Assume:
(1) Assumptions (1)-(8) for Sr-90.
(2) Period between fallout and growth into plant for human uptake is 2 half-lives
oe yoy SOUT
b. Sr-89 (calculations for infant):
(PE-=factor of 2).
(3) Total skeletal weight of infant is average of at least 5 times increment of skeletal
weight during period of dose (PE—factor of 1.5). For a 6 month fetus or young infant,
However, for these conditions an extra discrimination against
Sr is present in the placenta (for rats, a factor of 2) or in the predominance of milk in the
diet. A much greater dilution would take place for adults.
The infinity dose received by an infant for a fallout of 1 KT/mi? is shown in Table 5.
c. Other fission products.—The plant uptake of all other fission products is so low, or their
half-lives are so short, that they represent a much smaller hazard than the Sr isotopes.
3. Debris ingested after direct fallout on leaves:
One important parameter required in this calculation is an estimate of the fraction of
fallout retained on the leaves. This may be expressed in terms of the square feet of fallout
retained per pound of vegetation, or ft?/lb.
There are several pieces of experimental data
which bear on this parameter, although they do not support a reliable calculation. In the following computations, a basic assumption is made that the activity is retained on or in the
vegetation for 20 days (PE=factor of 3).
The alfalfa analyzed by Libby showed an average Sr-90 activity of 20 dpm Sr-90/gm Ca,
or about 180 dpm/Ib dry vegetation. This represents accumulation approximately through the
latter 24 of September, during which very roughly 20 dpm/ft? Sr-90 is thought to havefallen
out.
This gives 180/20=-9 ft?/Ib.
Grass measured under similar conditions by Harley gave a value of 3 ft?/Ib.
The sheep and calves analyzed for Sr-90 (Appendix C) integrated the fallout over the
summer of 1953. Making several rough assumptions about fallout, sources of Ca in diet, etc.,
an average value is obtained of 5 ft#/Ib.
The cattle and sheep analyzed for I-131 in June, 1954, led to very rough values of 1 ft?/Ib.
From these figures a value of 4 ft?/Ib (PE=-factor of 3) is selected as the basis of further
computations.
This value corresponds to 10% retention by the vegetation on a typical pasture
with a stand of 1000 Ibs/acre at time of fallout.
a. Sr-90 (calculations for infant or child):
Assume:
(1)
(Sr-90/Ca) food
=-0.4 (PE=factor of 2) as a result of selection against Sr
(Sr-90/Ca) vegetation
in milk component of diet.
(2) Ca content of vegetation is 0.5% on dry weight basis (PE—factor of 1.5).
(3) Intake extends through period of 20 days (PE—factorof 3).
(4) Biological half-life long compared with radiological half-life. (Most of excretion
which takes place occurs within a few weeks or months of ingestion. Assumptions (5) and
(6) on retention apply to long term retention.)
(Sr/Ca) skeleton
(5) ”(Sr/Ca)
food ==0.5 (PE—factor of 1.5).
(6) Skeletal Ca=15% of skeletal weight.
18
(PE-=factor of 1.2).
:
LINEG
there would he less dilution.