rainwater with the soil.
This interaction creates conditions of pH,
mineral content, and organic content of the water, some of the organic
components possibly having a complexing capability, The second phenomenon
is the solubilization of radionuclides as a result of their direct interaction with vegetation or body fluids.
This is a situation which is
extremely difficult to simulate in the laboratory, whereas simulation of
the interaction of ‘fallout with soil and rain can perhaps reasonably be
simulated in environmental chambers or otherwise (Larson et al., 1960).
The reported solubility studies show how the various investigators have
struggled with these problems.
Solubilities in water and in 0.1 N HCl
have been used most often as indicators of biological availability (Tamura,
1976), but other liquids have also been used, such as nitric acid, sodium
thiosulfate, citric acid-sodium phosphate buffer, seawater, ammonium
oxalate, chelating agents (EDTA and others).
Various types of experiments
also have been conducted:
leaching through columns, leaching in a static
system, series leaching, leaching of single particles on watch glasses,
even a single experiment in situ on the Test Site. Variables that have
been considered are shot environment, particle size, activity per unit
weight, distance from GZ. The measurements that were made were almost
exclusively gross activity measurements with a determination of the
fraction of the activity that was solubilized.
Rarely were analyses
performed for individual nuclides such as barium-140 and/or strontium89,90.
Larson et ai. (1960) has summarized significant findings from experiments
with Nevada Test Site fallout during the decade prior to 1959.
Debris
from balloon shots appears to be more "soluble" than debris from tower
shots; smaller particles are more "soluble" than larger particles.
Quantitative numbers on the fraction of activity leached are quite variable.
It would appear that leaching or solubility experiments that provide data
only in terms of gross activity are not very useful. The chemical characteristics of various nuclides and the distribution of nuclides in and on
the particles will result in different dissolution rates and different
solubilities. For example, incidental data obtained at our laboratory
with cloud samples from ground-surface bursts showed strontium-90 to be
mote leachable than promethium-147.
il.
Radioactivity. The prime concern for local fallout from shots was originally the external radiological hazard of the fallout field over a period
of time after each shot that such hazard may have been significant.
Thus, measurements were usually restricted to beta and gamma radiation
and most of the data generated before the early sixties are of only
limited use for the assessment of hazards that are now present on the
test sites, these hazards being associated with the presence of only a
very few long-lived nuclides, particularly alpha-emitters. Alpha-emitting
daughter products such as americium-241 were not considered,
240
Data are often reported as specific activities, that is, as counts or
disintegrations per minute per unit volume (um) or per unit mass (g).
The measurements were sometimes made on samples as collected (or size-separated) or on single particles.
In the latter case, a modal activity or
an average activity {s calculated or determined graphically from frequency
curves.
Some standardization was effected by reporting, when possible, .
the activities at T + 25 days (600 hours).
This time was chosen to
allow the decay of variable quantities of induced activities.
The measurements of gross activities are of some use in one respect;
where size fractions or individual particles are involved, some average
behavior of activity with particle size is indicated, or a yield-dependence
of radionuclide concentrations can, and sometimes had been, inferred.
Thus, a large amount of data have been reported and plotted, mostly in
the form of Frequency plots, showing gross activity as a function of
particle size.
For one or two tests, fallout particles between 300 and
2,000 um were separated by morphology into five classes and their activity
was measured and plotted as a function of size.
Data for individual
radionuclides were not reported with very few exceptions, and then mostly
in the form of fissions per gram (equivalent to molybdenum-99 concentrations), or as strontium-89, barium-140, or induced activity data.
After the early sixties, a different approach was taken to the analysis
of fallout and cloud samples (Nathans, 1970).
The isolation and analysis
of individual particles from shots of interest was virtually abandoned.
The limitations of gross activity measurements was more realized in the
experimental programs and individual radionuclide analysis was emphasized
more.
Thus, a more fundamental approach was being used with the expectation
that as a result, particle and radionuclide behavior could be generalized
into a predictive system for various types of shots.
This approach was
justified also by the improvements in computer hardware and software
capabilities, allowing much more sophisticated transport and fallout
models to be developed. An example is DELFIC (DOD Experimental Land
Fallout Interpretive Code (1966)) which considers cloud rise, transport,
and settling, under different meteorological conditions including rainout.
As a result, shots of interest such as Johnie Boy, Sedan, and others were
analyzed in much more detail than had been done before.
In addition,
many shots from the fifties were revisited.
Thus, cloud samples from
virtually all ground-surface shots and from many tower and balloon shots
were reanalyzed in considerable detatl, fallout samples not having been
available.
During this work, samples were separated into size fractions.
Radionuclide specific abundance was determined as a function of the
particle size, but related only to the volume or mass of radioactive and
nonradioactive particles together,
In the case of these old shots, the
data acquisition was obviously limited, because of decay, to only a few
nuclides.
241