The Candu reactor does use U235, it just doesn't require as much. Candu reactors can utilize unenriched Uranium (0.711% U235) or slightly-enriched Uranium. The heavy water moderator doesn't absorb nearly as many neutrons meaning it can utilize fuel with a lower percentage of fissile material.
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we have a lot of uranium 235... we don't really need to enrich it to 238