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 the reactor would be unbearably high
D. substances with heavy nuclei do not occur in liquid or gaseous state at room teperature

Answer

b

Explanation

According to the question the moderator used have light nuclei (like proton). When protons undergo perfectly elastic collision with the neutron emitted their velocities are exchanged i.e., neutrons come to rest and protons move with the velocity of neutrons.
Heavy nuclei will not serve the purpose because elastic collisions of neutrons with heavy nuclei will not slow them down.