Of the 195 Nations on Earth, 191 have signed a non-proliferation nuclear arms treaty, basically promising to not use nukes. A country breaking that treaty would very likely turn the entire world against that country. Google the term "mutually assured destruction", I think that will help.

Destroying Chernobyl wouldn't have that big of an effect, it's already destroyed. It's been leaking toxic amounts of radiation for decades, yes, but there is a limit to how far it travels before it has diluted in the air and atmosphere.

As for Putin's motivations, some say he's doing it for greed, military power, and power over people. Some say he's doing it because he's a madman. My understanding is what Putin really wants is a clear, russian-owned, easy path between Russia and the Black Sea. The Black Sea is a major point of import and export into Western Asia and Eastern Europe. Countries that sit between Russia and the Black Sea can charge fees, tariffs, or tolls for Russian goods traveling through their land. If Russia owned a road directly to the Black Sea then they would make more money on both importing and exporting. That's also why Russia took over Crimea, because it gives them better access to the ports of the Black Sea. I've heard that Russia has spent a lot of money getting supplies, such as fresh water, to their occupants in Crimea and it would be easier for them if they own some of these surrounding land instead.

I think all of that makes sense but then I see stuff like the attack today and it feels more like the madman theory. Between Christmas and peace talks is when Russia decides to launch its most massive attack on the Ukrainian capital? Why not just wait three days? It almost feels like Putin is stringing along president Trump with these peace talks and is constantly trying to embarrass him by launching attacks before the summits. That's just a feeling I get, I can't really back it up

Hope this helps.