-2percentage probably receiving doses of 2 to 3 rads. The iodine isotopes
of shorter half-lives contributed an additional 10% to 40% of the dose
from iodine-131.
From strontium-90, persons born during these years will experience a
70-year dose to the bone of about 1 rad to which would be added about
0.055 rad from strontium-89.
It will be recalled that in 1957, the ACBM prepared a statement for the
Commission on the chief problems presented by radioactive fallout.
This
was subsequently published by the American Scientist, and a reprint for
your information is forwarded with this memorandum. It is interesting
to find that the assessments of eventual doses to the whole body and to
bone which were made at that time are still reasonable. The actual
figures at the present time which are based on the total fission yield
of all tests up to the cessation of tests in 1958 come reasonably close
to our original estimates of the situation based on the assumption that
all tests were to have stopped in the fall of 1957. This comes about
because it is now apparent that the stratospheric burden is substantially
less than had been previously estimated, and that in fact most of the
,
material is already down.
With radioisotopes of long half-life, such as sr’? there is a slow
diffusion downward in the soil into an ever larger pool of their stable
counterparts, With the passage of years the radioelements of chief
concern not only become increasingly diluted but more firmly bound in
relatively insoluble forms.
Both mechanisms decrease their availability
to the root systems of plants and account, in part, for the tendency of
root uptake to be less than that usually predicted,
In soil contaminated by fission products, cesium-137 and strontium-90
are the most important isotopes in plant absorption. This is because
these isotopes act chemically very much like potassium and calcium,
respectively. The cesium-137 therefore is in competition with potassium
and the strontium-90 with calcium.
Accordingly, plants take up more
cesium-137 from soils poor in potassium and more strontium-90 from soils
poor in calcium. Or, stated in reverse, uptake of cesium-137 in poor
soil can be inhibited by applying potassium and uptake of strontium-90
by applying calcium.
Cesium-137 and strontium-90 are often adsorbed tightly onto the clay
particles in soils.
If they are bound tightly, they are less prone
to be absorbed by plants, but also less likely to be leached downward
into the lower soil and out of reach of roots,
Conversely, if weakly
bound, the isotopes enter plants more readily and leach away more
rapidly.
Some soils,
therefore, give up isotopes to the plants fairly