Depends on bomb type, but Ukraines and russias region is gigantic compared to the region affected by bomb fallout.

If it wouldn’t be so bad for the wildlife, it would be a decent strategy for Ukraine to dirty bomb along the whole border to russia, so crossing it by land would mean cetain death within a week. It could even be on Ukraines side to demonstrate it’s a defensive measure. Dirty bombs are less difgicult to make and Ukraine has a lot of experience dealing with radioactive contamination, too.