the bombs dropped on Hiroshima and Nagasaki where about 1 megaton
Nope, not even close. Fat Man (Nagasaki's bomb) was about 21 kilotons. Little Boy (Hiroshima's bomb) was about 18 kilotons. One megaton wouldn't have even left the city standing, hell, they wouldn't have even known for a couple days.
The effects of a massive nuclear exchange would be devastating. Humanity, be it for better or worse, would survive however. No amount of death short of one bomb per every household on earth could wipe us out.
Let's start with radiation. This is a side effect of the bombs, generated by the uranium or hydrogen core being turned into energy. Radiation has a nasty way of getting into people, causing irreversible genetic damage, and making you a threat to everyone you encounter. It is also a terrible way to go out, but as it stands, the least lethal part of a nuke.
The next part, is the flash and flame (lulz, that sounds like a death metal song), which is the second deadliest part of a bomb. The flash, which would go on for several seconds, is powerful enough to burn through clothing, and ultimately skin. The area directly beneath ground zero would be vaporized instantly, from the 100 million degree heat. Instant death, so just hope that that kills you.
Finally, the deadliest aspect. The pressure wave. This wave, traveling at 600mph, is capable of shredding even the toughest buildings. These buildings, where you would be at, would land right on your head, killing you from the compression.
Or course, I'm not mentioning the long term effects. Famine, disease, radioactive water supplies, not a world anyone short of a psychopath would want to live in.
However, if some terrorist ever gets his or her hands on a nuclear device, and you know it is coming, get your a** into the sewer. Immediately. It's a makeshift nuclear shelter, impervious to fallout or blast wave. Unless it lands right on your head, that will be your home for the next very long time.
I've already got myself a pipeline out in from of my house I can fit in, in such an event.