Operation of all fission power plants, outside of countries with nuclear weapons, is subject to very strict controls by International Atomic Energy Agency, which reports to United Nations Security Council. Also because each civilian nuclear power reactor breeds enough plutonium to produce multiple nuclear weapons each year, spent nuclear fuel is subject to the same very strict controls. (I know the the isotopic composition of this plutonium is miserable for weapon production, but nuclear weapons using reactor-grade plutonium have been build and tested).
https://en.wikipedia.org/wiki/Reactor-grade_plutonium
I don't know any thermonuclear weapon using highly enriched uranium. Highly enriched uranium is used only in few designs of nuclear weapons: nuclear weapons which you build when you don't have access to weapon grade plutonium, or nuclear weapons which have to survive strong external shocks - nuclear bunker busters. Plutonium Pu-239 based nuclear weapons are physically smaller and lighter, easier to place into missile nose cone.
