Alimentary tract absorption @ S. A. IBRAHIM ET AL. and Bennett 2002). In general, fractionation decreases with the magnitude of the device yield and increases with proximity of the detonation to the ground surface. Radionuclides will fractionate to varying degrees due to differences in fission products’ volatilities. In the fireball, bomb material (FeO) and earth/water-surface components form molten droplets in which refractory elements dissolve and distribute uniformly through the particles. As the fireball cools (below 1,500°C) and the particles form and solidify, the more volatile elements and their progeny condense anddistribute according to the available particle surface, where the amount of activity varies with 7° (Adamset al. 1958; Crockeretal. 1965; Hicks 1982). The degree of fractionation can then be defined as the ratio of refractory (R) to volatile (V) nuclides, where an R/V value of | represents the relative ratio of unfractionated debris (Beck et al. 2010). Various particles in the debris can also agglomerate to form a complex heterogeneous mix of particles of different shapes and sizes (Norman and Winchell 1970; Vajda 2001). *’Sr, '°’Cs, and *’Sr are examples of radionuclides that, having gaseous precursors, are subject to additional fractionation and, thus, are partially depleted in local fallout occurring at close-in locations compared to other volatile nuclides. **Zr and '“Ce are refractory in fallout and are frequently used as reference radionuclides to determine the degree of fractionation for other nuclides. In general, volatile radionuclides exhibit a high degree of solubility relative to the refractory radionuclides. In terms of particle size distribution, the fallout process hasthe effect of sorting the debris by diminishing particle size along the forward path of the cloud. Consequently, the refractory and volatile phases are also partially separated over the initial period following detonation. It has been observed that fallout particle size and total radioactivity are correlated and both decreased with distance after the 1954 Bravotest at Bikini Atoll (Lessard 1986). About 20-30% of the total fallout activity is deposited at close-in locations and is mostly associated with large particles dominated by the refractory phase. About 90% of the more volatile nuclides deposit on small particles at more distant locations. This is particularly evident in high-yield explosions with a high cloud top, often moving at high velocity due to upper atmosphere winds. For low-yield explosions, at low cloud height and low windvelocity, a significant fraction of small particle size can deposit near ground zero. As with distance, time of arrival of fallout may also be a surrogate for change in particle size (Simon 1990; Anspaugh 2004). Detonations at altitudes sufficiently high to eliminate incorporation of ground soil into the fireball tend to produce small, spherical and highly radioactive particles with activity distributed more evenly throughoutparticle 235 volume. These particles tend to be more soluble than for ground surface detonations (Crocker et al. 1965). If soil and other on-site materials are incorporated in thefireball, the resulting particles tend to be large in size, of irregular shape, and of low specific activity. Solubility and leachability of radionuclides from fallout particles are governed by several factors including the following: a) the physical and chemical characteristics of the host particle (size, surface area, surface chemistry, surface orientation of radionuclides, age of fallout material, etc.); b) radionuclide speciation and oxidation states; c) conditions associated with detonation including the degree of fractionation during particle formation; and d) environmental factors including soil/ water chemistry. The most extensively investigated solvents used in leaching studies have been water, sea water, and 0.1 N HCl (Crocker et al. 1965). Dilute HCl is generally considered as a simulant for stomach fluids. Data on leaching of fallout debris have usually been reported in terms of the fraction of gross beta or gross gammaactivity found in the soluble phase. Results in terms of individual radionuclide solubility are much more useful, but these data are very scarce (Anspaugh 2004). Solubility of fallout particles in HC] at pH 1.0 from the NTS was greater for smaller particles (LeRoyetal. 1966). Reducing particle size by approximately one-half resulted in an increase of the fraction leachedactivity by a factor of two (Table 1). Also, acid leaching was much more effective in dissolving fallout particles than was water or alkaline solutions and solubility increased with multiple extractions and the duration of contact. Measurements of total gammaactivity leached by water from coral samples under different test conditions were reported by Crocker et al. (1965) and the information is summarized in Table 2. In surface-water tests, fallout particles were much more water soluble than from ground(coral)-surface tests by approximately an order of magnitude. Data from the NTS (Baurmashetal. 1958) on fallout particle solubility in 0.1 N HCI showedthat, in tower tests, when the fireball interacted with surface material, the soluble fraction of the formed material was much lower (0.20—0.30) than for higher altitude air Table 1. Solubility of total radioactivity from local fallout as a function of particle size; data from the Nevada Test Site (NTS) on particles collected six days after detonation (from LeRoyet al. 1966). Particle size (zm) Fraction of activity leached in HCl at pH 1.0 ~500 180 90 ~0.06 0.19 0.42