You know it's a bad day when a thunderous explosion is followed by a mushroom cloud on the horizon.

Decisions you make in the next few minutes will determine your and your family's fate.

		Graphic: In case of nuclear explosion ...

Do you hop in the car and drive furiously in the opposite direction? Do you have time to get your children at school and duck into a basement shelter? Or do you head straight to the fruit cellar with cans of SpaghettiOs and jugs of water to hunker down for days to let nuclear fallout dissipate?

To help Americans with such lifesaving decisions, two Carnegie Mellon University professors have outlined the science, psychology and rationale for survival -- that is, for those far enough from ground zero to survive the initial blast.

"Individuals' Decisions Affecting Radiation Exposure After a Nuclear Explosion," written by H. Keith Florig and Baruch Fischhoff, was published in the May edition of Health Physics.

The paper provides radiation experts and government officials the tools to help people survive a 10-kiloton explosion -- roughly equal to 10,000 tons of TNT or nearly the size of the 1945 blast that devastated Hiroshima. A blast that size would erase one square mile with fallout causing additional deaths and cancer risk 20 miles downwind and beyond.

Detonation of a stolen or improvised nuclear device in an urban area would pose the greatest threat, the study says.

Within 20 miles of ground zero, survival and subsequent health impacts would depend on quick wits and sound decisions, said Dr. Florig, a senior research engineer in the Department of Engineering and Public Policy in CMU's Engineering College.

For now, he said, the Department of Homeland Security's Web site: ready.gov/america/_downloads/nuclear.pdf provides useful albeit limited information on surviving such a blast. It generally advises people to seek shelter, limit exposure and, if possible, flee to safety. But it leaves key questions unanswered.

"No one took the time to sit down with people and ask them what they want to know," Dr. Florig said. "We're trying to look at the problem from the user point of view."

Dr. Fischhoff, a psychologist who specializes in decision making, said government officials should use the study to develop a strategy to help groups of people, especially those without money or transportation to survive such catastrophic events.

"I think we should plan in advance an evacuation strategy," he said. "I would like to see someone systematically work through the decisions and see a planning effort that works through the decisions of different groups who could not protect themselves, to help them work through the hazards and provide support systems."

He said no one strategy is for all people, so strategies are necessary for different groups.

Most people might be surprised to find out how low the odds are that they will ever encounter a situation as described in the first paragraph.

What are the odds that your city would be hit, and if so, that the wind would be headed in your direction, and that you would be home and face the prospect of finding shelter in a basement?

Probability would rise for anyone living near likely targets, including Washington, D.C., New York City or San Diego, the nation's largest West Coast port. While it's less likely that a city like Pittsburgh would be targeted, certain scenarios make the probability more than negligible, Dr. Florig said.

Here are important details to embrace:

Taking shelter in existing space with supplies available is better than nothing. One must evaluate the options and react immediately.

One question is whether people should forsake personal safety to help family, friends and neighbors. Such decisions have risks and consequences, including the chance of finding a better shelter.

Other questions include whether to prepare a shelter in one's basement in advance; whether to risk leaving one's home for safer, more distant shelter; and how long to remain in a makeshift shelter before evacuating.

The study does not encourage construction of nuclear shelters. Instead, it lists supplies people should store in the most walled-in segment of one's basement. Generally it suggests that people must decide for themselves whether the risk is sufficient to prepare a space in the basement and keep supplies on hand.

A key decision is whether space is available for a temporary shelter and to store supplies. The entire project would cost several hundred dollars a year, Dr. Florig said.

Those who decide to stock supplies should consider having food, water, clothing, utensils, medicines, first-aid and sanitation supplies, a battery-powered radio, a flashlight, dust masks, duct tape, plastic sheeting, bedding, pet needs and even entertainment, since one might be stuck there for days.

"Some people may view the probability of an attack occurring in their city as so small that they dismiss the entire prospect," the study says.

A 10-kiloton bomb would be lethal to most people within a mile from ground zero. For those surviving the initial blast, survival depends on radiation exposure to fallout that could cause death or pose a serious cancer risk.

It raises the question whether one should forgo immediate shelter and try outrunning the fallout.

Key to that decision is learning the blast location quickly via radio or media, if electricity is available. Those close to the blast should secure shelter as quickly as possible. Those 2.5 miles from ground zero can expect fatal levels of fallout in less than 15 minutes. That means one's best bet is to head to the deepest underground shelter within minutes, if wind is blowing in their direction.

From 2.5 to 6 miles, fallout will arrive in 15 minutes to an hour. Radiation levels would be fatal for most people. Travel is advised only if one quickly can find better shelter before fallout arrives, then use remaining seconds to fortify that shelter.

Those in an area 12 to 30 miles from the blast face an additional 20 percent cancer risk, so seek better shelter if time permits. Failure to reach shelter increases cancer risk, which at this distance would not be immediately fatal.

Thirty to 60 miles from the blast provides time to collect the family and flee to safety.

Arrival of fallout depends on wind direction. If the wind -- and fallout -- is headed straight toward you, one choice is to get into a car and travel at a 90-degree angle from your location to avoid the "fallout footprint." Dr. Florig said that footprint likely would be long and narrow in the direction of the wind. So traveling out of the footprint as fast as possible would be one way to avoid or limit danger.

The big problem is figuring out precise wind direction, especially high in the atmosphere.

"We advise people that if they choose to evacuate the path of fallout, there's the chance that roads will be clogged [with other vehicles], and you might be caught above ground without a shelter," Dr. Florig said.

Once in shelter, people finally must decide when best to leave and limit radiation exposure.

Each person would have to decide based on individual circumstances, family considerations and available shelter.

"People will do sensible things if they have the right information," Dr. Fischhoff said. "We want to provide concrete advice that people can know in advance and figure out what to do.

"The logic is pretty simple," he said. "If you are close to the blast, find shelter immediately. If you are an intermediate distance, do preparations but don't evacuate. If you are farther out, you probably can outrun the problem, if you have transportation."

But the decisions, he said, are "for everyone to judge."