Ukraine keeping the nukes was never going to happen. The US, EU, and Russia were all in agreement on that. Ukraine was in shambles at the time, and no one wanted the risk of nukes getting transferred or sold outside of the existing nuclear club.

Ukraine had physical possession of the nukes, but their ability to actually use them was highly suspect. They might have been able to circumvent the security measures given enough time, but if anything such an attempt would have sparked an international "peacekeeping operation" to make sure the nukes didn't fall into the wrong hands.

https://cisac.fsi.stanford.edu/news/budapest-memorandum-myth...