The function of heavy water in a nuclear reactor is to
Answer & explanation
Answer: (a) Slow down the speed of neutrons
Heavy water (D2O) is used as a moderator. Fission releases fast neutrons, and the moderator slows them down so they can cause further fission. Heavy water absorbs very few neutrons, so reactors that use it can run on natural uranium.
- ✓ (a) A moderator slows down the fast neutrons produced by fission. Heavy water does this while capturing very few neutrons, which keeps the chain reaction going.
- ✗ (b) A moderator never speeds neutrons up. Its job is to remove their kinetic energy through collisions.
- ✗ (c) Heavy water can also carry heat away as the coolant in pressurised heavy water reactors, but any fluid could do that. Its defining function, the reason heavy water specifically is used, is slowing neutrons while absorbing very few of them.
- ✗ (d) The chain reaction is regulated or stopped by control and shut-off rods, which absorb neutrons. Heavy water absorbs very few neutrons, so it sustains the reaction rather than stopping it.
Remember · Moderator = slows fast neutrons (heavy water, graphite, light water). Heavy water absorbs few neutrons, so PHWRs use natural uranium. Control rods absorb neutrons to regulate or stop fission.
Sources
- IAEA Technical Reports Series No. 407 – Heavy Water Reactors: Status and Projected Development ↗ “Additional heat is generated by moderation (slowing down) of the fast neutrons produced by fission in the fuel”
- NPCIL (Govt. of India enterprise) – Light Water Reactor note ↗ “Compared to heavy water, light water as moderator has a higher tendency to capture neutrons, so natural uranium, with its low fissile content (0.71% of U235), is insufficient”
Question: UPSC Civil Services (Preliminary) 2011, General Studies Paper I. Explanation: Minimalist IAS, checked 25 Oct 2026 (how we verify). ·
