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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 unbearbly high
D. substances with heavy nuclei do not occure in liquid or gaseous state at room temperature

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Correct Answer - B
The moderators used have light nuclei (like proton). When protons undergo perfectely elestic collision with the neutrons emitted, their velocities are exchanged, i.e., neutrons come to rest and protons move with the velocity of neutrons. Heavy nuclie will not serve the purpose because elestic collision of neutrons with heavy nuclie will not slow them down. Choice (b) is correct.

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