What is the use of heavy water in nuclear reactors?

Heavy water plays an important role in the functioning of
nuclear reactors that use uranium which has not undergone enrichment to increase the
percentage of uranium-235 as the fuel.

Uranium consists of
many isotopes, the largest percentage being U-238 followed by U-235. Only atoms of U-235
can undergo fission. The fission of U-235 is initially started by bombarding it with
neutrons. When atoms of U-235 breakup it generates more neutrons which lead to the
fission of more atoms of U-235.

The role of heavy water
here is to reduce the speed of neutrons that are produced. This is important as it
increases the probability that the neutrons will react with U-235 and lead to more
fission reactions rather than get absorbed by U-238 which would not cause any reaction.
As heavy water has the isotope deuterium instead of normal hydrogen it also does not
absorb neutrons like normal water does.

Heavy water due to
these properties is essential to ensure that a chain reaction is started when normal
uranium is used as fuel. The alternative to using heavy water would be to enrich the
uranium and increase the percentage of U-235 which is an expensive
process.