They never owned any. No Soviet nuke was under the control of Kiev. And thank god. Otherwise the world would have nazi bombs out there as a menace.
Ukraine was a successor state to the Soviet Union. They retained control of those nukes on their territory just as Russia did for those on their territory (Russia is not the old Soviet Union, as so many here like to remind us). They gave them up in return for guarantees that Russia would respect its borders. We can see how well that work out.