Question

the function of heavy water in a nuclear reactor is to

Answer 1

Heavy water also known as neutron moderator is used in nuclear reactors to slow down the excited neutrons that is produced during a nuclear fission reaction.
By slowing down the neutrons, it promotes safe and slower nuclear fission.

Answer 2

Answer:

Heavy water (D2O) - NRC Water containing significantly more than the natural proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.