Wouldn’t one or two of those former Soviet republics have the facilities and the scientists to whore one out to the terrorists? The number of bombs Ukraine had until 1996 was huge- about 5000 units or so.
And Kazakhstan is islamist...
My understanding (and I admit that my understanding may be wrong and/or incomplete) is that only Russia had all the facilities on its own soil to go from mining the ore through enrichment through bomb manufacture through delivery system manufacture. Ukraine had a stockpile of bombs, but no way to completely manufacture new ones from start to finish, and since they weren’t likely to get technical assistance from either Russia or the United States to fill in the gaps they decided NOT to spend the money necessary to become a Nuclear Weapons Weapons power, since it would be so expensive.
So the Ukrainians turned their nukes over to the Russians. And since it was in the Russians' and the Americans' interest to make sure that all the weapons were turned over, I expect this was verified by both nuclear powers together and separately to their own satisfaction.
Maybe the Ukrainians figured out a way to fool both, but I'm inclined to be a skeptic. If you are not so skeptical, well that's your right.