Can't Uranium be used for Nukes? I have no idea about weapons, but I think Plutonium is sometimes found as a result of Uranium refining and fission, and that it is more powerful than uranium, but can't Uranium simply be used for nukes?
Uranium IS used in Atomic(fission) and even Hydrogen bombs (fusion) for that matter, the thing is, you need enriched Uranium as you might know each element has different isotopes, and in the case of uranium for nukes U-235 is needed for it fission properties (U-233 is even better in theory but it has reaction issues), the natural uranium is U-238 (with around 1% of U-235), for a nuke you'll need an enriched Uranium with 90%+ of U-235.
So no uranium can't be used for nukes like that hence the difficulty
And yes Plutonium can be found in nuclear reactor, as when some of the U-238 acquires a neutron during the fission process it's becomes U-239 which decays to Pu-239, and Plutonium is even better than Uranium in the aspect that the critical mass needed for an A bomb is way lower than the one needed with Uranium, but again the amount of Plutonium needed is in dozens of kgs, and you need over 94% of Pu-239 (which is the highly fissile isotopes of Pu)
As for China, they have no problem producing either, and they already have a good stock of Hydrogen bombs (Fission-fusion) and A-bombs (Fission)