M.A.D. not very likely

Mutually assured destruction has been an idea on the political map since the days of the cold war between Russia and the West.

If the nuclear arsenals on both sides of the fence were to be unloaded simultaneously on the opponent, nuclear winter would ensue and those that didn't get fried by nuclear radiation would basically die from starvation and hypothermia in the years following the nuclear exchange.

The war in Ukraine is now building in intensity and the Russians are finally beginning to target transportation infrastructure, like railways and airports, bridges included, that are used to move foreign military aid and politicians to Ukraine for grandstanding photo ops with Zelenski.

Meanwhile the military/industrial complexes of both sides are the ones gaining from this conflict, including the financiers that make it possible, who in turn control the politicians on both sides.

And those folks are not interested in winning or losing a war. They just want to keep it going for as long as possible, and get another one started when this one peters out: Vietnam, Afghanistan, Iraq, Libya, Syria, Yemen, and now Ukraine.

So, it is very unlikely that nuclear mass destruction will be unleashed unless one party is pushed into a corner to the point of acting in desperation.

And the powers that be will make sure that this does not happen because it does not serve their interests.