UPTAKE OF RADIOACTIVE NUCLIDES
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again likely to be the dominant source of concern, followed probably by
strontium 90 (6, 7).
Characteristics of radioactive substances which cause them to be wmportant sources of internal radiation—Therelative importance of individual
radioactive nuclides as sources of internal radiation depends on many factors apart from the quantities in which they are released into the environment. Of these the major are: (a) the extent to which they are transferred
through food chains into diet; () the extent to which they are absorbed
I
From NATURAL BACKGROUND AND
RATES IN MREM/YEAR?
World-wide fallout from nuclear
weapons tests up to 1961
July 1959|
UK only
fraction of
total dose
not
1954-2000
until after
delivered
dose rate
AD
2000 AD
from the gastrointestinal tract into the body; (c) the extent to which they
FPneile oi
dose rate
average
annual
275
are accumulated and retained in tissues; (d) their half-lives; and (e) the
type and energy of the radiation they emit.
In practice the nuclides which are readily absorbed from the gastrointestinal tracts of animals are either isotopes of elements important in
metabolism or closely similar to them. Potassium 40 and caesium 137, both
alkali metals, are absorbed and circulate freely throughout the body and,
10
4.2
0.65
3%
0.2
1.5
0.16-0.26
0.24-0.42
90%
Nil
8.1
1.6
9%
2.7
0.81
9%
6-14
1.3
—
because of the gammaradiation they emit, irradiate all tissues; they are potential sources of genetic injury. Iodine 131 accompaniesstable iodine to the
thyroid gland. Strontium 90 and radium, both alkaline earths like calcium,
pass with it to the bone. By virtue of their long biological retention and long
half-lives (strontium 90, 27 years; radium 226, 1620 years; radium 228, 6.7
years) the bone and the bone marrow are irradiated for long periods.
Carbon 14 becomes distributed throughout all living tissues. Because the
basic nature of metabolic processes in plants and animals is similar, nuclides
which are readily accumulated in animal tissues are usually those which pass
most freely through food chains.
Of the nuclides mentioned in the previous paragraphs, iodine 131 and
potassium 40 can be excluded from the present discussion. The short half
life of iodine 131 (8 days) makes its absorption by plants of little importance; it enters food chains mainly through the direct contamination of
plants, lodging on them in a mannersimilar to strontium 90 (8). Although
potassium 40 is the main source of internal radiation to which man has yet
been exposed, the considerable literature on the absorption of this element
Report (2), p. 21.
makes unnecessary its inclusion in this review. Attention will therefore be
).
id on UN Report (2), p- 27.
given mainly to strontium 90, caesium 137, radium and other members of
the uranium and thorium series, and carbon 14. Other nuclides, however,
cannot be always ignored; they are considered briefly in the final section.
however, not the only source of en-
.
rom nuclear reactors may also occur
wee er ee nee ee TR Sree oe
aa
SrroNTIUM. 90
This nuclide has received considerably more attention than any other in
food chain studies. Before its behaviour had been studied experimentally it
was natural to speculate on the basis of its chemical similarity to calcium.
Mee
From some respects this analogy was misleading. It undoubtedly encouraged
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blishments, iodine 131 is likely to be
ed into the atmosphere. Dilute radioproducts and induced radioactive sub-ontrolled levels which make it an in: population. Accidents at nuclear re
higher levels of local contamination.
studies indicate that internal and not
risk to the population; iodine 131 is
the belief, now disproved, that strontium 90 would always enter man’s diet
mainly, like calcium, from the soil. The comparison of the two elements,
however, led also to the view, since amply vindicated, that the major sources
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