Table 1— POTENTIALLY HAZARDOUS RADIONUCLIDES IN FALLOUT FROM NUCLEAR DETONATIONS Fission* Radionuclide Type of radiation abundance, % Radiol. half life Pua Sr*? Cs Pm‘? Celt a B By B Bay 5.0 6.2 2.6 5.3 24,000 yr 27.7 yr 26.6 yr 2.64 yr 285 day yi B Bal? ps! By By zs sr Nb" By B By 6.4 5.9 4.6 6.4 6.0 2.8 65 day 58 day 51 day 35 day 13 day 8 day Totalt production, megacuries 0.3 5.5 7.2 30 200 1100 1150 950 2000 5000 4000 Abs. on ingestion, % Body MPL, pe 3x 103 30 100 1x 107? 1x 107? 0.037 1.0 54 60 5 1 x 107? 1x 107? 30 1x 107? 5 100 26 5 4 76 4 0.7 *Slow neutron fission of U™*; abundance in weapon debris is somewhat different. { Total initial activity in megacuries produced by all weapons tests to mid-1957. fission by neutron interaction with bomb components. Since 1 kiloton of fission yield is pro- duced by 1.4 x 10° fissions,® each of which results in the production of a Pu’** atom, 55 mega- tons of fission would produce 0.2 megacuries of Pu?**_ Other isotopes of plutonium, when con- verted to equivalents of Pu*®®, bring the total production to about 0.3 megacurie equivalents. The production values given in Table 1 are not a measure of the relative biological importance of the various nuclides, but merely provide some general idea of the relative initial activities produced by all weapons tests through mid-1957. Development of sufficiently sensitive detec- tors should result eventually in detection of most of these radionuclides in foods and man. Strontium-90 (references 5 and 6), Cs'*" (reference 7), and I'*! (reference 8) have been meas- ured quantitatively in the human body, and the presence of Cein pooled urine samples has been reported.® In addition, Ba!° (reference 10) and Sr®™(reference 11) have been observed in milk, and other radionuclides have been detected in air and other materials composing man’s environment. The extent to which they pose a potential threat to man’s health and well-being depend on their rate of production and on their individual physical and biological properties. 3 3.1 DISTRIBUTION OF FALLOUT FROM NUCLEAR DETONATIONS Postulated Mechanisms of Distribution Libby®’was first to propose a model explaining fallout and distribution of atomic debris from nuclear weapons detonations. His model is based on three kinds of fallout —local, tropospheric, and stratospheric. Local fallout is deposited in the immediate environs of the explosion during the first few hours. This debris consists of the large particles from the fireball and includes partially or completely vaporized residues from the soil and structures which are swept into the cloud. Tropospheric fallout consists of that material injected into the atmosphere below the tropopause which is not coarse enoughto fall out locally. This debris is sufficiently fine that it travels great distances, circling the earth from west to east in the general latitude of the explosion, until removed from the atmosphere (with a half-time of 20 to 30 days) by rain, fog, contact with vegetation, and other meteorological and/or physical factors. Stratospheric fallout is composed of fission products that are carried above the tropopause and can result only from large weapons (of the order of 1 megaton and greater). Libby!3)> has postulated that atomic debris, once it is injected above the tropopause, is mixed rapidly throughout the stratosphere and falls back uniformly into the troposphere with a half-time of about 7 years. As it returns to the troposphere, it is deposited over the earth’s surface in relation to meteorological conditions. He attributed the higher Sr” soil concentrations in the rn 284