234 Health Physics estimate” values are needed to calculate realistic population-average or typical mdividual doses The purpose of this paper 1s to (a) review and evaluate historical data on the physical and chemical characteristics of radionuclides associated with fallout debris includmg fractionation, solubility, and other fac- tors which may affect the1r absorption by humans, (b) review and evaluate f, values for key fallout radionuchdes from the available hterature as they pertain to fallout m general and to the unique coral environment of the Marshall Islands, mm particular, and (c) provide an evaluation and present our “best estimates” for f, values for use im reconstruction of realistic mternal doses from acute imtake of local and regional fallout from nuclear tests, and for chronic intake of radioac- tive materials following their mcorporation ito terrestrial and aquatic foodstuffs In this paper, local fallout 1s assumed to have occurred at close-in locations that are typically associated with large particle sizes (250 jum) and extend roughly up to 300 km from ground zero, whereas regional fallout that occurs at distant locations 1s composed of only smaller particle sizes and 1s assumed to extend from 300 to 3,000 km from ground zero Justifications for the choice of f, values for fallout radionuclides were made takmg mto account the particular circumstances of various nuclear test conditions and the expected nuchdes’ solubility and biological availabil- August 2010, Volume 99, Number 2 BACKGROUND AND METHODS Unique characteristics of the Pacific Proving Ground The basic components of topsoil at coral atolls lke the Marshall Islands are coral-based sand, hard and weathered coral, and marme shells Coral-based soil 1s typically low m organic matter, potassium, clay, and many other nutrients, but ennched m lime (CaCO,) content (Srmon et al 2002) In the case of detonations over coral or coral-based soils, calc1um carbonate m the ground is heated to very high temperatures or completely vaporized, resultmg m the formation of relatively fme particles When such particles mix with bomb debris m the cooling fireball, they adsorb fission radionuchdes within and on the surface of particles (Norman 1973) Fresh coral debris 1s largely CaO, CaCO,, and Ca(OH), (Herdt et al 1953, Maxwell et al 1955) and, hence, raises the pH considerably (pH 10-11) when it dissolves m water (Crocker et al 1965) The maximum soil adsorption and retention of Sr, Cs, Y, Ce, and Pu also occurs at this high pH range In addition, me rich soil may further mcrease the retention of Sr and Cs (NAS 1963) In fallout particles from water-surface tests, radioactivity 1s carned m water droplets which form a cloud of mustafter detonation In the presence of sea water, some fission products may be fused in sodium oxide particles or precipitated as sulfate ity, whenever such mformation was available Informa- on particle surfaces (discussed im detail later) For tests conducted on barges or towers, iron from the steel review of the available fallout hterature, declassified mixed iron, calcium, and fission-products oxides (Adams tion presented m this paper resulted from a thorough government documents, and various reports from national and mternational orgamzations Although emphasis wasgiven to nuclear weapons testmg conductedat the Marshall Islands, mformation provided here may be applicable to fallout from tests or detonations conducted elsewhere A large numberof fission and activation products are formed m nuclear weapons tests (Hicks 1981) Most of them are either produced m low yield or have short half-lives Key fallout radionuclides that contribute substantially to the mternal dose to humans resulting from nuclear tests conducted m the Marshall] Islands are isotopes of iodine (I), tellunum (Te), cestum (Cs), strontium (Sr), yttrum (Y), molybdenum (Mo) barium (Ba), lanthanum (La), cerium (Ce), praseodymium (Pr), iron (Fe), cobalt (Co), zme (Zn), and neptuntum (Np) (Simon et al 2010) Plutomum (Pu), americ1um (Am), uramum (U), zirconum (Zr), ruthemum (Ru), rubidium (Rb), and bismuth (B1) are among other elements of terest present in radioactive fallout Review and evaluation of f, values for the above elements from exposure to fallout are addressed im this document structures can mcorporate m fallout particles to form et al 1958) Small fallout particles from coral islands were found to be 10-50% water soluble and almost completely soluble m acids (Cohn et al 1954, Herdtet al 1953, and personal experience) Fractionation, solubility, and bioavailability of fallout debris Fractionation and solubility of fallout debris are important phenomena because of their relationship to bioavailabihty and the absorption of various radioactive species by humans The fractionation occurs due to both physical and chemical separation of fallout particles and/or fission products The degree of fractionation 1s a function of many mteracting factors mcludmg the physical and chemical properties of nuchdes formed m the fission process as well as those of progeny elements ma given decay cham, particle size and composition, distance from ground zero, the specific conditions of nuclear detonation (weapon composition, yield, burst height, meteorological conditions, etc ), and the propertes of the surroundmg environment after the deposition of fallout particles (Tompkins and Wermer 1955, Beck