data were obtained are truly representations of likely operational conditions, and to the difficulties involved in collecting an adequate num-
ber of samples at enough points to give a good degree of statistical
reliability.
The only surface test detonations of major operational
significance thus far have been made in the Pacific Proving Ground,
where the amount of land surf'ace available and the character of the
s0il involved are not representative of likely employment conditions
to be met in the operational use of nuclear weapons.
Many of the un-
certainties which now exist could be resolved if the opportimity is
ever presented to have a true, large yield land-surface detonation.
These gaps in the current fund of knowledge are not considered suffiecient to change the basic ideas presented here but only to provide refinements necessary to give a better understanding of the potentialties
involved.
H.
Evaluation of Fall-out Models.
A fall-out model is a graphical representation capable of
utilizing available information to predict the perimeters of the local
area that is likely to involve a residual radiation hazard to health
following the detonation of an atomic device.
Ideally, it should pre-
dict with maximum accuracy from a minimum of information and be work-
able in the least possible time before or after the device is detonated.
At present, some of the various models under development are used to
insure the continuity of the health and security of the personnel and
animals of the two nuclear weapon test sites and their surrounding
communities.
The predictions of any of the methods can be checked by
survey and sampling, and the degree of reliability determined for its
practical use in denying areas of danger following a hostile detonation.
At Nevada, local fall-out has accounted for approximately 10% to
20% of the fission yield of low tower shots.
With the super weapons
tested at the Pacific Proving Ground, local fall-out accounts for as
much as 50% of the fission products foumd and encompasses whole island
groups.
50