Ignorant question - how bad are nuclear weapons?
Do they wipe out cities, countries? What happens in the event of Russia launching one? Will there be time to react?
The launch of nuclear weapons from land/water is a serious event and pretty easily noticed, as it generates massive noise, vibration and heat.
Most rockets of that size can be easily destroyed/re-coded/intercepted until they reach a target as long as they're identified rather early. We must also take into consideration that Russia's technology is quite behind and, most likely, most of their nuclear weapons can be easily hacked/redirected/destroyed in flight. (I'm talking out of my ass here. My father, who used to be in the military, tells me there was some law that all nuclear weapons must be in constant communication with the ground, must have fail safe mechanisms and a way to be disabled remotely, but don't know if that's true or not these days).
If they're destroyed in flight, there will be some radiation, but it will generally disperse upwards because of the massive heat. If one goes on the ground, damage needs to be evaluated based on several factors, how large was the missile, on what type of soil it hits, does it hit in a city with a lot underground infrastructure, wind/climate, etc. Usually a nuclear explosion fecks everything in an area, from infrastructure, water sources, vegetation, animals, humans and for quite a while I would say.
In no fecking way one should ever be used again on this planet, no matter the reasons.
There quite a few other better/smarter weapons than nuclear when fighting a war, especially since once you've let a nuke go, the other big players will know quite surely from where you've launched it.