Our Nuclear Future: Facts, Dangers and Opportunities

by Edward Teller; Albert L. Latter

This hot region tends to be limited by a sharply defined boundary which is called a shock front and which is moving outward at a speed of several hundred miles per second. This front finally reaches the limits of the more or less dense material in which the whole bomb structure was originally encased. It then breaks through into the surrounding air. The air heats up in the immediate vicinity, and this is the beginning of the fireball.

From this point on, the energy spreads due to the push of the high-temperature air. A sharp shock front forms and keeps moving outward at a speed greatly surpassing ordinary sound speed. The radioactive material is contained within this hot and expanding sphere.

As the fireball expands and the temperature falls, more and more visible radiation is emitted. Actually, the surface is growing less brilliant as the structure expands and cools, but its greater size and the longer time that is available for the emission of radiation overcome this disadvantage. Finally, at a radius of perhaps a few hundred feet for a small bomb and a mile for a big one, the fireball expansion halts. This happens because the shock front is no longer strong enough to make the air luminous. The luminosity not only stops advancing but is actually partly dimmed by absorbing substances formed by the badly mistreated air molecules.

The time which has elapsed to reach this stage of the explosion depends on the bomb energy. If two explosions are compared, and the bigger one has a thousand times the explosive power of the smaller one, then the time needed to reach the extreme expansion of the fireball will be approximately ten times greater for the more violent event. In any case, a reasonably close observer has to use strongly absorbing glasses during this time if he is not to be blinded. For small bombs, the expansion of the fireball is too short to register. For the really big ones, you can see the expansion developing and you wonder when it will stop. To the unprotected eye the small bombs are almost as dangerous as the big ones, because there is not enough time to blink.

In the meantime, the shock wave, now separated from the fireball, travels through the air and carries with it a considerable fraction of the original explosive power. An important part of the damage which a bomb can cause is due to this invisible pressure wave which spreads with a speed close to that of sound, over a distance of miles, before it settles down into harmless rumbling.

The rest of the energy is still sitting in the fireball near the point where the explosion occurred and the hot air now commences to ascend, breaking up into a turbulent mushroom as it goes. The hot interior portions get occasionally exposed and the object gives the appearance of an enormous flaming mass, at least when seen in a motion picture which slows down the action and reduces the size. The radiant tongues are too big and too fast for any ordinary flames.

During this stage the display gradually pales sufficiently so that it can be viewed with the naked eye. The originally hot masses have now emitted enough energy in the form of light and mixed with a sufficiently great mass of cool air that they no longer glow violently. This mass of central and rising gas contains practically all the radioactivity, not only that originally formed in the explosion but also some produced by neutrons which leaked out of the bomb and got captured by a variety of nuclei in the air, water, or ground within the neighborhood.

And now the aftermath of the explosion is turning into a display growing rapidly and yet in a measured manner so that not only the eye of the observer but his mind and his feelings can follow the events. The mushroom which has been formed by the first updraft develops into a column with more and more agitated boiling masses added on the top and with slanting skirts of a snowy appearance descending toward the sides. What is this white mass that looks just like a cloud of peculiar shape and that has grown up to the high heavens (or as the meteorologists call it: the stratosphere) in a few minutes before our eyes?

It is actually a cloud: a collection of droplets of water too small to turn into rain but big enough to reflect the white light of the sun. And it is formed in a similar way to the cumulus clouds of a thunderstorm. Indeed it is a beautiful example of a many-storied castle of cumulus upon cumulus. But strangely enough what makes this cloud is not the heat of the bomb. It is the cooling of the air masses that have been sucked in as the remnants of the fireball rush upward like a giant balloon. Under this balloon air is drawn upward. As this air rises, it cools and water vapor contained in it condenses into droplets: precisely the same mechanism which gives rise to thunderheads on a hot summer day.

The white skirts (which are not always present) do not consist of any material that is falling out of the cloud. On the contrary, a moist layer of air is sucked up into the cloud from the side and the droplets which form in this layer give rise to a cloud-sheet with the appearance of a skirt.