Unlike with Turkey, Ukraine could have easily taken complete possession of the nukes existing on its territory and they had enough resources to also take complete control, by replacing the control parts of the nukes, perhaps in at most a few months of work.
The only way in which the Russians could have stopped this would have been to nuke Ukraine before they would have been able to modify the captured nukes, but it is unlikely that this would have been permitted by the other nuclear powers.
The only way in which the Russians could have stopped this would have been to nuke Ukraine before they would have been able to modify the captured nukes, but it is unlikely that this would have been permitted by the other nuclear powers.