[user104658]

I mean ... if you don't die in the blasts your next choices are;

1) Radiation poisoning (relatively quick, not pleasant)
2) Cancer (medium, not pleasant)
3) Starvation because of nuclear winter (slow, not pleasant)

... ... ... best to go in the fireball really. You don't need a switch to die painlessly before the bombs hit either, you aint gonna feel it if you're in the blast radius of a nuke, you won't even know it hit.