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II. GENERAL The nature and magnitude of the fallout hazard to agriculture depends upon the chemical and physical properties of fallout, characteristics of the soil and land sur- face, and the type and density of vegetation, as well as upon the amount of fallout. Thus, the choice of reclamation and decontamination measures would also be influ- enced by these factors. The radioactivity in fallout is derived principally from fission products, and therefore depends on the fission yield of a nuclear explosion. If the fission yield gives energy equivalent to the explosion of one million tons of TNT, the gamma radiation activity of the fission products would be as shown in Table 1 (31). The beta radiation activity would be 2-20 times as great as the gamma activity (51), but unless it is in direct contact with the body, it is of less physiological significance. Alpha activity from unfissioned materials and the radioactivity from neutron- activated products is usually negligible by comparison. TABLE 1 Total Gamma Radiation Activity of Fission Products from a 1-Megaton Explosion (31) Time After Activities Explosion (Megacuries) 1 hour 300,000 1 day 6,600 1 week 640 1 month 110 1 year 5.5 The rate of decay of fission products is rapid at first and becomes progres- sively slower with increasing time after the explosion (Table 1). This change in rate of decay is caused by the presence of a mixture of short- and long-lived nuclides in fresh fission products. The known fission products include 170 isotopes of 35 elements, ranging from zinc-72 to terbium-161 (9). Some short-lived nuclides of importance in agricultural products are iodine-131, barium-140, and strontium-89. Many fission products of interest have radioactive daughters by decay. The chemical and biological properties of these daughter nuclides are different from those of their parents. If the half-life of the daughter is sufficiently great, its dis- tribution in the soil or plant depends upon its characteristics, not those of its par- ents. Some possible effects of this phenomenon are discussed in detail|elsewhere (47). The half-lives of 13 parent nuclides and 8 daughters are listed in Table 2.
TABLE 2 Half-Lives of Fission Products of Possible Significance in Food Chains and of Some Radioactive Daughter Nuclides Atomic Mass Parent Nuclide Daughter Nuclide Number Element Half -Life Element Half- Life 91 Strontium 9. 7 hours Yttrium 58 days 131 Iodine 8. 0 days 140 Barium 12. 8 days Lanthanum 40 hours 86 Rubidium 18. 6 days 141 Cerium 28 days 103 Ruthenium 41 days Rhodium 5. 4 minutes 89 Strontium 54 days 95 Zirconium 65 days Niobium 35 days 144 Cerium 275 days Praseodymium 17 minutes 106 Ruthenium 1. 0 year Rhodium 2 hours 147 Promethium 2. 3 years 137 Cesium 26. 6 years Barium 2. 6 minutes 90 Strontium 27.7 years Yttrium 64 hours The discussion of possible fallout patterns is beyond the scope of this report, but it should be stated that fallout distribution depends on many parameters. These include meteorological conditions, yield of the explosion, elevation of the burst, and the nature of the terrain. The fallout from a particular surface nuclear explosion may be classified in four categoriesâdropout, close-in, tropospheric, and stratospheric. These cate- gories differ in distance and time from the point of detonation. Dropout occurs at or very near ground zero, where the prompt effects of the burst are greatest. Close-in fallout consists of solid particles settling to earth under gravity within a few hours after the explosion. It may extend several hundred miles downwind from the site of a large nuclear explosion. Tropospheric and stratospheric fallout con- sists of very small particles which may remain suspended in air for a long time. The scavenging action of precipitation is important in bringing these particles to earth. High concentrations of radioactive materials are found in areas receiving close-in fallout, and their subsequent distribution in soils and crops is therefore of special significance. Yet, it may be possible to take remedial actions in these areas, whereas such actions might be precluded in areas affected by dropout be- cause of the vast physical destruction. Less than one fourth to more than one half of the fission products formed in a nuclear explosion at or near the ground surface may return to earth as close-in fall- out (51; 135, pp. 105-106). If early rain is associated with the fallout cloud, the amount of close-in fallout increases. Explosions that are so high that the fireball does not touch the ground may produce little close-in fallout.
The fate of the radioactive isotopes in deposited fallout depends on the physical properties of the fallout and the chemical behavior of the nuclides. Surface bursts in the kiloton range, over continental soils, yield predominantly siliceous radioactive particles (3, 83, 92). Particles from tower bursts in the same energy range reflect the incorporation of tower materials (83). Megaton bursts over coral islands have produced primarily calcareous particles (92). It has been reported that the dust from the Castle Bravo burst of 1954 was mainly calcite. Presumably, aragonite was evap- orated, recrystallized as calcite, and precipitated as aggregates (44, 126). This wide range in gross chemical composition, considered along with the observed range of particle sizes, leads to the conclusion that the biological availa- bility of the constituent radioactive isotopes cannot be predicted for a particular ma- terial without some knowledge of its characteristics. The solubility in distilled water of selected particles from a continental detonation ranged from 0. 28 to 1.2 per cent of the total radioactivity. One to 74 per cent was dissolved in 0. 1 N_ HC1 (83, 51). In another study (8), it was found that the solubility in 0. 1 N HC1 of deposited particles from four tower shots ranged from 20 to 30 per cent and that of airborne particles from 65 to 85 per cent. Some of the nuclides of agricultural importance, notably strontium-90 and cesium-137, may be partially depleted in the local and close-in fallout. This frac- tionation results from the fact that precursors of these nuclides are noble gases early in the condensation of fallout particles (136, p. 72). A major part of the biological experimentation with fallout constituents has been conducted with soluble sources of the respective isotopes. Consequently, the observed effects exceed those that would be obtained from the same amount of the isotope in the less soluble fallout. It is presumed that the use of soluble sources generally provides maximal effects. In addition to the variability in the composition and solubility of fallout, the soil and plant aspects of the food chain contamination are complicated by variations in soil properties and differences in the structure and physiology of plant species. This will be the subject of discussion in the following sections.
