Oooh I've been inspired by this talk of nukes being plot devices.
What about an enchanted sword in a game or a lightsaber or something that spells instant death if it makes contact?
Like... a soul sucking sword or something.
That would be intense! I just don't know whether my players would like it or hate it. Probably hate it... but then again most of them want to play a new character now that they know the system better.
What better way to kill off a character than be killed by a Jedi?
If you're looking for something which kills instantly and absolutely, it already exists. It's called mordite.
Also, something which people keep seeming to miss or misunderstand, is that when the device detonates, the 'explosion' is actually the reaction of the surrounding environment to the sudden release of massive amounts of energy. All this discussion of overpressure, pressure waves, etc is essentially how the energy released via fission or fusion ends up dissipating, via raditation (EM/thermal and particle) kinetic, etc.
Now the RAW might allow for a character with enough Fate points to be at Ground Zero when a nuclear device detonates and have them survive... In reality, no, that wouldn't happen. As Jim Butcher has commented on more than once, while the works are fiction, things like magic still follows basic laws of physics. Magical fire is still fire, it's hot, it burns , it spreads, etc.
Also, to parapharse a Q&A I read years ago in Dragon magazine, back when TSR still offered the Marvel Superheroes RPG...
Q: "But what if Aunt May stabbed Galactus with a kitchen knife and spent 100 Karma points to achieve a Kill Result, Aunt May would've killed Galactus."
A: "Aunt May would never be in a position to have 100 Karma points to spend stabbing Galactus..."
Basically, things don't survive being 'hit with nuclear weapons' which when you get down to it, are devices that breakdown and/or reassemble the basic building blocks of the universe. Instead, things survive by avoiding being hit/directly hit but them.
-Cheers