In all seriousness, all you really need to do if the area you are in has been irradiated is to leave, even if that means on foot. Getting just a few miles away from a nuclear event is more than enough distance to mitigate the worst of the effects of radiation.
If you don't inhale or ingest radioactive particulates and don't die in the blast, then odds are good that if you're not seriously injured as a result of the blast then you'll live a long, unhappy life afterwards.
The Alpha particulates are blocked by your skin.
The Beta particulates are blocked by heavy clothing, any clothing really. This would be the stuff in tritium watches and night sights. Not something you want to ingest or inhale, but otherwise relatively harmless.
Gamma can only realistically be blocked by mother earth and the same goes for neutron radiation. If you know it's coming, find a hole… and cars don't do bean dip against gamma. On the bright side, if you're close enough to receive a lethal dose the blast will probably get you first.
A simple rag over your mouth is sufficient. Don't eat and don't drink, if possible, until you're well away from the irradiated area. Hint, walk into the wind and away from where you saw, or didn't see since it would've blinded you, the miniature sun on earth.
Obviously a gas mask with a CBRN filter is better, but the only real reason to carry one with you is if you know you're going into places that are irradiated. If you were assigned cleanup duties, you'd be issued a mask and dosimeter to measure the total dosage of radiation you've received.
In case anyone here wanted the gory details:
400-500 Roentgens - About 50% will die in about a month.
500-750 Roentgens - Very few survivors and immediate medical attention will be necessary for the few who do survive, which won't likely be available.
1000 Roentgens - Death pretty much guaranteed within a week or two.
5000 Roentgens - Immediate incapacitation and death within hours to a week.
About how much would you receive from a 1MT burst?
2 miles - about 44 Roentgens
1.5 miles - about 700 Roentgens
1 mile - about 14,000 Roentgens
.5 miles - about 500,000 Roentgens
Distance is your friend.
Now, those same doses received over time are not significantly damaging to the human body and you naturally receive radiation from a variety of sources, most notably the sun, over time. Understand that radiation measurement is a function of the magnitude of the dose and the time over which it is received. The uniformity of your radiation exposure and the time over which you receive the dose have a lot to do with how sick you will or won't be.
If the rate of exposure is over 10 Roentgens per hour outside and unless you are trapped out in the open in the fallout zone, then STAY INSIDE. Fallout radiation decay is exponential and within a week or two very little of the initial radiation will remain.
Here's a quick and dirty primer on how all this nasty stuff works:
If that's a bit too technical, I can post some other freely available resources.
For whatever crazy reason, they teach you about this stuff in the Navy because they still think the Russians or Chinese are going to invade, but I realize most people have no idea about this kinda thing.