*Without using their nukes
FTFY
Or just used the threat of doing so to take Ukraine piece by piece.
If Russia were to use nuclear weapons in Ukraine, it would be a pariah for centuries, assuming it didn't lead to global thermonuclear war.
This seems to be the case, but no one knows for sure. It's likely that there are people in power who think they can gain an advantage using tactical nukes. Some weapons are specifically designed for this purpose, such as the American ER weapons (PDF) https://www.llnl.gov/sites/www/files/1975.pdf
Chernobyl released ~400 times as much radioactive material into the environment as the Hiroshima bomb. The nuke isotopes are also much shorter lived than what Chernobyl released.