Science: Measured Fall-Out

The exact implication of a few cryptic sentences by Atomic Energy Commission Chairman Lewis L. Strauss will long be debated in scientific, military and diplomatic circles, but their gist was clear: the U.S. had found a way to control, at least in some measure, the deadly and indiscriminate fallout produced by large nuclear explosions.

“Real progress,” said Strauss last week, “has been made with respect to … achievement of maximum effect in the immediate area of a target with minimum widespread fallout hazards.

“It has been confirmed that there are many factors, including operational ones, which do make it possible to localize, to an extent not hitherto appreciated, the fallout effect of nuclear explosions.

“Thus, the current series of tests [in the Pacific] has produced much of importance, not only from a military point of view, but from a humanitarian aspect. We are convinced that mass hazard from fallout is not a necessary complement to the use of large nuclear weapons.”

The Army’s Lieut. General James M. Gavin recently alarmed all Europe by predicting that an all-out nuclear attack on the Soviet Union might kill several hundred million people, as the fallout drifted capriciously with the wind, falling on friend and foe alike. If the AEC has achieved a “large nuclear weapon” with greatly reduced fallout, it will enable atomic strategists to lay down their pattern of death with greater precision, make the H-bomb a far more useful military weapon. A bomb exploded, for instance, over a Polish air base would be less likely to depopulate Berlin.

Operational Factors. The AEC did not explain how it controls H-bomb fallout, but it pointed the way to some speculation. Strauss’s “operational factors” presumably refer chiefly to the altitude at which the weapons are exploded. The 1954 H-bomb test that made “7,000 square miles of territory … so contaminated that survival might have depended on prompt evacuation” (according to the AEC’s own reports) was exploded on a tower on a small coral island. Its fireball dug a deep crater and tossed millions of tons of pulverized coral into the air. This material, made highly radioactive by contact with the fireball, was the poisonous “atomic snow” that settled on boats, islands and water 220 miles away.

The high-yield H-bombs of the current test program were dropped from aircraft and exploded high above the surface. Thus their fireballs did not concentrate their fury on a small area of coral, but spread it over miles of water. As a result, not much pulverized material was carried upward. The total radioactivity produced by such a bomb may be large, but most of the potential fallout is distributed high in the stratosphere in the form of extremely fine particles or even single molecules. Such impalpable stuff is slow to fall. Not much would fall in any one place, and its strength would be much reduced by mere passage of time.

“Clean” Bombs. The Strauss statement implies, however, that H-bombs have been made “clean” by something besides “operational factors.” Nuclear pundits are already speculating about how the bombs themselves may have been changed so as to yield less fallout.

The radioactivity produced by a nuclear explosion comes chiefly from two sources: 1) fission products (fragments of plutonium or uranium atoms) and 2) free neutrons, which enter atoms of many common elements and make them radioactive. It does not seem likely that much can be done to reduce the total quantity of the fission products. But bomb physicists may have learned how to make the explosion produce fission products that are less radioactive, or that lose their activity before the fallout reaches the ground.

It is more likely that something has been done to control the bomb’s free neutrons. One device would be to make sure that the bomb’s casing and mechanism do not contain material that is made radioactive by neutrons. Another would be to provide material that soaks up neutrons without becoming dangerously radioactive itself.

A third possibility is that “clean” H-bombs may not rely on fission at all. The mighty bomb of 1954, although involving a fusion (hydrogen) reaction of some sort, got most of its energy from fissioning uranium, and therefore produced a gigantic amount of fission products. The latest models may be designed to use only a little fissioning uranium for a detonator, and to get the bulk of their energy from a fusion reaction whose end product is a stable element such as helium. In that case they need produce hardly any fallout. “Clean” H-bombs may be unusually expensive, and not necessarily ideal in the case of an all-out nuclear war. A good strong fallout enables a few explosions to put a whole nation out of action, and is therefore a military weapon too valuable to ignore. Fallout can be maximized as well as minimized, and more easily. Presumably, the AEC has given at least theoretical attention to this possibility too.