question_answer 38)

  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 (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 temperature

Answer:

  (b)The moderators used have light nuclei (like proton). When protons undergo perfectly elastic collision with the neutrons 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. Choice (b) is correct.