In nuclear reactor, heavy water is used as A. Coolant B. Fuel C. Moderator D. Atomic smasher

The correct answer is: C. Moderator

Heavy water is used as a moderator in nuclear reactors to slow down the neutrons produced by the fission of uranium-235. This is necessary because the neutrons produced by fission are too fast to cause further fission. By slowing them down, the moderator increases the probability that they will interact with a uranium-235 nucleus and cause another fission event. This is called a chain reaction, and it is the basis of nuclear power generation.

The other options are incorrect:

• A. Coolant: The coolant in a nuclear reactor is used to remove heat from the reactor core. This heat is then used to generate steam, which drives a turbine to generate electricity.
• B. Fuel: The fuel in a nuclear reactor is the material that is fissioned to produce the neutrons that sustain the chain reaction. In most nuclear reactors, the fuel is uranium-235.
• D. Atomic smasher: This is not a term that is used in nuclear physics.