Why is heavy water used in some nuclear reactors?

Heavy water, or deuterium oxide ($D_{2}O$), is used in nuclear reactors primarily as a moderator to facilitate the fission chain reaction.12

In a thermal reactor, neutrons produced by fission are released at high speeds and must be slowed down—or "thermalized"—to effectively induce further fission in other nuclei. Heavy water is highly effective for this purpose because it slows down these fast neutrons through elastic scattering while absorbing very few of them.324

By minimizing neutron capture, heavy water allows reactors to maintain a sustainable chain reaction even when using natural uranium as fuel, which does not require the expensive enrichment processes needed for reactors that use ordinary water as a moderator. Additionally, in designs like the CANDU reactor, heavy water is also utilized as a coolant to transfer the heat generated by fission to the steam generators.5