The most commonly used moderator in nuclear power plants is

ANS:A - graphite

Graphite is a form of carbon consisting of layers of carbon atoms arranged in a hexagonal lattice structure. Each carbon atom in graphite is covalently bonded to three other carbon atoms, forming strong, stable bonds within the layers. However, the layers themselves are held together by relatively weak van der Waals forces, allowing them to slide past each other easily. This unique structure imparts several notable properties to graphite:

1. Lubricating properties: The weak interlayer forces in graphite enable its layers to slide over each other with minimal resistance. This property makes graphite an excellent lubricant, commonly used in applications such as automotive engines, machinery, and industrial equipment.
2. High thermal conductivity: Graphite exhibits high thermal conductivity along its layers due to the delocalization of electrons within the carbon-carbon bonds. This property allows graphite to efficiently conduct heat, making it suitable for use in thermal management applications such as heat sinks, crucibles, and thermal insulation.
3. Electrical conductivity: While graphite is not a metal, it possesses significant electrical conductivity along its layers due to the presence of delocalized π-electrons. This property makes graphite useful in electrical applications such as electrodes, electrical contacts, and as a conductor in batteries and fuel cells.
4. Chemical inertness: Graphite is chemically inert under normal conditions, resisting reaction with most acids, alkalis, and organic solvents. This chemical stability makes graphite suitable for use in corrosive environments, such as chemical processing equipment, as well as in high-temperature applications.
5. High temperature stability: Graphite exhibits excellent stability at high temperatures, maintaining its structural integrity and properties even at temperatures exceeding 3000°C (5400°F). This high-temperature stability makes graphite valuable in applications such as refractory materials, crucibles, and heating elements.
6. Moderating properties in nuclear reactors: In nuclear reactors, graphite is used as a neutron moderator to slow down fast neutrons produced during the fission process, thereby facilitating sustained nuclear chain reactions. Graphite's ability to effectively moderate neutrons arises from its low atomic number and the presence of carbon-12, which has a high scattering cross-section for neutrons.