Now, that’s the before – what about during the fireworks themselves?
Whatever happens, do not be in the fireball blast radius, because anything in this region, both above and below ground, will be wiped out, like disinfectant killing off tabletop bacteria. A bunker will not save your skin, quite literally.
You need to be several kilometers away from the epicenter, at least. Avoid key landmarks, and if you're in a city that's a key target, we'd advise getting out as quickly as possible.
Now, assuming you’re far away enough, you’ll see a bright light when the nuke explodes. It seems obvious, but don’t look at the light – you’ll go blind, as it’ll be like staring into a man-made Sun that’s a lot closer than the real one. Remember, cool guys/girls, walk away from explosions, not stare at them.
If you’re near a window or in a tall building, get to the core of it and as far down into it as possible. You will have just a few seconds before the pressure wave hits, and ideally, you’ll be far enough out that the structure you’re in won’t be flattened. Don’t stand near any windows, as these will explode inwards and send shards of glass into your face at supersonic speeds.
Put your hands over your ears too. If the pressure wave is powerful enough, it’ll cause your eardrums to burst otherwise.
If the building remains upright, you need to stay in the center of it for several hours, perhaps even the entire day. That way, the ionizing radiation, and the subsequent cloud of radioactive fallout won’t be able to reach you through so many layers of concrete or brick.
If you’re downwind of the blast, you may be in trouble. Breathe in enough of those irradiated soot particles and you’ll get radiation sickness. The best thing you can do is find somewhere that’s poorly ventilated and put a cloth over your mouth and nose whenever you can. This is particularly tricky to avoid, though, so fingers crossed the wind blows another direction.
Mathematical models have suggested that if you’re far away enough and you’re in a flimsy house or building, it’s best to run to higher quality shelter – so long as you do so in no more than 30 minutes, or the radioactive fallout will get you.