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


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
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



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