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. 



