Only a mad man would use a nuke and Putin is clearly not a mad man.  That is why NATO/Ukraine can attack Russia with impunity.  If you guys want to survive, you should replace Putin with someone a lot more unhinged, a lot more likely to do something rash and vindictive.  Only then will NATO actually fear the nuclear threats and back off. 

 It's not so important who will carry out this explosion. Whether it will be done by Putin or by the Ukrainians claiming that Putin did it, the main thing is the event itself.