It's not that easy. There are multiple problems with that approach:
1- Acquiring plutonium: You need a high flux of neutrons to convert uranium to plutonium, something which cannot be done without a working nuclear reactor. Mathematically speaking, for producing 1 kilogram of Pu-239, you need an extremely powerful beam of neutrons in the order of 10^(27) particles per second. Most countries that use plutonium for nuclear weapons have breeding reactors which have their own technical difficulties and proliferation restrictions. Most breeding reactors use liquid sodium as the coolant, which like all alkali metals, it is extremely reactive and can explode if it comes into contact with water / high humidity. I suppose it's more expensive than heavy water too, but I'm not sure.
2- Plutonium reprocessing: Plutonium is highly toxic and lethal and you will need advanced reprocessing facilities. You need to keep these facilities secret obviously because you cannot come up with a good justification if you're a member of the NPT. Most people forget that the very reason that Iran's case was referred to the UN Security Council by the IAEA was because Iran admitted to experimenting with PUREX in research scale, in the scale of milligrams if I'm not mistaken.
3- You will still need uranium: You will still need to mine/import uranium. And uranium mines shouldn't be that hard to find, particularly if you have already revealed them to the IAEA. I suppose if they know where your uranium mines are, learning where the mined uranium goes shouldn't be an impossible task to do.
4- Justification: If you're a member of the NPT, or the world does not want you to have nuclear weapons, it's harder to come up with civilian applications of plutonium reprocessing than uranium enrichment. You will probably get your facilities bombed to the stone age before you can achieve what you want with plutonium reprocessing.
5- It must be covert: Obviously, you cannot use plutonium produced by a nuclear reactor under the supervision of the IAEA. So, you'd better keep your nuclear reactor secret or pull out of the NPT. Even if IR-40 was operational today, without a strong political willingness and a capable military deterence, it couldn't have been used for anything but civilian applications supervised and monitored by the IAEA.