I love nuclear reactors and CANDU are quite cool. But I don't think that today we have any reasons to build CANDU reactors, except possibly that Canada can demonstrate they can build them for cheaper than others can build light water reactors. The ability to build is something that has little to do with the technical merits of a nuclear reactor design. But all things being equal, a PWR or a BWR should cost less per GW than a CANDU reactor and have other advantages too:

- main problem with CANDU: proliferation. India was able to build nuclear weapons after using a Canadian built heavy water reactor (basically a CANDU reactor) [1]. There is no guarantee that another country will not try something similar in the future, the design has no built in proliferation resistance. An operator can remove irradiated fuel at any time, and if the IAEA discovers they engage in plutonium manufacturing and they get on a black list, they can manufacture their own fuel quite easily, because CANDU uses non-enriched uranium. With light water reactors, you need enriched fuel, so if you are flagged as a proliferator no fuel manufacturer will be allowed to sell you fuel, and it's going to be much harder for you to manufacture your own fuel, since you can't enrich. If you can enrich uranium, you might as well try to build a uranium bomb (like Iran is trying to do). Also, with light water reactors, you refuel only at discrete times, generally about 18 months apart, so it is much more difficult to extract lightly irradiated fuel without being caught by the IAEA.

Now some less important problems:

- because CANDU uses non-enriched uranium, it produces much more nuclear waste per GWh compared to light water reactors. Nuclear waste is not the boogeyman nuclear anti-advocates make it to be, but still, if you generate 5-10 times more nuclear waste than the mainstream alternatives, it is less than ideal.

- there is one positive reactivity feedback loop in a CANDU design. Because of that CANDU designs are not licenseable in the US. The Canadian nuclear regulator is comfortable that the design is stable [2], but if you can choose between a design with one positive feedback loop and one without any positive feedback loop, why would you choose the first?

- heavy water is a worse moderator than light water (by a large factor). It [1] https://en.wikipedia.org/wiki/India_and_weapons_of_mass_dest...

[2] https://www.cnsc-ccsn.gc.ca/eng/resources/research/safety-an...