The Radioactive Fallout Problem
In a matter of (say) 10 to 15 minutes, small metallic or solid spheres made radioactive by neutron irradiation will fall from the mushroom cloud much like hail. The more intense radioactive fission product fallout will come down somewhat later as dust or powdery flakes. Downwind, the fallout may take several hours before it arrives on the ground.
The first thing to do is to look at the anvil top of the mushroom cloud. If it is pointing at you, you need to act fast, and get to an emergency shelter, or get out of the way of the oncoming fallout, if possible.
If the anvil top is not pointing at you, then you have more time to assess the threat of fallout to you, and/or find a suitable fallout shelter.
Remember 3 feet of dirt, 1 foot of concrete, will shield the external radioactive flux by a factor of 10. The more shielding, the better. Deep basements, caves, even culverts under a road can be useful fallout shelters. You should plan to stay in the emergency shelter for about 2 days minimum, and then get to a better shelter for 2 weeks or more.