TABLE 2 



Half-Lives of Fission Products of Possible Significance in 

 Food Chains and of Some Radioactive Daughter Nuclides 



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 categories — dropout, 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. 



