(a) What is the function of the moderator in a nuclear reactor? (b) What substance acts as the moderator in a pressurized water generator? (c) What other substances are used as a moderator in nuclear reactor designs?

The primary function of a moderator in a nuclear reactor is to slow down the neutrons produced during the fission process. By slowing down the neutrons, the moderator increases the likelihood of further reactions and ensures a controlled and sustained chain reaction. Slow neutrons are more likely to cause fission events in the fuel, which in turn generates more neutrons, continuing the nuclear chain reaction.

In a pressurized water generator (Pressurized Water Reactor or PWR), the substance that acts as a moderator is light water (ordinary water, H2O). Moreover, in PWRs, light water also serves as a coolant and transfers the heat generated during the fission process to the steam generators, which is later used to produce electricity.

Apart from light water, there are several other substances that can be used as moderators in nuclear reactor designs. Some examples of these substances include: 1. Heavy water (D2O): Heavy water is a form of water containing a larger than normal amount of the hydrogen isotope deuterium and is used as a moderator in certain types of reactors, such as the CANDU reactor. 2. Graphite: Graphite is a commonly used moderator in gas-cooled reactors (such as the Advanced Gas-cooled Reactor, AGR) due to its excellent neutron moderation properties and high thermal conductivity. 3. Beryllium: Beryllium has some advantages as a moderator due to its low neutron absorption cross-section and high scattering cross-section. However, its high production cost and the issue of handling beryllium-associated health risks have limited its use in commercial reactors. These substances slow down neutrons by scattering collisions with their nuclei, thus increasing the probability of further fission events and maintaining a sustained chain reaction within the reactor.