The scenario: Satellite data fed into Pentagon computer relays to White House. . . Soviets have launched nuclear bombs toward United States. . . 40-80 megatons headed for Washington. . . President has four minutes to react. . . He turns the key. . . Second key is turned. . . U.S. bombs launched. . . The Last World War.

What does this scenario, considered feasible by experts, mean to the readers of this newspaper? If "ground zero"--the point where a nuclear bomb impacts--were the White House itself, what would happen to you if you lived in a Dupont Circle condominium; an Adams Morgan row house; a Kenilworth Courts public housing apartment; a tree-shaded Gold Coast bungalow? Suppose you live in Prince George's County, or Fairfax, or even in Baltimore. What then?

Let Earl Callen explain it. A research physicist at the Naval Surface Weapons Center at White Oak, formerly with the National Security Agency, a professor of physics at American University, Callen knows the numbers and the jargon. As a member of Ground Zero, Callen says he wants to make the complex language of nuclear horror understandable to all those who would be its certain victims.

"It's simple," he said yesterday. "Everyone would be killed. You'd die lots of different ways. But everyone would die. Maybe three million people."

Ground Zero describes itself as a nationwide project committed to educating the American public on nuclear war. Its headquarters is here in Washington. Officials of the organization take pains to separate themselves from groups advocating a nuclear weapons freeze as well as other nuclear policy stands.

Starting Sunday, Ground Zero will conduct a week of informational activities in 150 cities, 500 smaller towns, 350 college campuses and 1,000 secondary schools throughout the country.

Organizers anticipate that at least half a million people will take part in the week-long effort. Officials, church leaders, educators and community organizations around the country have joined in sponsoring a wide range of educational events. These are a sampling:

The Rev. Billy Graham is to speak at Yale University. Rep. Morris Udall is scheduled to speak at the University of Arizona and Sen. Daniel P. Moynihan will speak at Cornell University.

At one minute after midnight Sunday, a bicycle marathon is to begin at Champaign, Ill, to demonstrate to residents that a nuclear strike on the nearby Minuteman missile base would cause the deaths of at least half the people living in the Champaign-Urbana area.

On Monday, signs and markers warning of the dangers of nuclear war will be posted along the route of the Boston Marathon, which is being run that day.

In Wichita, Kan., on Monday, demonstrators will meet at the local "ground zero" and march out to points marking the circles of destruction.

In Spokane, Wash., physicians will meet at Gonzaga University Friday to discuss "the medical consequences of nuclear war in Spokane."

Saturday, there will be a tour of nuclear weapons plants in California's Santa Clara County.

Yesterday, Callen drove with some reporters from Lafayette Park out 16th Street to the D.C. line at Silver Spring, stopping to explain what would happen if a one-megaton nuclear bomb were to hit the White House.

"The first thing to bear in mind is that a one-megaton bomb is a small bomb," said Callen. "It's the sort of weapon they'd use on small cities, like Scranton, Pa. If the Soviets were to bomb the Washington-Baltimore area, they'd be more likely to use bombs up to 25 megatons. Between 40 and 80 megatons would hit the metropolitan area. But we'll be extremely conservative and illustrate the effects of a one-megaton bomb."

With "ground zero" the White House, and the bomb like a pebble dropped into a pond, this is what Callen said would happen as the deadly, broadening ripples rolled out across the city:

* The blast would dig a crater 200 feet deep and a quarter of a mile across. Within the crater would be the White house itself, Lafayette Square, the Treasury Building and the Old Executive Office Building.

But, any image of these buildings collapsing or tumbling into the crater, any thought of rubble remaining, is false. "The buildings and their occupants would vaporize," said Callen.

What does that mean? How does a huge building of concrete and steel vaporize?

"It's what happens to water when you boil it," he explained. "It disappears. An entire building would be reduced to its individual protons. Nothing would be left to see or touch."

* In the next ripple of destruction, a radius of seven-tenths of a mile from the White House, a fireball would, quite literally, cook to a crisp most of downtown--the District Building, the Martin Luther King Jr. Library, the Washington Post building, the Greyhound and Trailways bus terminals, the new Convention Center, George Washington University, the State Department, the FBI Building.

To understand how this could happen, hear Callen: "The temperature of the fireball would be 100 million degrees. The surface of the sun is 6,000 degrees. For simplification, call it 10,000 degrees. Thus, the fireball is 10,000 times as hot as the sun."

* Within the next ripple, 1.7 miles away from the White House, 98 percent of the people would die instantaneously. Individual forms of death would vary: some bodies would be crushed by a pressure of almost 2,000 pounds on every square foot of the body's surface; some would be hurled through the air by winds of over 300 miles an hour; many would be incinerated; others would be lethally irradiated.

Buildings, too, would be crushed or hurled into the air. Among them would be the Capitol, the House and Senate office buildings, the Supreme Court, the Smithsonian museums along the mall, Union Station, most foreign embassies, L'Enfant Plaza, Howard University Hospital, the Dupont Circle, Shaw and Adams Morgan neighborhoods, the Roosevelt, Fourteenth Street and Memorial bridges.

Meridian Hill Park marks a point on this circle of devastation. Callen, silvery moustache, ruddy face, squinting narrow eyes, stood on the park's rampart before a lifesize statue of Joan of Arc in armor mounted on a horse, and peered out over the city, toward the White House. "From here to 'ground zero' no one would survive. Perhaps the cockroaches. They seem to have survived everything history has offered. But nothing else would make it."

* The next circle, three miles out, is marked by the First Church of the Nazarene at 16th and Webster streets NW. Half of all the people living within this band would die immediately. Another 40 percent would be injured initially and die shortly afterward by fire or radioactive poisoning. All but the strongest commercial buildings would be destroyed.

Within this circle are National Airport, the Pentagon, Fort Myer, Fort McNair, Washington Navy Yard, Key Bridge, South Capitol Street Bridge, Rosslyn, Howard University, Gallaudet College, Georgetown University and its hospital, Children's Hospital, Washington Hospital Center, all Southwest D.C.

* Five miles out from "ground zero," marked on the 16th Street axis by the entrance to Walter Reed Medical Center, only 5 percent would die immediately; 45 percent would be injured, generally burned severely.

Heat would be intense enough to melt metal and glass and to cause combustible materials--curtains, furniture, trees, grass, clothing, carpets, wood houses, automobile interiors--to burst into flame. These individual fires would be swept into a mass congflagration by the 300-mile-an-hour winds.

Lying within this wave of destruction are most of Arlington County, American University, Catholic University, University of the District of Columbia, all bridges crossing the Anacostia River, St. Elizabeth's Hospital, Sibley Hospital, Cleveland Park, Congress Heights, Hillcrest, Brookland and much of Northeast Washington.

* Seven miles out, 25 per cent of all people would be injured. Buildings would be damaged. The greatest danger would be from radiation sickness. Within this circle are such suburbs as Bethesda, Falls Church, McLean, Suitland.

Beyond the Capital Beltway: as far away as Baltimore a person in a tall building looking in the direction of 'ground zero' would be struck blind by the intense light of the fireball. Radioactive fallout would extend as far as 200 miles downwind.