The first thing you need to do is asses is your risk. If you live over 100 miles from a nuclear facility, you're not in much danger, but take prevailing winds into consideration. Radiation can travel hundred of miles under the right conditions. If you're not downwind and far enough away, your only radiation risk is from nuclear weapons.
In this day and age its not likely that we will face the detonation of a nuclear device on American soil, but we should consider the possibility. There is also the risk of a dirty bomb. I think these are a more likely scenario than tactical nukes, barring proliferation in countries like North Korea and Iran. If you live in an urban center, you should be prepared for a terrorist attack. A dirty bomb is not a nuclear explosion, but rather an improvised explosive packed with radioactive material. The purpose of which is to cause damage to the population through radiation sickness. This would damage a population far beyond what a normal explosive could accomplish. In many countries such as Iran, it probably wouldn't be very difficult to obtain these materials. In this scenario, a gas mask and iodine tablets are a necessity. You can purchase military surplus gas masks from cheaperthandirt.com for about ten bucks. Iosat radiation tablets can be bought from Amazon.com from many distributors. You may have some trouble finding them nowadays, I've heard most places are sold out.
The gas mask is not as much of a necessity in the meltdown scenario, as there is no explosion (nuclear or otherwise) to kick up radioactive dust and debris. But fires caused by a meltdown can cause clouds of irradiated ash and smoke. There is also the possibility of an attack on a nuclear facility. In case of a terrorist attack, you should hopefully have a plan already.
A few key things to know, radiation, gamma radiation specifically, radiates out in all directions from whatever particle its attached to, it does not turn corners. So the best place to shelter is in the basement of a building with at least two floors. The idea is to get as much material and space as possible between you and radioactive particles. If in a city, a central room at least two floors up and with two floors above should shield you from most radiation. If possible pile as much stuff between you and the outside as possible. For example, filing cabinets, mattresses, blankets, whatever you can find. Make sure to mark your shelter for search and rescue with the number of people inside. Do your best to make your shelter airtight, particles of dust and debris can carry radiation into your shelter. This is most important in the first couple of days. Most radiation will have dissapated to a safer level after three days, but in the case of a meltdown this isn't true. A power plant has so much material it will spew radiation for weeks. It is best to wait for rescue until you can't wait any longer.
If you have to leave, have a plan. Take into consideration wind and terrain. If the plant is in the west and the wind is blowing east, travel north or south out of the swath of radiation. You wont be able to outrun it going east. If you have no water you're going to need to get moving sooner. You can go three days without water, and three weeks without food. If you don't have water, don't eat, digestion uses up water. If you're out of water and there's little hope of rescue, get moving by the second day, if you wait too long you'll be too weak to move. So stock up on water, have at least a couple bottles in your car, in your desk at work, and have at least 10 gallons per person at home. Don't forget, your hot water heater has about 30 gallons or more in it. So at the start of the disaster, shut off the valve that supplies water in, and drain out the bottom. Make sure you have a battery or hand crank radio at home or in your car as well. Hope this helps out, and hopefully you wont need it.
