Ncert Exercises & Solutions

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

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

Hint: Use the principle of conservation of momentum.

Explanation: In a nuclear reactor, the primary role of a moderator is to slow down fast neutrons produced during the fission process, making them more likely to induce further fission reactions in the fuel (usually uranium-\(235\) or plutonium-\(239\)).
Neutrons lose energy effectively through elastic collisions with nuclei of the moderator. According to the principles of collision mechanics, a neutron transfers more energy to a nucleus if the nucleus has a mass similar to the neutron’s mass. Light nuclei (such as those in hydrogen, deuterium, or carbon) are ideal for this purpose because the mass ratio is close to \(1,\) resulting in significant energy loss per collision. In contrast:

Heavy nuclei (such as lead or uranium) are much more massive than neutrons, so the energy loss per collision is minimal. As a result, neutrons retain most of their energy and are not slowed down effectively, making them less likely to cause further fission.
According to the question, the moderator 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.
Hence, option (2) is the correct answer.

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