How does heavy water help CANDU reactors use natural uranium?

CANDU (Canada Deuterium Uranium) reactors are specifically designed to utilize natural uranium as fuel, eliminating the need for the expensive and complex enrichment processes required by other reactor types.1

The key to this capability is the use of heavy water as a neutron moderator. In a nuclear reactor, a moderator is essential to slow down high-energy neutrons produced by fission, increasing the probability that they will strike other fissile atoms (such as uranium-235) to sustain a chain reaction.123

Ordinary water is an effective moderator, but it also absorbs a significant number of neutrons. Because natural uranium contains only about 0.7 percent of the fissile isotope uranium-235, the additional neutron absorption by ordinary water would prevent a chain reaction from being sustained. Heavy water, composed of deuterium and oxygen, is an extremely efficient moderator that absorbs very few neutrons. This high efficiency allows the reactor to maintain a self-sustaining chain reaction using the low concentration of fissile material found in natural uranium.321