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Inside The Mushroom Cloud

Part I
The "Physics Package"1


IMAGINE A FREIGHT TRAIN rumbling past for eight hours. The train is 250 miles long. Imagine every car is fully loaded with dynamite.

Such a train's explosive potential is equal to that of a one-megaton nuclear weapon.

More than 50,000 nuclear weapons are deployed around the planet today.

A one-megaton weapon weighs at most a few hundred kilograms, made up of a few kilograms of lithium deuteride and tritium, some kilograms of plutonium, and about 100 kilograms of uranium-238.

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Very large nuclei like uranium-238 are not very hard to break up; above atomic weight 242 there are no stable nuclei.

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It so happens that every time a uranium-235 or plutonium-239 nucleus breaks up into two fragments it releases, on the average, two neutrons. These have such energies that if they hit another uranium nucleus they can split it. This is the chain reaction that makes nuclear weapons possible.

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How fast can this chain reaction happen? To figure that out we must find how long it takes for one doubling step to occur. That is as long as it takes the neutron to travel to the nucleus it splits.

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On the average the neutron travels three centimeters in uranium before it hits a nucleus. Its speed is comparable to the speed of light. So it takes about a tenth of a nanosecond (a nanosecond is one billionth of a second) to complete a step. There are 70 doubling steps. So fission is completed in about eight nanoseconds.

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During the first seven nanoseconds only about one percent of all available nuclei have fissioned: 99 per cent of the energy in a nuclear explosion is released within the last billionth of a second.

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Because the sphere has no time to expand in the eight nanoseconds it takes to release all this energy, the pressure inside it will have to rise in direct proportion to the temperature, which is about 130-million degrees Centigrade. Since originally it was one atmosphere, the pressure will rise to more than 100-million atmospheres.

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The energy released takes many forms. Some is released as kinetic energy. But the largest fraction of energy is released in the form of electromagnetic radiation: gamma rays, x-rays, ultraviolet light, visible light and, eventually, infrared radiation.

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The fireball is completely transparent to all electromagnetic radiation, so this radiation escapes from the fireball and heats up the adjacent air.

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But the x-rays can't go very far because they are absorbed by air molecules. This heats the air around the original fireball to such a high temperature that in turn it becomes transparent, allowing the radiation to move out and heat up additional layers of air farther away from the expanding fireball.

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This process removes energy from the interior of the fireball and cools it down uniformly; it also makes the fireball expand at supersonic speed.

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As the fireball expands and cools, the radiation that escapes from it changes from x-rays to visible light to thermal (infrared) radiation. When the fireball temperature is reduced to about 300,000 degrees centigrade, the speed of its growth becomes equal to the speed of sound in the air.

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At this point, two things happens: First, the superheated weapons debris that travels inside the fireball with supersonic speed catches up with the outer edge of the fireball; and second, a shock wave develops at the outer surface of the fireball that begins to shock-heat the air around the fireball, making that air incandescently hot.

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Since hot air absorbs visible light, one cannot see the fireball until the gases around it have cooled enough to permit visible light emitted by it to escape. The obscuring of the fireball by the shocked air is the cause of a characteristic "double flash" of light that a nuclear detonation in the air displays. This, incidentally, is the signal by which monitoring satellites detect nuclear weapons tests in the atmosphere.

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How big does the fireball eventually get and how high does it rise into the air? The fireball keeps on growing after breakaway and reaches its maximum size minutes later. A megaton-size weapon exploding on the ground will generate a fireball about three miles in diameter before it starts lifting 25,000 meters into the air.

*    *    *

To summarize, the explosion has created a number of physical effects, some common to all explosions, others characteristic only of a nuclear detonation.

First there is a very intense burst of neutrons and gamma-rays coming from the fission fragments and the fusion of the light nuclei.

Then comes a silent giant wave of intense heat and a flash of light hundreds of times brigher than the sun.

A shock wave of very high pressure follows, pushing down on everything with crushing force. This shock wave travels outward from the point of detonation like an ever-expanding ring, slapping down on the ground.

It is followed by intense winds that reach speeds of hundreds of miles per hour and die down slowly as the shock wave travels farther and farther away from the point of detonation.

As the fireball rises from a ground explosion, it entrains with it millions of tons of vaporized dirt that cools, condenses and starts falling toward the ground as the winds at the upper level of the atmosphere sweep the huge cloud downwind from the point of detonation.

A billion billion billion million (1033) oxygen and nitrogen molecules in the air have been combined by the heat of the blast into nitrogen oxides, which then rise with the cloud to the upper levels of the atmosphere.

An even larger number of liberated electrons start spiraling along the lines of the geomagnetic field of the Earth.


1 Nuclear warheads are known in the language of the military industrial complex as "the physics package."

During the trial of the Ploughshares Eight in 1981, representatives of General Electric refused to acknowledge the name or purpose of the dark, conical 4-1/2-foot-tall MK12A re-entry vehicle introduced as an exhibit.

Each MK12A can accommodate 10 nuclear warheads, each with the power of 17 Hiroshima bombs. The company spokesperson would only refer to the cone as "the product."

For an exploration of the language of nuclearism, see separate article.


Inside The Mushroom Cloud Part II: The Human Package
Inside The Mushroom Cloud: Introduction

Published in Sources Winter 1983 

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