Heavy water is used in nuclear reactors as a

Heavy water (deuterium oxide, D2O) is primarily used in nuclear reactors as a moderator. Its primary function is to slow down fast-moving neutrons produced during fission to thermal speeds, which is necessary to sustain a chain reaction [t1][t3]. Heavy water is considered an excellent moderator because it has a high moderating ratio and a very low neutron absorption cross-section [t1][t8]. This low absorption allows reactors, such as the Pressurized Heavy Water Reactor (PHWR) and CANDU designs, to utilize natural uranium as fuel instead of enriched uranium [t2][t5]. While heavy water also serves as a coolant to transport heat away from the reactor core [t1][t5], its defining role in the classification of these reactors is its function as a moderator [t8]. It is not used as a fuel, catalyst, or solvent in the nuclear fission process.

Sources

1. [1] https://hwb.gov.in/frequently-asked-questions-faqs
2. [2] https://en.wikipedia.org/wiki/Heavy-water_reactor
3. [3] https://www.sciencedirect.com/topics/engineering/heavy-water-reactors