowling ball hitting a wall, it bounces back with almost the same speed, losing very little energy.
3. Neutrons behave the same way. Colliding with light nuclei, such as hydrogen in water or deuterium in heavy water or carbon in graphite, they lose a large fraction of kinetic energy per collision, so after several collisions they slow down to thermal speed.
4. This slowing down does not add or remove any neutrons, the same neutrons are simply moving slower afterward, so it is wrong to say the moderator increases or decreases the neutron count.

That is different from control rods, which physically absorb neutrons to reduce their number, so the moderator's only job stays speed reduction, matching option C.