Ukraine was never a nuclear power any real sense. The USSR's bombs were parked there, and Ukraine merely had physical (but not operational) custody over them after the USSR fell. Ukraine could have kept them to bootstrap a nuclear arms program, but they didn't, so they were never had a nuclear deterrent to give up in the first place.
Nice russian talking point. UA designed developed and maintained most top tier soviet nuclear weapons. The largest nuke plant in USSR was Yuzhmash in Dnepr and largest design bureau again in UA Dnepr KB Yuzhnoe. UA had to help maintain russian nukes after the collapse of USSR cause russia lacked tech. capability.