Question

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

Answer 1

The correct option is C elastic collision of neutrons with heavy nuclei will not slow them down.

Definition:

Moderator: A moderator is a material used in a nuclear reactor to slow down the neutrons produced from fission.

During a fission reaction, the nucleus of a heavy fuel such as Uranium absorbs thermo neutrons and become unstable thereby splitting into fission products. Now for a moderator, we only use light nuclei because according to conservation of momentum and energy, the fast particle hitting the moderator acts as elastic collision with the particles of moderator thus transferring kinetic energy from the fast neutron to the moderator thereby helping in splitting of atom. In case of a heavy nuclei it will not split any atom but will absorb all the kinetic energy of the fast neutron thereby rendering no fission reaction. Therefore, for a continuous fission chain reaction, we use a light nuclei moderator in the nuclear reactor.

Final answer: (b)