Thus, it is likely that a technique has been developed which will make possible test operations which contribute much less fallout. In addition, the nonmilitary applications of atomic explosives, which the underground shot on September 19 last year disclosed, appear to be so promising that for them alone we must continue certain tests in order that these benefits may be available to the human race. For example, in the underground shot, just mentioned, we produced an earth shock which was very revealing to the seismologists in its clarity and sharpness within a considerable distance from the Nevada test site and it is certain now that from atomic detonations we will be able to determinethe internal structure and character of the earth with a clarity and detail never possible with earthquake shocks because of their diffuseness in both time and location. A second possibility is the applicability of nuclear explosions to moving earth, if the fallout hazard can be controlled. Craters produced in the Pacific Islands are convincing testimony of the possibility of making harbors in regions where the local fallout hazard is tolerable. Per- haps with the devices of reduced fallout which are now being developed such applications will be possible in more populated regions. A third most intriguing possibility is that of shaking and breaking subterranean structures by nuclear shock. The underground detonation, despite its small 1.7 kiloton yield, is estimated to have crushed about 0.4 million tons of rock. It happened that the mountain selected consisted of rather soft rock, but nevertheless it was consolidated and supported its own weight. After the explosion, a sphere 260 ft in radius was crushed so it could easily be mined. It was not rendered radioactive because the radioactivity was contained in thin rock shell mentioned earlier, which weighed only 700 tons and which was visually distinguishable from the ordinary rock and thus can be separated easily. It is clear that this type of application has great promise. A fourth example is the containment of the heat generated from large atomic explosions in rock structures which are dry and therefore free of the pervasive thermal conductive charac- teristics of steam and water. This affords a definite possibility for generating atomic power; if detonations, which are large enough to make such power economical, are practical and if the subsequentdrilling and removal of the heat by injection of water to produce steam proveto be practical. A fifth example is the possibility of making radioactive isotopes by surrounding the explosive devices with appropriate materials so that the neutrons which always escape in atomic explosions can be utilized at least in part. A sixth example is the potential utilization of the radiation and heat of the bomb to cause chemical reactions. These six possible nonmilitary applications show that nuclear explosions may have peaceful applications of real importance and that the understanding of the phenomenaof radioactive fallout is useful not only in conducting a weapons test, but in the promotion of important peaceful applications. The radioactivity produced by the detonation of nuclear weapons has been extensively studied and reported upon.'~!” From this work we have learned about the amountof radioactive fallout which occurs, and the mechanismsfor its dissemination in a broad and general way. Let us consider a few of these generalpoints. 1. The stratosphere plays an extremely important role for the fallout from megaton yield weapons, and the troposphere is the medium which disseminates the fallout from kiloton detonations; thus, speaking broadly, stratospheric debris is from megaton yield detonations and the tropospheric fallout is from those of lower yield. It is not that the yield of the detonation is determinative, but rather that the altitude to which the fireball arises before its average density is equalized with that of the surrounding air determines the fallout rates. The megaton yield fireballs are so enormousthat they stabilize at levels only above the tropopause, the imaginary boundary layer dividing the upper part of the atmosphere, the stratosphere, from the lower part, the troposphere, while the kiloton yield fireballs stabilize below the tropopause. The tropopause normally occurs at something like 40,000- to 50,000-ft altitude, although it depends on season and location. In other words, low-yield bombsfired in the stratosphere would be expected to give the same slow fallout rates as high-yield weapons do whenfired in the troposphere, or on the surface if attention is focused on the part of the fallout which does not come down locally to form the oval shaped pattern pointed in the downwind direction. 254