ala rate greater than radioactive decay from
1954 ta the presen: (Ne7?: Ne?9: Br&2). Activitv concentrations of '*’Cs and *’Sr tn vegetation

were observed to decline at a rate greater than
that predicted bs. radioactive decay alone (Ne77,.
Ne’?9} Body burdens and urine acuvity concentrauons were observed to increase tapidiv and
to decline slowiv throughout the residence ume
of persons at Rongelap and Ltink Atolls (Co73:
Le8Qb). These observations led to the seiection
of a dechming conunuous intake mode’.

An exponentia, decline in the amount of

activity ingested each
sources was assumed.
equations were derived
applied to each nuchde

dav from the dietan
The foilowing general
(LekOb!. They mav be
of concern.




AP =



H—. (])



mie wm Alt

pe kn


Using adult average data, two consecutive
urine or bodv-burden measurements were used
to estimate the unknown value of 4. a rate
constant describing the removal of radioactivity

in diet items. This vielded n — | estimates of k

where x as the number of measured adult

averaze data points for body burden or urineacuvity concentration during the residence in-

terval, An average value of & was assigned for
the enure residence interva] during which activity Was measured. After the average & was
obtained. an estimate of the atom ingesuon rate
on dayof return was calculated based on a value
for adult average body burden or urine-activity
concentration and the time since dav of return.

Tats generatec n values of the atom ingestion
Tate on day of return where m was again the
number of adult average data points for body

burden or urine-activitv concentranon.
As indicated by equations (1) or (2). a single
exponentiai relationship was used to model the

decline of radioactivity in diet items. Use of
these equations led to an estimate of the dietary


removal rate constant, k. over the enure residency inter-al. The average percent decrease in

g-9' (Sve)
tA mei
f,a (SAQe
e moh

the vearly activity ingested was determined from
this dietary rate constant as follows:


day of return of each atoll population), d;
A= instantaneous fraction of atoms decaying

per unit time. d~'; P = initial daily atom inges-

tion rate on dav of return. atoms s7~';
A = instantaneous fraction of atoms removed
from compartment / in the body by biological

processes, d~*; y;= compartment 7 deposition

fraction; y’ = the numberof radioactive atoms
in compartment / relative to the numberin all

compartments on the day of return (some persons returned with body burdens); U = 24-hr or
I-l. urine-activity concentration at any time
post-return, Bq !~—'; U, = subject urine excretion

rate, |}d~'; f, = fraction of element transferred

from GI tract to blood; f, = fraction of element
reaching extracellular fluid that is excreted
through the urine pathway: k = instantaneous

fraction of atoms removed from the atom inges-

tion rate per unit time, d~', due to factors other

than radioactive decay; g = body burden at any
time post-return, Bq; and g° = body burden on
the day of return, Ba.

vo = 100(1 ~*~ *),


where °, = average percent decline in the atom

mgestion rate during the residence interval

The definitions of the other quantities in
equation (3) were the same as previously given.
The value of 1 was taken at 365 days and the

percent reflected the average yearly decline averaged throughout the interval during which a

nuclide was observed in people. Thus for "Cs,
the average was for a period of 24 yr at Ron-

gelap and 27 yr at Utink.

In the development of the three equations,
several assumptions were made. For instance,
decay of nuclides during transit through
the stomach and small intestine was
assumed to be negligible relative to their
decay within the systemic organs. This was
because of the long half-life of the nuclides

relative to the transit time through the upper

portion of the gastrointestinal tract. Urine activity and body-burden data were assumed to


where ¢ = time post-onset of intake (time from

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