Question

What is the role of a moderator in a nuclear reactor?

Answer 1

A moderator may be a material utilized in a reactor to hamper the neutrons produced from fission. By slowing the neutrons down the probability of a neutron interacting with
235-U nuclei is greatly increased thereby maintaining the chain reaction

Complete step by step solution:
In engineering, a neutron moderator may be a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely more susceptible than fast neutrons to propagate a nuclear chain reaction of uranium-235 or another fissile isotope by colliding with their atomic nucleus.

Materials used for moderation got to a really specific set of properties. First, a moderator cannot absorb neutrons itself. This suggests that the moderator should have a coffee neutron absorption cross-section. However, the moderator should be ready to hamper neutrons to a suitable speed. Thus, in a perfect moderator the neutron scattering cross-section is high. The neutron scattering may be a measure of how likely a neutron will interact with an atom of the moderator. If the collisions between neutrons and nuclei are elastic collisions, it implies that the closer in size the nucleus of an atom is to a neutron, the more the neutrons are going to be slowed. For this reason, lighter elements tend to be more efficient moderators.

"Light water" is the most ordinarily used moderator (roughly 75% of the world's reactors) although the term is slightly ambiguous, usually meaning natural water, but could also ask for actual light water. Solid graphite (20% of reactors) and deuterium oxide (5% of reactors) are the most alternatives. Beryllium has also been utilized in some experimental types, and hydrocarbons are suggested as another possibility

Note:
In many cases, an equivalent substance functions as both coolant and a moderator. The moderator slows the fast (high-energy) neutrons emitted during fission to energies at which they're more likely to induce fission. In doing so, the moderator helps initiate and sustain a fission chain reaction.