UPSC Prelims 2011 question on Heavy Water Moderator

Analysis

Heavy water acts as a moderator in nuclear reactors. Its main function is to reduce the speed of fast neutrons and convert them into slow or thermal neutrons.

1. Slow down the speed of neutrons — Correct

Neutrons released during nuclear fission are very fast.

Heavy water slows them down without absorbing too many neutrons.

Slow neutrons are more effective in causing further fission of Uranium-235, helping sustain a controlled chain reaction.

2. Increase the speed of neutrons — Incorrect

Fast neutrons are less likely to be captured by U-235 nuclei.

Increasing neutron speed would reduce the efficiency of the fission chain reaction.

3. Cool down the reactor — Incorrect

Heavy water can also act as a coolant in some reactors, but its specific role as a moderator is to slow down neutrons.

Do not confuse moderator with coolant.

4. Stop the nuclear reaction — Incorrect

Stopping the nuclear reaction is the function of control rods.

Control rods are usually made of boron or cadmium, which absorb neutrons and reduce or stop the chain reaction.