(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 moderator is an important component in a nuclear reactor. Its main function is to slow down the neutrons produced during the fission process. Fast neutrons, generated when the nucleus of a fuel atom undergoes fission, have too much energy for them to effectively sustain the chain reaction. Slowing down these neutrons increases the probability of them causing further fission in neighboring fuel nuclei, thus maintaining a controlled chain reaction.

In a pressurized water reactor (PWR), the most common substance used as a moderator is ordinary water, also known as light water or H₂O. The water in the reactor core not only acts as a coolant, removing heat generated during the fission process but also serves as a moderator, slowing down fast neutrons to sustain the chain reaction.

Apart from ordinary water, there are other substances that are used as moderators in various nuclear reactor designs. Some of these substances include: 1. Heavy water (D₂O): Deuterium oxide, or heavy water, is used as a moderator in heavy water reactors such as the CANDU (Canadian Deuterium Uranium) reactor. It is more effective at slowing down neutrons than ordinary water without absorbing many of them, thus allowing the use of natural uranium as fuel. 2. Graphite: Graphite, a crystalline form of carbon, is used as a moderator in certain types of reactors, such as the RBMK (Reaktor Bolshoy Moshchnosti Kanalnyy) design, originally developed in the Soviet Union. Graphite is an excellent moderator because it can slow down neutrons effectively without capturing many of them. 3. Beryllium: Beryllium is another substance used as a moderator in some reactor designs due to its excellent neutron reflection and low neutron absorption properties. Beryllium is used primarily in research reactors and some advanced reactor designs.