In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose because

Options

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

Answer

B

Explanation

If nuclei are heavy, they will not carry enough momentum after to collision and hence neutrons will not get allowed effectively.