The function of heavy water in a nuclear reactor is to

The correct answer is A. Heavy water (D2O) is used as a moderator. Fission releases fast neutrons, around 2 MeV, but the U-235 nucleus is most likely to fission with slow, thermal neutrons. Collisions with deuterium nuclei, which have nearly the same mass as neutrons, remove energy from the neutrons efficiently. Because deuterium absorbs very few neutrons, heavy water lets natural uranium sustain the chain reaction. This is why India's PHWRs, which run on natural uranium, use heavy water.

• B is wrong. Moderators reduce neutron speed. Nothing in the reactor is meant to speed neutrons up.
• C is wrong. Stopping the chain reaction is the job of control rods, made of boron or cadmium, which absorb neutrons. Heavy water does not do this.
• D is wrong because A is correct.

Core concept: moderation and neutron economy in thermal reactors. For Mains GS-III, this links to nuclear energy, the three-stage thorium programme, and why India chose PHWRs for its fuel cycle.

Trap: students mix up moderator and control rod. Heavy water slows neutrons, while control rods absorb them to shut the reaction down.