Putin and his pals keep noisily threatening about using them. It seems he sees their value mostly in the terror he can stir up in his threats.
Not cornering a snake is always a concern, but once Putin is trapped in a losing circumstance, the nukes don't fix it, but make it harder and more costly to escape.
Pundits and 'experts' talk more and more about it becoming a stalemate, and it's a pretty good possibility that stalemate will seriously settle in along a contact line not tremendously different from what it was at the start of the invasion. In that circumstance Putin could find an exit opportunity by declaring the operation successful in the liberation of the pro-Russian separatist areas. That sort of result might be just a few months away.
All that depends on 1) Ukraine being able to fight back to near the original contact line and holding it (they seem capable of doing that if supplied with war materials from the West) and 2) Ukraine accepting the loss of the separatist areas in the east and northern shore of the Sea of Azov.
But the Russian war atrocities to date present a huge problem, it's going to be hard for Ukraine to end the war with a cease fire and treaty with the nation whose 'special operation' victimized it in an encyclopedic collection of war crimes.
Lighting off nukes in Ukraine (or elsewhere) makes those atrocities all the worse. International punishment would be extremely severe, likely making the Treaty of Versailles look like a parking ticket, and leaving Russia downsized physically and economically in the same way as Austro Hungary was after WWI.