Ukraine might have some possible retaliatory options in that case though. Far from ideal, but they could for example load a big ship full of explosives and blow up much of St Petersburg.

Of course, other options such as biological weapons have been explored in the past. Ukraine wouldn't necessarily have to invest all that much to prepare retaliatory operations capable of killing millions of Russians in the case of a nuclear attack.

The only problem with such less orthodox means is that they're almost necessarily covert, and therefore can provide limited deterrence. "We have ways to impose immense costs if necessary" just doesn't sound that scary when the means are a secret.