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Why can CANDU reactors use natural uranium fuel?

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Enrichment of uranium — uranium processing How uranium is enriched to make a nuclear weapon.

CANDU (Canada Deuterium Uranium) reactors are able to utilize natural uranium as fuel because they are moderated and cooled by heavy water.1

In most nuclear power reactors, the concentration of the fissile isotope uranium-235 must be increased through enrichment to sustain a chain reaction, as natural uranium contains only a small percentage of this isotope. However, heavy water is an extremely efficient moderator that absorbs fewer neutrons than the light water used in other reactor designs. By minimizing neutron loss, the CANDU system allows a sustained fission reaction to occur using natural, unenriched uranium. This design choice allows countries to operate nuclear power plants while remaining independent of foreign enrichment services.21