Which one of the following pairs of materials is used as moderators in nuclear reactors ?

Explanation:

Moderators in Nuclear Reactors:

• In a nuclear reactor, a moderator is a material used to slow down the speed of neutrons produced during nuclear fission. Slowing down neutrons increases the probability of successful fission reactions, particularly in reactors using uranium-235 or plutonium-239 as fuel. Moderators are critical for maintaining a sustained nuclear chain reaction.
• During nuclear fission, fast neutrons are released. These neutrons must be slowed down to "thermal" speeds (low kinetic energy) to effectively collide with fissile nuclei like uranium-235. Moderators achieve this by allowing neutrons to lose energy through elastic collisions with the nuclei of the moderator material, which have comparable mass to the neutron.

Properties of an Ideal Moderator:

• Should have low neutron absorption to avoid capturing neutrons and disrupting the chain reaction.
• Should be effective in slowing down neutrons (high scattering cross-section).
• Should be chemically stable and able to withstand high temperatures and radiation exposure.

Common Moderator Materials:

• Heavy Water (D₂O): Contains deuterium, which has a lower neutron absorption cross-section compared to hydrogen and is highly effective in slowing down neutrons.
• Beryllium: A lightweight metal with excellent neutron-scattering properties and low neutron absorption.
• Graphite: A form of carbon that is commonly used as a moderator in reactors such as the RBMK and Magnox types.

Beryllium and heavy water

• Beryllium: Beryllium is an excellent moderator due to its low neutron absorption cross-section and high neutron-scattering capability. Its lightweight nuclei are comparable in mass to neutrons, enabling effective energy transfer during collisions. Beryllium is commonly used in specialized reactors where high performance is required.
• Heavy Water: Heavy water (D₂O) is another widely used moderator, especially in reactors like the CANDU (Canada Deuterium Uranium) reactors. The presence of deuterium instead of regular hydrogen reduces neutron absorption significantly while maintaining high scattering properties. Heavy water is particularly advantageous in reactors that use natural uranium as fuel.