Second best in on the middle floor of a building, away from windows. In an actual nuclear attack, where nuclear weapons are detonated at high altitude (the most effective way to distribute radiation), electromagnetic pulses (EMPs) will shut down everything from vehicle engines - most of which are now electronically controlled - to cell phones, computers and even electrical power.

A nuclear plant that goes offline because of a failure will cause a momentary reduction in power supplies or an actual power loss, quickly followed by a resumption in normal power levels as other sources are re-routed through the electrical grid via substations. Don't assume a momentary dimming of the lights is a cause for alarm. Generating plants (both nuclear and fossil-fuel fired) are routinely taken offline for repairs. No such "flicker" will alert the public to a terrorist's dirty bomb, however.

If you can't find shelter, cover your face and exposed skin (a coat, a jacket, even a newspaper will help) and shut your eyes. The flash will blind you, either momentarily or permanently. The pressure wave that follows can actually rupture internal organs, so in the absence of a building or even a sheltering wall, lie down in a gutter or other declivity facing away from the blast and protect your head and eyes from flying debris. Do not assume that structures like fire hydrants, newspaper stands or bus shelters provide protection. None of these is designed to stand up to the 100- to 500-mile-per-hour winds generated by a nuclear explosion.

Once the blast wave has passed, try to get out of the open air, which is distributing mass quantities of lethal radiation. If you were outside during the blast, you will already have picked up a large dose, but radiation sickness is cumulative. The more you can prevent, the greater the likelihood you will survive.

If the wind is blowing after the blast wave passes, move into the wind. Radiation is thickest downwind. Avoid low-lying areas, which accumulate radiation in the same manner as they collect frost or fog (by subsidence inversion). Get to a source of water, preferably a broken water hydrant or water main that will remain somewhat uncontaminated for fifteen minutes to half an hour because the source is usually enclosed or distant. Remove all outer clothing, including shoes. Do not pull items over your head. which will only "shed" the radiation farther. Cut off your clothing, if possible, or strip it slowly down your legs. Then bathe yourself, but do not put the clothing back on; cold and wet is better than dead. Continue looking for shelter. Do not touch anyone who has been exposed to radiation other than to check for vital signs and search pockets for identification and foodstuffs or matches, and do not eat or drink anything that is not wrapped or containerized.

If you managed to find shelter before the blast, either in a building or - better yet - a basement or subway tunnel, seal yourself off with whatever is handy - wooden panels or signage, plastic bags, even bed sheets or thick layers of newspaper - anything that will delay the infiltration of radiation. You can't block off a subway tunnel, but these tunnels usually have utility or maintenance rooms off the main tunnel structure where you can take refuge.

If you're in your basement, you should have stockpiled some food and water against normal emergencies like tornadoes or hurricanes. If not, run water in the washing machine or laundry tub as quickly as possible, preferably hot from your water tank. In the basement of a building or a subway, try to find water and food near at hand, as from the pockets or purses of other victims (being careful to make as little contact as possible), a vending machine, a lunch room or even water from standing puddles.

Remember than a human can survive about two weeks without food, but only about 72 hours without water. If the area around you has mostly collapsed from the blast, try to mark the entryway before you seal yourself in, so rescue workers can find you. Use discarded, brightly colored clothing, as this is a traditional signal to rescue workers that someone is buried. Use the universal distress signal (SOS), or Morse code, to signal your presence to rescue crews. This is three sharp taps, three heavy, sustained blows, and another three sharp, quick taps (dot-dot-dot, dash-dash-dash, dot-dot-dot).

Once you are more or less safe in your shelter, either as a family unit or a collection of strangers, you may have to decide whether to accept additional survivors. This will depend on how close your shelter was to the initial blast and how intensely irradiated these survivors are likely to be. If you have access to large amounts of water in your shelter, you can help these unfortunates decontaminate themselves and then bag or bury their outer clothing. Food and water brought into your shelter are suspect, because they - like their carriers - have been exposed.