question_answer

Give reasons for (a) Lighter elements are better moderators  for a nuclear reactor than heavier elements. (b) Cadmium rods are provided in a reactor. (c) Very high temperatures as those obtained in the interior of the sun are required for fusion reaction to take place.

Answer:

                (a) The nucleus of a lighter element contains a relatively larger number of protons. When fast neutrons are passed through such an element, they make elastic collisions with its protons, which have smaller velocities. After few interactions, the final velocities of the neutrons become equal to the low velocities of protons and hence they get slowed down. (b) For a controlled chain reaction, the average number of available neutrons should never exceed one per fission. Any excess neutrons over this critical unit should be absorbed. This is what the control rods do. They are made of cadmium because cadmium has a high cross-section for neutron absorption. (c) To overcome coulomb repulsion, the fusing nuclei are given enough thermal energy by raising their temperature to\[\text{1}{{0}^{\text{6}}}-\text{ 1}{{0}^{\text{7}}}\text{K}\]. Such a high temperature is available in the interior of the sun.