ANS:A - graphite

Graphite is a form of carbon that consists of layers of carbon atoms arranged in a hexagonal lattice structure. It is a non-metallic, crystalline material with unique properties that make it useful in various industrial and technological applications, including its role as a moderator in nuclear reactors. In the context of nuclear reactors, graphite serves as a moderator. A moderator is a material used to slow down fast neutrons produced by nuclear fission reactions to thermal energies, thereby increasing the probability of further fission events. Graphite is particularly effective as a moderator because of its low atomic weight and ability to efficiently slow down neutrons through elastic collisions with carbon atoms. Here's how graphite functions as a moderator in nuclear reactors:

1. Neutron Moderation: In a nuclear reactor core, fast neutrons released during fission reactions are slowed down (or moderated) as they collide with the carbon atoms in the graphite moderator. These collisions transfer kinetic energy from the fast neutrons to the carbon atoms, reducing their speed to thermal energies.
2. Enhanced Fission: Slowing down the neutrons increases the likelihood of them being captured by fissile nuclei (such as uranium-235 or plutonium-239), leading to additional fission reactions. This process helps sustain the nuclear chain reaction necessary for power generation in a controlled manner.

Graphite offers several advantages as a moderator material:

• High Moderating Efficiency: Graphite is highly efficient at slowing down neutrons due to its low atomic weight and the spacing of its carbon atoms.
• Chemical Stability: Graphite is chemically stable at high temperatures and in the presence of radiation, making it suitable for use in the harsh environment of a nuclear reactor core.
• Mechanical Strength: Graphite has good mechanical properties, including high strength and thermal conductivity, which contribute to its reliability and longevity in reactor applications.

Graphite-moderated reactors, such as the RBMK reactors used in some Soviet-designed nuclear power plants, utilize graphite as both a moderator and a structural material within the reactor core. However, the use of graphite as a moderator is less common in modern light water reactors (LWRs), where ordinary water serves as the primary moderator.