SMOKY external gamma-exposure fields based upon the individual
radionuclides and their gamma emissions, and has shown that po #38 is
a better approximation over longer time periods.
|
_ Also, the use of this infinite exposure model does not represent
realistically the exposure received by people because no provision is
fade for the shielding provided by residences, workplaces, schools, or
automobiles.
These twoproblems were recognized and addressed by Dunning
(Du57a,b). ‘Based upon measurements of the external gamma-exposure
field on the [stand of Rongelap over a two-yr period (reproduced in
Fig. 1), Dunning developed the following mode] as a more realistic
fpN
expression of the externgl\ exposure rate in a real open-field situation
where fallout is weatheringtinto soil:
a
a.
cr
=
R(1) t"'*4 fom”st 168 hr
R(t) = <bR(1) 7°? “1.3 for 168.hr< t < 336 hr
[7]
cR(1) t “1.4 for 336 Hr <_t
where b and c are constants required forcontinuity.
The estimated exposure (EE) experiencedby people over a one-yr
period is then calculated as
FE =5§
~
8760
a
.
R(t) dt
TT
_
where 3 is a building shielding factor of 0.75.
(8)
Tirts-was_based upon
the experimental observation that buildings reduce exposure by an
average factor of two (DuS7a) and the assumption that people are in
buildings half the time.
The solution of the above is
207