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How does heavy water help CANDU reactors use natural uranium fuel?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

In CANDU (Canada Deuterium Uranium) reactors, heavy water serves as a moderator that allows the system to utilize natural uranium as fuel.1

Unlike other reactor types that may require enriched fuel, the CANDU design uses heavy water because it is highly effective at slowing down neutrons to the speeds necessary to sustain a fission chain reaction while absorbing very few of them. Because heavy water captures neutrons only slightly, it preserves the neutron economy of the reactor, making it possible to maintain a chain reaction using the lower concentration of fissile material found in natural uranium. This design choice allows countries to operate nuclear power plants without relying on foreign uranium enrichment services.231