I think that would be just the excuse the Russians would need to use nukes on Ukraine.
If the Ukraine is going to lose, what does it matter? That's the problem with desperation. There's plenty of pithy quotes through the ages: "Rather die on my feet than live on my knees", "Make a desert and call it Peace", "Victory at all costs, victory in spite of all terror, victory however long and hard the road may be; for without victory, there is no survival."...take your pick.
My point is if it looks like the Russians are going to win, and at this point we all know Putin will only accept complete capitulation. He will have to punish a vassal state for this much trouble, The Ukrainians have nothing to lose by getting dirty. At that point the only difference is how much of Russia is radioactive when they are done, because there won't be a Ukraine, except as a nice fiction on treaties somewhere.
Hell, I'd say Ukraine has got to be taking stock of what fissile material they have, and if it can be enriched. It's not like a fission bomb is hard to make if you have the material.