Q. The function of heavy water in a nuclear reactor is to (UPSC Prelims 2011)
Answer: Slow down the speed of neutrons
Notes: The correct answer is [A] Slow down the speed of neutrons.In a nuclear reactor, heavy water (D2O) acts as a moderator. When a Uranium-235 atom splits (fission), it releases high-energy "fast" neutrons. However, these fast-moving neutrons are actually less likely to be captured by another Uranium nucleus to keep the chain reaction going—they often just fly right past.

• Slowing Down: For a sustained chain reaction, these neutrons need to be slowed down to become "thermal neutrons" (slower neutrons that are in thermal equilibrium with their surroundings).
• Elastic Collision: When a fast neutron hits the deuterium atoms in heavy water, it transfers some of its kinetic energy to the deuterium, much like a billiard ball hitting another. This slows the neutron down to a speed where it can effectively trigger the next fission event.