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UPTAKE OF RADIOACTIVE NUCLIDES
ELL
ions
s in which the ratio of the two
as materials are leached from the plant-base zone into the soil, plant-base
absorption gives place to indirect entry through roots.
Whereas the extent of foliar or floral absorption will depend on the
quantity of fission products deposited during the growth of the individual
een
4 influences the relationship betw
in
both
wn
sho
been
has
It
.
ered
nsid
of
n
atio
: variations in the concentr
that
rption of strontium 90 provided
ing on
iters plants as a result of lodg
soil.
istinction to absorption from the
ues
tiss
of
n
atio
to the direct contamin
.
ial lodgement.
studies
eresting aspects of food chain
90 can
m
ntiu
stro
h
whic
to
nt
exte
tive
sidepo
us
inder conditions of continuo
this
in
rs
ente
h
lout. The amount whic
rption
ly in the recent past while abso
lknow
t
hou
Wit
ein.
ther
l
tota
mulative
to
le
ssib
ese two processes it was impo
de fallout.
m consequences of world-wi
ge
th dietary contamination would chan
the
on
nds
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,
tion
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occurs particularly with perennial plants which are the main components
of many pastures.
The extent of direct contamination is also influenced by the physical
form of the deposit, especially its particle size and its solubility. Large
particles usually rebound from leaf surfaces, being retained only if they
lodge in re-entrant angles; it has been found that, in general, particles in
excess of about 40, in diameter are little retained (28). Particles con-
siderably in excess of these dtmensions will occur in the near-in fallout
from nuclear weapons but not at considerable distances. Insoluble materials
are a source of superficial contamination only. The greatest contamination
of plants will therefore be caused by finely divided soluble deposits; worldwide fallout is in this form. It is important to realise that the findings on
its behaviour are not applicable to large, sparingly soluble particles, which
may be the major constituent of near-in fallout.
Perennial pastures—-The mechanisms responsible for the contamination
of pastures have received particular attention because of the importance
of milk as a source of strontium 90 in diet. Information has been obtained
both from experiments and from widespread surveys of strontium 90 in
milk; since the OR (milk/diet) is about 0.11 (2, 19) changing levels in
grass can be reasonably inferred from those in milk.
Observations at nuclear weapons trials and experiments both in the
laboratory and in the field suggest that 20 to 30 per cent of finely divided
deposits or solutions may be initially retained on the edible tissues of pasture 4 to 6 inches high (29, 30). Bartlett et al. (31) have analysed the results of a series of experiments in which strontium 89 was applied as a fine
spray to pastures which were exposed to the normal summerclimate of the
United Kingdom after the spray had dried. Losses were variable, but the
quantity remaining on the grass could be expressed by a simple exponential
relationship of the form y = e-**, x being the numberof days after spraying
and y the fraction of the initial deposit which remained on the herbage.
The value of the constant k was computed as 0.05 with the standard error
of +0.02; thus some 50 per cent was on average lost within 14 days and
about 90 per cent after 50 days. It appears that strontium is removed from
leaves to about the same extent as other carrier-free nuclides (8) and the
extent of loss computed by Bartlett ef al. (31) is close to that estimated
after the accidental release of iodine 131 from Windscale Works in 1957.
In two weeks after that accident the levels of iodine 131 in herbage
appeared to decrease by about twice the amount attributable to the decay
aRion
condistinguish three types of direct
es,
leav
by
n
ntio
‘ar contamination : rete
conbase
t
plan
(¢)
by inflorescences,
ace
by the basal parts of plants, or surf
.
soil
the
into
ated
rpor
inco
be
ats by the first two routes will not
with
ly
eous
ltan
Icium may enter simu
when
it to a much smaller extent than
these
of
ce
rtan
impo
ive
relat
soil. The
reas
Whe
s.
on the growth form of plant
tical
prac
of
is
on
pecies, floral absorpti
imarly
icul
part
is
on
rpti
plant-base abso
is
It
.
them
to
ined
conf
ily
ssar
1 not nece
and
on
ndaries between foliar absorpti
of conring rain the downward washing
rpabso
with
ntly
urre
lant will occur conc
,
wise
Like
y.
entr
se
t-ba
plan
m merge into
MSNLmaR ae + WEBI « =!
S.
n” 1s
The term “direct contaminatio
OE mE tend 8
can be
itium are toxic, the carrier ion
voir after it has been retained there for an appreciable period of time. This
fe bape ok
kaki BMSDat GGNER Ce TE BEET + oe seagate
Since
tion of strontium plus calcium.
less,
or
100
to
1
lly
usua
is
ium
calc
to
leaf or inflorescence, material may be absorbed from the plant-base reser-
Jen
appears
m is low (10, 25, 26). Entry
2/79
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