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

Question

In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderators used have light nuclei. Heavy nuclei will not solve the purpose because (A) The net weight of the reactor would be unbearably high. (B) Heavy nuclei will break generating unwanted smaller nuclei. (C) Elastic collision of neutrons with heavy nuclei will not slow them down. (D) Substances with heavy nuclei do not occur in liquid or gaseous state at room temperature.

Tardigrade Admin · Accepted Answer

During elastic collision between two particles, the maximum kinetic energy is transferred from one particle to the other when they have the same mass. Consequently, a neutron loses all its kinetic energy when it collides head on with a proton. For this reason materials which are abundant in hydrogen such as paraffin and water, are good moderates for neutrons.

Q. In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderators used have light nuclei. Heavy nuclei will not solve the purpose because

The net weight of the reactor would be unbearably high.

Heavy nuclei will break generating unwanted smaller nuclei.

Elastic collision of neutrons with heavy nuclei will not slow them down.

Substances with heavy nuclei do not occur in liquid or gaseous state at room temperature.