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.

Answer 2

Step 1: Identify the Concept

Moderators in nuclear reactors slow down neutrons. The efficiency of a moderator depends on the mass of its nuclei relative to the neutron's mass. The closer the masses, the greater the energy transfer during collisions.

Step 2: State the Issue with Heavy Nuclei

Elastic collisions between neutrons and heavy nuclei result in minimal energy transfer to the heavy nuclei. The neutrons essentially bounce off without slowing down significantly.

Step 3: Deduce the Answer

Since heavy nuclei do not effectively slow down neutrons in elastic collisions, they are not suitable as moderators.

Final Answer: Elastic collision of neutrons with heavy nuclei will not slow them down.