General Considerations 



11 



large, megaton devices, half or more of the total 

 fission products are injected into the strato- 

 sphere from which there is a slow leakage into 

 the troposphere (of the order of 10 per cent 

 per year) and subsequent fallout fairly evenly 

 over the entire northern hemisphere, with lesser 

 amounts in the southern hemisphere (Libby, 

 1956a, b). Of this long-term fallout, up to 71 

 per cent falls on the oceans, since this is the 

 proportion of the earth's surface covered by 

 them. (The proportion of land to sea is higher 

 in the northern hemisphere than in the south- 

 ern, and since most of the long-term fallout 

 occurs in the northern hemisphere, the amount 

 entering the ocean will be less than 71 per 

 cent.) On the other hand, some of the fallout 

 on the land will eventually reach the sea 

 through land drainage or river runoff. 



Except in the case of deep underwater bursts, 

 all of the fission products reaching the sea 

 from weapons tests are deposited in the upper 

 layer of the ocean. Removal into the deeper 

 water is relatively slow. Despite the rapid mix- 

 ing within the upper layer by vertical and hori- 

 zontal wind stirring, the products from a large 

 weapon remain in measurable concentrations 

 over many months. A survey made 13 months 

 after the 1954 weapons tests in the Pacific 

 showed low-level activity over a vast area (Har- 

 ley, 1956). 



Radio isotopes in fallout on the land remain 

 largely in the upper few inches of the soil. Fall- 

 out on the sea, on the contrary, is rapidly dif- 

 fused through the upper mixed layer, some 75 

 meters deep on the average. Consequently, for 

 conditions of equal fallout, the concentrations 

 of radio isotopes in the part of the sea from 

 which they are taken up by man's food organ- 

 isms are less than in the soil. Thus, even 

 though the calcium concentration of sea water 

 is lower than in most soils, the ratio of stron- 

 tium 90 to calcium in the marine environment 

 is now much less than in agricultural lands of 

 the mid-western United States. In 1955 (Libby 

 1956b) these soils contained about .025 micro- 

 curies of strontium 90 per kilogram of calcium 

 available to growing plants. Revelle (1957) 

 has calculated that for an equal amount of 

 widely distributed fallout (from approximately 

 25 megatons TNT equivalent of fission) about 

 .00015 microcuries of strontium per kilogram 

 of calcium would be present in the upper mixed 



layer of the sea, half of one percent of the 

 amount in soils. 



In addition to fission products, neutron ir- 

 radiation of elements in the environment im- 

 mediately after the detonation produces other 

 radioactive isotopes. With ordinary land or 

 marine materials, the amounts of this neutron- 

 induced radioactivity are small (Libby, 1956a). 

 However, soon after the 1954 tests in the 

 Pacific, quantities of zinc 65 were discovered in 

 marine fishes, and subsequently cobalt 60 was 

 recovered from clams in the Marshall Islands. 

 These isotopes probably originated from neu- 

 tron irradiation of metals, other than the fis- 

 sionable materials, in the test device. 



Comparison of table 2 and table 4 demon- 

 strates that the mass of radioactive isotopes in- 

 troduced into the sea from weapons tests, or 

 which might be introduced from disposal of 

 waste products, will be very small compared 

 with the amounts of their normal isotopes al- 

 ready present. The introduction of the radioac- 

 tive material does not, therefore, appreciably 

 modify the chemical and physical properties of 

 normal seawater, so that the chemistry of the 

 introduced radioactive substances is the same as 

 for the corresponding non-radioactive isotopes 

 in the sea. 



Introduced radioactive isotopes will partition 

 into a soluble and an insoluble fraction. The 

 physical states of a given element under equi- 

 librium conditions depend upon whether or not 

 the solubility product of the least soluble com- 

 pound has been exceeded. Since the ionic ac- 

 tivities of the elements in the complex chemical 

 mixture that is sea water are not accurately 

 known, it is difficult to attack this problem from 

 theory. Greendale and Ballou (1954) have de- 

 termined the distribution among soluble, col- 

 loidal, and particulate states of important fission 

 product elements by simulating the conditions 

 of an underwater detonation; their results are 

 given in Table 5. Elements of Groups I, II, V, 

 VI and VII usually occur as ionic forms, while 

 other elements, including the rare earths, occur 

 as solid phases. Some of these results have been 

 confirmed by field observations following weap- 

 ons tests (see Chapter 6 of this report by Carritt 

 and Harley, and Chapter 7 by Krumholz, Gold- 

 berg and Boroughs). Those elements in Table 

 5 that have sufficiently long half-lives to con- 

 tribute a significant share of the total activity 

 after one year of decay are marked with an 

 asterisk. Cesium 137 and strontium 89 and 90 



