Having looked around my archives I came upon this little scenario from
the late 70's.

After about 0.1 millisecond (one ten-thousandth part of a second) has
elapsed, the radius of the ball of fire is some 45 feet,14M and the
temperature is then in the vicinity of 300,000 degrees Centigrade. At
this instant, the luminosity, as observed at a distance of 100,000
yards (5.7 miles 9+Km), is approximately 100 times that of the sun as
seen at the earths surface.... the ball of fire expands very rapidly
to its maximum radius of 450 feet 140M+ within less than a second from
the explosion. -Los Alamos

The day the Bomb went off YOU are there

It was a sunny Summer morning in Chicago loop the usual crowd of
shoppers, tourists, messengers, office workers heading out to an early
lunch. It was Friday.

At 11:27, a 20-megaton nuclear bomb exploded a few feet above street
level at the corner of La Salle and Adams. First the incredible. flash
of light and heat. In less than one-millionth of a second, the
temperature rose to 150 million degrees Fahrenheit-more than four
times the temperature at the center of the sun.

The roar followed immediately but there, in the center of the city and
for miles around, no one was left to hear it. There was only the heat.
And the dust.

Imagine that it happened. We will not speculate here on why it
happened-on whose fault it was, on the series of diplomatic bluffs and
blunders and miscalculations here and there that made it happen. It
happened.

Even in the macro-magnitudes of nuclear weaponry, a 20-megaton bomb is
large -the equivalent of 20 million tons of TNT, though such
comparisons have little meaning. The yield of a 20-megaton bomb is
some 1,500 times as great as the yield of the bomb that was dropped on
Hiroshima in 1945.

The U.S. does not admit to deploying any 20-megaton bombs in its
nuclear arsenal. With its superiority in missile numbers and missile
accuracy, the U.S. prefers weapons of lower yield. But the Soviet
Union's 200 SS-9 intercontinental ballistic missiles are believed to
carry warheads in the 20-megaton range, and they-along with lesser
bombs-are presumably targeted on the 50 largest cities in the U.S.

In the event of a nuclear war, a total of some 100 to 200 megatons
would be directed at a metropolitan area like Chicago's.

The bomb that exploded in the Loop left a crater 600 ft. deep and
nearly a mile and a half in diameter. The crater's lip, extending
almost to the shore of Lake Michigan on the cast, was 200 ft. high and
would be, after the cloud of radioactive debris and dust had settled
or dissipated, the tallest "object" visible in the area of the blast.

For the moment, though, there was just the incandescent fireball,
rising and expanding outward at enormous speed, reaching a height and
breadth of three or four miles, illuminating the sky, so that 100
miles and away, over Milwaukee, the flash blinded the crew of a
Chicago, bound airliner.

Around Ground Zero, everything-steel-and-concrete skyscrapers, roads
and bridges, thousands of tons of earth, hundreds of thousands of
people-instantly evaporated.

At the edge of the fireball, a thin shell of superheated,
super-compressed gas acquired a momentum of its own and was propelled
outward as a blast of immense extent and power, picking up objects
from disintegrating buildings, snatching huge boulders and reducing
them to vapor that would solidify, eventually, into radioactive dust.

Three seconds had elapsed since the bomb went off.

A high-altitude blast at one to three miles above ground level would
have inflicted considerably greater blast damage, but the surface
blast has its own "advantage": by maximizing the amount of debris
sucked up in the nuclear explosion, it multiplies the long range
radiological effects, threatening the survival of living things
hundreds of miles from the target area. And even the blast radius of a
surface detonation is powerful enough to ignite fires more than 20
miles. from Ground Zero-more than 30 miles. if clouds help to reflect
the flash.

Within a minute, the familiar shape of the mushroom cloud began to
form over Chicago, symmetrical and strikingly beautiful in various
shades of red and reddish brown. The color was provided by some 80
tons of nitric and nitrous oxides synthesized in the high temperatures
and nuclear radiations. In time these compounds would be borne aloft
to reduce the ozone in the upper atmosphere.

The mushroom cloud expanded for 10 or 15 minutes, reaching a mature
height of 20 to 25 miles. and extending 70 to 80 miles. across the
sky.

To a distance of five miles from Ground Zero-to affluent Evanston on
the north, well past working-class Cicero on the west, beyond the
University of Chicago campus on the south, there was--nothing. A few
seconds after the bomb went off, the fireball appeared, brighter than
5,000 suns. Those who saw the sudden flash of blinding light
experienced instant and painless death from the extreme heat long
before the noise and shock wave reached them.

Glass melted. Concrete surfaces disintegrated under thermal stress.
Anything combustible exploded into raging flames. Even reinforced,
blast-resistant structures collapsed, along with highway spans and
bridges.

The blast wave arrived about 15 seconds later, buffeting the few
man-made remnants that had not been pulverized. With the shock came
torrid wind, traveling at some 300 mph, carrying dust and embers and
fragments, blowing down vents and tunnels to suffocate the few
surviving human beings who had been sheltered below ground level.
After about 10 seconds, the wind reversed direction, drawn back toward
Ground Zero.

The enormously high temperatures from the fireball of a nuclear weapon
generate enough light and heat to ignite simultaneous fires over huge
areas. In these areas the heated air forms a rising column, resembling
on a vast scale the airflow in a fireplace. Cool air drifts into the
fire zone to replace the rising hot air. As the fires gain strength,
burning hotter and more violently, the chimney effect intensifies,
sucking in more air and causing the fire to burn hotter still.

About 20 minutes after the atomic bomb attack on Hiroshima, a mild
wind began to blow from an directions toward the center of the city.
Within two or three hours, the wind developed a speed of 30 to 40 mph
and air temperatures rose steadily toward 2,000'F as fires burned out
of control for a distance of 1.2 miles. from Ground Zero. The wind was
accompanied by light, radioactive rain over the center of the city and
heavier rain around the periphery. It was a firestorm, and it
destroyed about 2,800 acres.

A 20-megaton bomb could, under similar conditions, generate a
firestorm that would devastate an area some 500 times as large.

On the freeways radiating from the Loop, automobiles, trucks, and
buses were simultaneously evaporated and blown away, their particles
sucked up into the fireball to become components of the radioactive
cloud.

Along the Stevenson Expressway, some seven or eight miles from Ground
Zero, scores of oil storage tanks exploded-ruptured by the shock wave
and then ignited from the grass and shrubbery burning around them.

At this range, too, aluminium siding on homes evaporated and some
concrete surfaces exploded under thermal stress. The few buildings
still standing were in danger of imminent collapse-and all were
engulfed by flames. Highway spans caved in. Asphalt blistered and
melted.

Clothing caught fire, and people were charred by intense light and
heat. Their charcoal limbs would, in some instances, render their
shapes recognizably human.

With greater distance from Ground Zero, the effects diminished. About
10 miles. from the Loop, in the area around the Brookfield Zoo, the
fireball was merely brighter than a thousand suns. Glass did not melt,
but shattered window fragments flew through the air at about 135 mph.
All trees were burning even before the shock wave uprooted most of
them.

Railroad bridges collapsed, and railroad cars were blown from their
tracks. Automobiles were smashed and twisted into grotesque shapes.
One- and two-story wood frame homes, already burning, were demolished
by the shock wave, which also knocked down cinder-block walls and
brick apartment buildings.

Those who had taken shelter underground-or, more probably, just
happened to be there-survived for 15 minutes or a half hour longer
than those who were exposed. They suffocated as oxygen was drawn away
by the firestorm that soon raged overhead.

At O'Hare Airport, the world's busiest, aircraft engaged in landing or
takeoff crashed and burned. Planes on the ground were buffeted into
each other and adjacent hangars, their fuselages bent and partially
crushed by the shock wave. Some 30 seconds before the shock wave
struck, aluminium surfaces facing the fireball had melted and the
aircraft interiors had been set aflame.

The enormous temperatures associated with all nuclear weapons,
regardless of yield, result from fission, the process in which certain
atomic nuclei become unstable and disintegrate. (Even a fusion bomb
like the one here described gains about half its energy from fission.)
As the nuclei break up and form new atoms, they yield neutrons and
immense amounts of energy. The atoms created by fission are so
radioactive that if one could collect two ounces of them one minute
after their creation, they would match the activity of 30,000 tons of
radium and its decay products.

When a 20-megaton nuclear bomb goes off, it produces more than half a
ton of this material. One minute after detonation, it is as
radioactive as 30 million tons of radium. Though this radioactivity
declines within one day by a factor of 3,000, the material still has
the radioactivity of 10,000 tons of radium.

If one could instantly remove the entire fission inventory from the
largest commercial nuclear power plant (3,000 megawatts thermal) and
simultaneously detonate a 20-megaton nuclear bomb, 30 minutes after
the "experiment," the activity from the bomb would be about 100,000
times as great as the activity contained in the reactor's fission
inventory.

The astronomically hot fireball indiscriminately incorporates all
those materials into a superheated gas and mixes them with minions of
tons of earth and target debris. The mixture condenses into droplets
of liquid and then solidifies into particles ranging in diameter from
one-thousandth to one-fiftieth of an inch. The particles incorporate
all of the extremely dangerous radiological residues, and are borne
aloft to deliver death hundreds of miles from the target.

In addition, many neutrons escape the exploding weapon to he absorbed
by the earth and air in the immediate blast area. This leads to the
production of a wide variety of neutron-activated radioactive isotopes
of such elements as sodium, chlorine, manganese, zinc, copper, and
silicon, as well as radioactive carbon transmuted from nitrogen in the
air.
All of these substances, dangerous to varying extents, remain active
in the blast area to jeopardize survivors and would-be rescuers.

In the pleasant western suburb of Hinsdale, some 16 miles. from the
Loop, the manicured lawns surrounded by wooden picket fences on
tree-shaded Chicago Avenue caught fire first. Leaves in the trees
ignited next, and then the picket fences themselves. Paint evaporated
off house exteriors. Children on bicycles screamed as they were
blinded by the flash of the fireball. An instant later, their skin was
charred. Autos collided, as their tires and upholstery burst into
flame.

The white wooden cupola on the brick village hall blazed, and even the
all-stone Unitarian Church on Maple Street was burning--ignited by the
curtains on the windows facing east.
The shock wave arrived some 50 seconds later, tearing the roofs off
houses, blowing in side panels, spreading burning debris.

At about the same distance north of the city, Ravinia Park's summer
festival was to have featured an all-Mozart program that Friday
evening. There would be no Mozart and no Ravinia Park. By 11:30 AM,
that agreeably green place was a burning wasteland.

About 21 miles. south-west of the Loop, the Argonne National
Laboratory sprawls on some 1,700 acres of parkland. Its 5,000
employees had engaged in a broad variety of research efforts, many of
them centered on the development of nuclear power. Argonne and its
predecessor, the Metallurgical Laboratory of the University of
Chicago, were instrumental in developing the atomic bomb.

Argonne researchers who happened to be looking out a window on that
Friday morning-gazing, perhaps, toward the Sears Tower barely visible
on the skyline to the north-east suddenly saw a flash that filled the
sky with the brightness (from their vantage point) of 50 to 80 suns.
They were blinded, their clothing ignited on their bodies, and exposed
skin areas suffered extremely severe third-degree bums.

Here, too, leaves and grass and many readily combustible materials
caught fire at once. The shock wave, which arrived a minute and a half
later, caused only minimal damage, except as it spread burning debris.
But the fires soon raged out of control, for here, as for many miles
around, there was neither power nor water pressure nor emergency
equipment nor any human will but the impulse to surrender to the
hysteria of total disaster.

And soon after all this happened, the radioactive cloud, carried by
the prevailing winds, began drifting toward the east at about 20 mph.

By the time the mushroom cloud has completed its 15-minute process of
stabilization, it is directly overhead for distances up to 40 miles.
from Ground Zero. Fires are still burning as radioactive particles
begin settling on the landscape. The radiation level rises rapidly to
exceed 4,000 to 5,000 roentgens per hour, delivering a lethal dose
within seven to eight minutes. Individuals driven out-of-doors by fire
are directly exposed.

Within an hour or so, elements of the cloud begin to arrive about 40
miles. downwind. The density and activity of the particles is such
that a belt four to five miles wide quickly develops radiation levels
of more than 3,000 roentgens per hour. By this point, activity is
diminishing, so that it requires an exposure of 10 to 20 minutes to
absorb a deadly dose. Within a larger belt, up to 10 miles. wide,
fewer particles am falling, allowing up to a half hour's exposure
before a fatal dosage is absorbed.

As the cloud moves downwind, expanding and dropping particles, the
fallout level becomes unpredictable, though it remains, in many
places, extremely high.

No one knows how many Americans might die from blast and fire and
radiation sickness in a nuclear attack. Casualty projections are a
matter of heated controversy within the government and outside it. A
reasonable conjecture is that an all-out nuclear attack might claim
160 million lives-about three quarters of the population. In a
particularly strategic concentrated metropolitan area subject to a
direct strike-Chicago, for example-virtually the entire population
could be expected to perish.

But American casualties would, of course, not be the only ones. No
matter -how it happened or whose fault it was, there would be a
counterstrike, and the indiscriminate murder of one nation's
citizens-ours or theirs,-would be avenged by the indiscriminate murder
of the other's. Moving slowly to the east, Chicago's radioactive cloud
brushed Indiana and was blown into Michigan, dropping silent death
along the way, drifting inexorably toward Detroit. But it didn't
matter, for at a few seconds before 11:27 that Friday morning, a
20-megaton bomb had exploded in Detroit, too.

SOURCE: By Erwin Knon and Theodore Postol. Reprinted by permission
from The Progressive, 408 West Gorham Street, Madison, Wis. 53703.
Copyright (D 1978, The Progressive, Inc.