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Moderators play a crucial role in nuclear reactors. Their primary function is to slow down the fast-moving neutrons that are generated in the fission process. By slowing down these neutrons to thermal or near-thermal energies, the probability of them causing further fission events within the reactor's fuel increases. This enables a self-sustained chain reaction to take place, which is necessary for producing continuous energy output in the reactor.\newline

A pressurized water reactor (PWR) is a specific type of nuclear reactor in which water acts as both the coolant and the moderator. The water, present in a pressurized form, slows down the neutrons, allowing the nuclear fission chain reaction to be sustained. It also absorbs some of the heat generated by the chain reaction, which is then used to produce steam in a secondary loop to spin turbines and generate electricity.\newline

Apart from water, there are several other substances that have been used as moderators in different types of nuclear reactors. Some commonly used moderators are: 1. Heavy water (Dâ‚‚O): Heavy water is water composed of deuterium, which has an extra neutron compared to regular hydrogen. Due to its high neutron-capture cross-section, heavy water is an excellent moderator and is used in heavy water reactors like CANDU reactors. 2. Graphite: Composed of carbon, graphite has low neutron absorption and high thermal conductivity, making it an effective moderator in certain designs, such as the RBMK (high-power channel-type) reactors and gas-cooled reactors. 3. Beryllium: Beryllium has a high scattering cross-section for thermal neutrons, making it an effective moderator. However, its use as a moderator is less common due to its high cost and limited availability. Beryllium has been used in some experimental reactors and compact designs like small modular reactors (SMRs). In conclusion, moderators play an essential role in sustaining the chain reaction in nuclear reactors, and different substances are used as moderators depending on the specific reactor design. Water, heavy water, graphite, and beryllium are some of the known moderator substances.