If it is nuclear war, some will survive. Even a super volcano, as you wrote, would probably leave some alive. If an event that bad happens though, there's no way to plan for it. You might as well not think about such things therefore.

However, I've studied nuclear war quite a bit as a residential security expert, and it is definitely survivable. There are certain things you need to do in advance of one happening though to greatly improve your chances.

Don't live in a large urban area, and do not live immediately down wind from one. The same applies to a nuclear power plant. Do not confuse the two either. Nuclear fallout decays quickly. Nuclear power plant radiation does not.

In a nuclear war though, the power plants will fail.

You do not want to live near any obvious nuclear target either. If your city does not get struck at first, you should still move away from it. Follow up strikes can take weeks to occur as submarines are moved into position.

Shelter in place from fallout, and the longer you can stay inside the better. Two weeks after the last blast occurs is a minimum. A month is safer. Once you do leave your shelter, a basement works in a pinch, make sure children are carried and kept off the ground.

Their growing bodies are more susceptible to lingering low levels of radiation than an adult's.