Some good comments above.
But a couple of misconceptions need to be cleared up.

The explosion of a nuclear bomb happens SO FAST - there is no reaction time. You cannot duck. You cannot say "What's that?". It is game over. If you are unlucky enough to be near the site of the blast, you will get the effects.

The principal effect is HEAT - terrible heat. If you look at the accounts of the scientists who witnessed the Trinity Test - first A-bomb test. They were about 10-miles from the test, and wearing protective glasses. They remarked that the heat, from that distance, was so intense that it felt like someone had opened the door to a very hot furnace. It was so strong, you almost couldn't handle it. That was from a distance of 10-miles.

There is an ONLINE site somewhere that shows eyewitness accounts from Hiroshima and Nagasaki, immediately after those weapons were dropped. You see images (sketches) with many people with terrible burns. Whatever part of your body is exposed, severe burns. The rivers were full of bodies from people who ran to the water because "their bodies were on fire" ... and they died floating in the water.

I wish I could find the ONLINE Web site. Maybe somewhere here can post it.

You can't "shelter" from the nuke. You just pray that you are lucky enough to be far away from it.

Pete