CHAPTER 3
EFFECTS OF SOIL TYPE AND BUILDING MATERIAL ON THE PORTION
OF RADIOACTIVITY DEPOSITED IN LOCAL FALLOUT
The effects of soil type and building material on the amount and
levels of radioactivity deposited in local fallout are of three types:
(1) induced activities can enhance the radiation levels over those of
fission products alone, and can change the shapeof the decay curve;
(2) the substrate material can influence fractionation effects among
the fission products through variation of melting and vaporization
temperatures and chemical reactivity with the various radionuclides;
and (3) differences in particle size, density, and other physical
properties of the soil or matrix material can influence specific
activity/particle-size distributions and hence fall-rate variables.
U. S. experience and hence data are limited to three substrates;
Nevada dry desert alluvium, wet coral rock and sand, and sea water.
In addition there is the experience with tower bursts (mostly steel,
but occasionally of aluminum or wood) referred to in Chapter 2.
Comparative analyses of the data are difficult because detonations
on coral have generally been larger in yield than those over Nevada
soil; the larger yield detonations were also in tropical atmospheres
having much higher humidity and other differences from temperate-zone
‘{—
atmospheres.
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Extensive work in the days of atmospheric testing 1-5 indicated
that the principal soil constituents that influence the production of
induced activities are sodium, aluminum, iron, and manganese contained
in the soil (these being the principal elements activated by bomb
neutrons), and the water content of the soil (a non-activated competitor
for neutrons).
The longer-lived of the first two induced activities,
Na-24, has a half life of only 15 hours, so it significantly affects
decay only up to about 4 days after the detonation.
It has, however,
a penetrating component of radiation that makes its presence important
while it does last. At later times, Fe-59 and Mn-54 may be found in
relatively large amounts in fallout from large-yield detonations.
In general, soil-activation products are not important contributors
to local fallout except for weapons with very low fission-fusion ratios.
With respect to the effects of soil type or substrate, no definitive
sets of data are available for comparing gross radioactivity/particlesize distributions of the fallout from comparable detonations on
dry desert soil and wet coral.
In pkinciple, the fallout from detonations
over deep water or over wet substrates such as coral should have a
smaller K-factor (Ky)
than fallout from detonations on dry soil, since
the radionuclides would be carried initially by smaller particles and by
particles with variable fall rates due to evaporation and condensation
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