760
MARTIN
Herbivorous mammals may be externally contaminated by direct
exposure to fallout or by contact with contaminated plants or soils.
Radionuclides may accumulate in animal tissues via inhalation, which,
in many cases, is relatively unimportant or via ingestion of contaminated materials. Although inhalation, ingestion of contaminated
soil or water, and ingestion of fallout particles while the animal is
preening cannot be
dismissed entirely, it is probably reasonable to
assume that externally contaminated plants are the major sources of
radionuclides for assimilation by herbivorous mammals (e.g., jackrabbits or dairy cattle) feeding in a fallout-contaminated environment
during the first 30 to 90 days after close-in fallout.
The summer of 1962 provided an excellent opportunity to study the
food-chain kinetics of Sr and ‘I in relation to desert shrubs and
rabbits in the Sedan fallout field. These studies included the formulation and testing of mathematical models that can be shown to provide
at least a partial explanation of the kinetic relations between initial
concentrations of Sr and '!I on fallout-contaminated plants and sub-
sequent concentrations of ®Sr in the bone ash or of '*I in the thyroids
of rabbits collected in the Sedan fallout field. Although the results of
these studies!!~!’ are not conclusive, partly because the causes of vari-
ation are imperfectly understood, they do provide evidence that math-
ematical models similar to those used by radiochemists to explain
decay-chain kinetics,'® by physiologists to explain tracer kinetics,'®
and by the International Commission on Radiological Protection (ICRP)
to establish maximum permissible concentrations’’ can also be used to
study food-chain kinetics under field conditions.
The objectives of this paper are (1) to summarize someof the data
related to ™Sr and "I on desert shrubs and in rabbit tissues following
fallout from Project Sedan, (2) to present the deterministic models that
provide a partial explanation of these results, and (3) to illustrate the
potential value of similar models in studying the food-chain kinetics of
radionuclides on pasture plants, in cow milk, and in human tissues
following a single fallout event.
METHODS
Project Sedan, a peaceful nuclear-explosives test, involved the
detonation of a 100 + 15 kt thermonuclear device at a depth of 635 ft in
alluvium and tuff at the north end of the Nevada Test Site on July 6,
1962. As predicted, the early fallout was relatively light and occurred
primarily within a 150-mile sector, N60°W to N60°E, from ground zero
in Yucca Flat.
Before and at various times after the detonation, plant samples
and rabbits were collected from representative locations in the Sedan
fallout field (see Fig. 2). Each sampling station was marked by a metal