human bone will be in equilibrium with regard to strontium-90/calcium
ratios peculiar to their particular position in the chain.
The continued availability of strontium-90 which has been
created but has not yetfallen out, looms large as one of ouwr most
important present uncertainties in the world-wide deposition problem.
Since new weapons tests continue to make available new strontium-90,
the question of the rate of uptake of strontium-90 into the biosphere,
year after year, of that deposited at any one time is difficult to resolve.
If the strontium-90 deposited is actively transported to bone
only for a relatively short period of time, say one or two years, then
the amount of strontium-90 which it is estimated may be deposited on
' soil without constituting a hazard can be raised considerably and, in
fact, the strontium-89 would be an important contributor of radiation
dose to bone on a gram-for-gram basis of produced material.
The very rapid uptake of strontium-90 by cattle due to direct
ingestion of fall-out material is not significant with regard to the
ultimate hazard calculation in terms of large scale fall-out over a
limited period of time.
The cattle bones wmder these circumstances
would be taking up strontium-90 rapidly for a limited time only.
mechanism would probably spare the milk to some extent.
This
After the
first cycle, the regular biospheric chain should take over.
The prob-
lem of continued uptake is thus more important than the short peak of
uptake immediately following fall-out.
This discussion has omitted serious consideration of the
problem of stratospheric storage and a continued slow fall-out over the
years.
If this mechanism is significant quantitatively, then a larger
fraction of strontium-90 produced must be considered available for
rapid uptake, than is available in the early fall-out period, even
though this availability in time may be many years post-detonation.
It must be remembered that the current 1.0 Sunshine Unit now
being found in the younger age group in this country should not neces-
sarily rise with continued growth.
Since this is a strontium-90/calcium
115