Question

In a nuclear reactor heavy nuclei is not used as moderators because

• A. They will break up
• B. Elastic collision of neutrons with heavy nuclei will not slow them down.
• C. The net weight of the reactor would be unbearably high
• D. Substance with heavy nuclei do not occur in liquid or gaseous state at room temperature.

Answer 1

The Correct Option is B

• Inefficient neutron slowing: Elastic collisions between neutrons and heavy nuclei (like uranium or lead) transfer minimal kinetic energy, failing to thermalize neutrons effectively.
• Physics principle: Momentum conservation dictates that light nuclei (e.g., hydrogen, carbon) slow neutrons best, as they absorb more energy per collision.

Why other options are incorrect:

• A: Heavy nuclei do not inherently "break up" due to neutron collisions.
• C: Weight is a practical concern but not the primary physics reason.
• D: State of matter (liquid/gas) is irrelevant; moderators like graphite (solid) are commonly used.