anything is possible in war. if Ukraine was on the brink of losing everything and they had nuclear weapons, would they not use them against Russia? that's the whole point. if your state is about to cease to exist, you are desperate. for now, the war has not reached this point. if Ukraine where to try to use nuclear material now they would lose allies. but you don't know what the war will look like in 2 weeks. so I don't think it's fantasy.