Precisely.

Although airburst strikes where the nukes are detonated high in the sky to maximize the damage caused by the blast wave generate very little or no fallout. So technically, the Russians could use nukes to hit targets in Ukraine with virtually no risk of blowback occurring.

The bottom line is this: ANY use of nukes, tactical or strategic by the Russians, especially if any civilian population centres are involved, will result in NATO intervening immediately.

Putin does not want a war with NATO, because it's a war he cannot win.