Fig. 10 DEPENDENCE OF RADIATION LEVELS UPON DISTANCE ALONG THE CENTERLINE OF THE FALLOUT PATTERN FOR DIFFERENT UNDERGROUND CRATERING EXPLOSIONS. The distribution with depth of radioactive isotopes in Test "1003" was analyzed by a layer-by-layer selection of soil samples with successive gammaspectrometric and radiochemical analyses in the laboratory. Figure 11 is a typical curve which characterizes the decrease in activity of radioactive products A(z) with depth, z, within the throwout zone. It can be seen from the figure that at a depth of around 1 meter, the activity has decreased approximately 100 times. This fact must have an important practical significance. On the average, for the throwout of the test "1003", 90% of the activity in the soil's upper layer is contained in its upper 52 centimeters. Decrease in activity is characterized by an exponential function: The total amount of activity in the surface layer of the throwout from the test "1003" comprises 10-15% of that formed. The amount of individual isotopes in this layer of earth is shown in Table 5 (in %). It can be seen from this table that the throwout zone has a scarcity of isotopes having gaseous precursors (89Sr, Sr, and 140 Ba). 2. Stream of radioactive gases In studying the radioactive atmospheric fallout caused by underground nuclear tests it should be considered that radioactive products, during the process of radioactive decay, "yield" isotopes of noble gases (133 Xe, 135Xe and others) and isotopes of iodine (1311, 1331, 1341, and 135I). As a result, a stream of radioactive gases with iodine admixtures is propagated in the atmosphere in the direction the wind is blowing. In view of the fact that the majority of fissionfragment products is found in the crater and the throwout zone, the crater may be considered a zone of discharge for a fundamental part of the radioactive gases. - Besides the isotopes listed above, such isotopes as 88Kr 88 Rb, 89Kr, 85mKr, 138 Cs, 85Kr may appear in the stream for up to 24 hours after the ex 138Xe plosion. The stream of radioactive gases and volatile elements after test "1003" was studied by successively sampling it at various altitudes with aircraft equipped with roentgen-meters and sample selection apparatus (filters impregnated with silver nitrate and activated charcoal in cartridges). Sampling was conducted over a time interval from 4 to 98 hours after the explosion. As a result of the gamma-spectrometric research on samples selected from the stream, it was discovered that several hours after the blast the iso topic make-up of the radioactive gases was represented practically entirely by a sum of xenon isotopes (133Xe, 135Xe) which were found throughout the sampling period in a ratio close to the computed one (i.e., obtained in calculating a nonfractionated isotope mixture). The 1311 and 1331 isotopes were also present in the stream, but their quantity was approximately 103-104 times smaller as measured by their activity. The altitude of the stream's upper layer varied on different days from 350 to 900 meters. The maximum radiation level was registered 4 hours after the explosion at an altitude of 50 meters, registering 2 mr/hr at a distance of 15 km from the epicenter. As a characteristic of the total amount of escaping gases it may be stated that the flow rate of the radioactive gases 74 hours after the explosion was 1260 Curies/hr. 3. Properties of radioactive aerosols contaminating the area in the case of underground cratering explosions The properties of radioactive aerosols formed in nuclear explosions are determined in large part by the conditions of particle formation. For underground nuclear cratering explosions the conditions for radioactive particle formation are considerably more complex than for aerial or groundlevel explosions. With such explosions there is no single prevailing mechanism for forming particles. Particles in this case may, on the basis of their fundamental properties, be divided into several groups. The portion of each such group is decided by a whole series of conditions; in particular the time, for example, which passes from the moment of detonation to the start of ejection of the pulverized rock out of the ground. Radioactive dust which falls in the throwout region in the wake of cloud and dust column movement, and also in the base surge propagation zone, is comprised of particles of rock pulverized by the explosion and subjected to various thermal effects. From this viewpoint such particles (discovered in the area of the "1003" explosion) may be tentatively classified into four groups. The first type consists of irregularly shaped fused particles, basically transparent and glass-like (Figure 12). In these particles there is a large quantity of occluded gases. Their specific gravity is therefore 1.1 to 1.7 times lower than that of neutral rock particles from the blast zone. The second type is particles which have not been entirely fused, and conglomerates consisting of several fused particles attached to a neutral particle carrier. Particles of this type are also irregularly shaped, but their activity is several times lower than that of particles of the first type. In outward appearance, particles of the third type do not differ from particles of neutral pulverized rock from the blast zone. Particles of the fourth type are fused or melted, globular or drop-like in shape and of various colors (transparent and dark with a shiny surface). In external appearance and structure they are analogous to particles characterizing ground-level explosions. Particles of these four types are unevenly distributed in various zones of radioactive fallout. Thus, the basic fallout zone is enriched with particles of the third type, while the dust column pattern, and especially the cloud pattern, are enriched with particles of the first, second and fourth types. The dispersed composition of radioactive dust falling in various zones of the contaminated area and at various distances from the explosion's epicenter can be sufficiently well approximated by the standard logarithmic rule. The distribution parameter—the median diameter of the particles—diminishes with increasing distance from the epicenter. However, in various fallout zones |