The Function of heavy water in a nuclear reactor is to

In a nuclear reactor, heavy water (deuterium oxide or D2O) primarily functions as a neutron moderator. Its critical role is to slow down the high-speed (fast) neutrons produced during the fission reaction [1]. Slowing these neutrons is essential because thermal (slower) neutrons are significantly more effective at inducing further fission in uranium-235 nuclei, thereby sustaining a controlled chain reaction [2]. Heavy water is an exceptionally efficient moderator due to its high moderating ratio and low neutron absorption cross-section, which allows the reactor to maintain a high neutron economy and even utilize natural uranium as fuel [1]. While heavy water also serves as a primary coolant to transport heat away from the reactor core in certain designs like the CANDU or PHWR, its defining nuclear function remains the moderation of neutron velocity [1].

Sources

1. [1] https://hwb.gov.in/frequently-asked-questions-faqs
2. [2] https://en.wikipedia.org/wiki/Heavy-water_reactor