50 



10 



< 



>10 Mt - 



1 to 10 Mt - 



0.1 to 1 Mt - 



0.02 to 0.1 Mt 



WORLDWIDE FALLOUT 75 



241pu/239,240pu 



• , Monthly samples 

 O , Yearly averages 



Denotations 



United States 



Union of Soviet Socialist Republics 



— Chinese 



I 



I I I 

 I II 

 i JJ 



I 



I 



I 



I I 

 1 .1 



I — 



1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 



YEAR 



Fig. 10 Concentration ratio of ^^ ' Pu and ^ ^ 'Pu in air at Richland, Wash. 



The most abundant plutonium isotope produced during nuclear detonation is the 

 weak beta-emitting '^^^Pu. The atmospheric concentrations observed in air at Richland, 

 Wash., from 1963 to 1972 ranged from 20 (d/min)/10^ m^ at standard temperature and 

 pressure in 1963 to 0.7 (d/minVlO^ m^ in 1972, whereas the ratio of ^"^^ Pu/^^^'^'^^Pu 

 was about 15 (Thomas and Perkins, 1974). These data are summarized in Fig. 10. 



Americium-241 , which is the daughter of ■^'^'Pu, in global fallout can be estimated 

 from the plutonium isotopic composition data. Americium-241 is an alpha emitter with a 

 half-life of 433 yr. It is a bone seeker, and, on the basis of the International Commission 

 on Radiological Protection (ICRP) maximum permissible concentrations in air and water, 

 its toxicity is comparable to that of ^^^Pu. In the nuclear power industry, ^'^^Am is 

 particularly important because it is a relatively large contributor to the total alpha 

 activity of higli-burnup nuclear fuel (Thomas and Perkins, 1974). Like ^'*°Pu, most of 

 the ^'^'Pu in large nuclear weapons test debris is produced in the detonation. By making 

 appropriate weighting and radioactive decay corrections, including in-growth from 

 parent— daughter relationships, the ^"^ ^Am/'^^^ '■^'*°Pu activity ratio of integrated global 

 fallout in February 1974 was estimated to be 0.22 (Krey et al., 1976). Two soil samples 

 analyzed for ^"^^Am fallout (Krey et al., 1976) gave ^^lAm/^^^'^^^Pu ratios of 0.25 

 and 0.22. 



Further calculations of the parent— daughter relationships indicate that the ^"^^Am 

 content of present integrated fallout in soil will peak in 2037 and will represent 42% of 

 the '"^''^•^Pu activity. 



The distribution of the ^^^Pu/^^^Pu atom ratio in soil is shown in Fig. 11. Similar 

 patterns emerge from the ^^^ Pu and ^^■^Pu data. There is a marked reduction in the ratio 

 in the southwestern United States and along the west coast of South America owing to 



