Nuclear Explosion Effects

They are talking about one gigantic nuclear explosion (81 Gt). Why couldn't multiple smaller explosions achieve the same outcome?

It depends: Real-world nukes, or Hollywood nukes.If real-world nukes, play with Nukemap to get an idea.https://nuclearsecrecy.com/nukemap/

Even nukes? Do H-bombs generate a lot of fallout?

I suspect any nuke smaller than the Tsar Bomb would leave top large pieces in the way.

It seems extremely exaggerated, such explosions require complicated bomb design, otherwise the first stages of the explosion throw the reacting matter away and it won't react completely.However, 5 megatons can destroy city completely, and if it explodes close to ground, it will create lots of contamination. Then it is up to the winds. Bad wind will make this contamination a major catastrophe thousands of kilometers away.

Nuclear bombs are strongly disinclined to "go boom" unlike say TNT. Turning the energy from splitting atoms into a large explosion, which is what you want from a weapon, will require precise timing. If you get it wrong either nothing happens, or you maybe create a small detonation, and cover a modestly sized area with dangerous debris from the failed attempt. Just throw a box of grenades into a waste water treatment plant or something instead for a fraction of the cost.

A nuclear explosion? No chance, it will "simply" leak, worst case it catches fire and goes Chernobyl but spreads its radiation over a more populated area, which is probably worse that a nuclear weapon going off.

Any scenario where anyone throws a nuke means the __world__ will be blown to pieces. That's a different scale of issue.

Nothing notably bad happens. Nuclear explosions are highly sensitive and engineered events. If you destroy the ignition mechanism, then it will become a dud.FWIW, there has been a number of nuclear explosion tests in our atmosphere: https://en.wikipedia.org/wiki/Tsar_Bomba

Nukes are not that powerful. See: https://what-if.xkcd.com/15/