Our Nuclear Future: Facts, Dangers and Opportunities

by Edward Teller; Albert L. Latter

Dosage in First 48 Hours After Fallout Began

This shot proved what had been argued for many years: that radioactivity is not just an incidental part of an atomic explosion. The people on Rongelap were far outside the area of danger from blast and thermal effects. But they received a sizeable dose of radiation. In fact, a person could have stood unprotected at a distance of thirty miles from the explosion and been perfectly safe from the blast and thermal radiation. But at that same distance in a downwind direction he would have accumulated a lethal dose of radiation within a matter of minutes after the fallout began.

Because of the radioactive fallout, the test sites must be located in remote parts of the world. It would be desirable if sites could be found which are so remote from populous areas that the tests could be conducted without regard to the direction of the winds. Unfortunately the bombs are too big and the planet is too small.

As a result the winds must be watched before every test; and the tests must be delayed until the winds are favorable. What happened to the Marshallese was an accident which might have been avoided if the winds had been blowing more directly toward the north at shot time. Since this accident the wind requirements for the tests have become far more stringent, our knowledge of the danger has increased, and the rules of safety have in all respects improved. Many large yield weapons have been tested since March 1, 1954, but no other accidents have occurred. We can be confident that accidents of this kind are now very improbable.

At the U. S. test site in Nevada there has been no instance of a major fallout on a populated area. Probably the most worrisome situation which has occurred there was in the spring of 1953 during the Upshot-Knothole test series. After the ninth shot of the series the cloud drifted eastward over St. George, Utah, a town of about 5000 people. Some fallout occurred shortly before nine o’clock in the morning. About nine-thirty AEC officials issued a warning advising the residents to stay indoors. By noon the warning was withdrawn and people were allowed to continue with their normal affairs. The incident left everyone a little bit scared, but no one had received a radiation dose greater than two or three roentgens.

We have been talking about the local fallout which occurs within a few hundred miles of the test site. Not all the radioactivity which is made in the explosion goes into this fallout. Some of it travels for really long distances, not hundreds but actually thousands of miles from ground zero. This part of the radioactivity is disseminated world-wide and completely escapes the control of man. To be sure, by the time this radioactivity is distributed over a large fraction of the earth’s surface, the dosage levels of radiation are very tiny, less than a ten thousandth of a roentgen for a megaton explosion. There is no danger whatever that a person would die or even become mildly sick from this amount of radiation. There is, however, the possibility of long-range effects such as bone cancer, leukemia, and genetic mutation.

The world-wide danger is, of course, primarily due to the big bombs. The little ones, such as are tested in Nevada, release about ten kilotons (TNT equivalent) of fission energy. Some of the big ones in the Pacific release a few megatons of fission energy. Since the amount of radioactivity is proportional to the fission energy released, one big bomb is equivalent to several hundred or possibly a thousand little ones. Altogether in Nevada, to date, there have been only sixty or seventy shots. It may be desirable to minimize the world-wide fallout from the big shots in the Pacific. But for the little shots in Nevada, it is probably more important to minimize the local fallout. How much radioactivity goes into the local fallout, how much into the world-wide, and how these relative amounts can be controlled, are the main topics for the remainder of this chapter.

Not all the radioactivity which is made in the explosion contributes to the fallout, either local or world-wide. Some of the radioactive fission fragments (gamma emitters) have such short half-lives[11] that they actually disintegrate before the bomb has disassembled. A great many others disintegrate in the first few minutes while the atomic cloud is rising. The energetic beta and gamma rays released in these early, rapid disintegrations are stopped in short distances and merely add to the havoc at the scene of the explosion.