{80
DIETARY RADIOACTIVITY INTAKE FROM BIOASSAY DATA
Uit)
instantaneous urine activity concen-
f,
fraction of atoms in GI tract reaching
blood
male or female urine excretion rate, |
U,
tration, Bq 1"'
dq
Assumptions
=0.7d0°!
=0.85
=0.15
=6.33x 10% d7!
U, = 1.41.d7' (male)
U, = 1.01d7' (female)
F, =0.9.
dN, _
7 (A+k) N,
P,
(1)
Given the previously described assumptions
gi(ty=3,4q,(t)
(2)
F=—,
3)
Substituting the above values into equation
(4) and summing over two compartments
yields an expression which relates the daily
k, +A-— N,
_ _p
E, (t)=
dq,tt)
Fa
(4)
E(t)=%3,E,(t)
(S)
=P,
P=
i,
6
(6)
U(t) U, = F,E(t)
(7)
ANALYTICAL SOLUTION
The instantaneous atom rate of change in
compartment i is described by assumption I.
‘Solving the differential equation for an
analytical solution yields.
=
Oo artaArkt
P,
Ni{(t)=N%e
(Ark)!
—
4
e Arigt ) (2)
and the body burden contribution from the
ith compartmentis,
gilt) =AN(t) = AN%e Oth
AP,
+ (A +k,)
a — etki)
(3)
From assumptions 3 and 4, the activity
excretion rate from the ith compartment is
k
) -AP,) ere,
EW) = VTE PIN, (q°(A +k
(4)
Assuming a two compartment model for
'7Cs, the following values are obtained from
ICRP Publication 10A (ICRP71) and ICRP
Publication 23 (ICRP75):
fi
ky
.
k.
kK,
X,
A
I
0.006 d-'
lett =Id,X,=1-X3, and X; = 0.002.
activity ingestion rate for "Cs to the '“Cs
body burden and excretion rate at the times
of counting and sampling respectively. The
daily activity ingestion rate cannot be algebraically isolated from the resulting equation
with ease. Therefore the r. h. s. of the equation is evaluated by using an estimate for this
quantity. This evaluation is compared to the
urine activity excretion rate and if they are
unequal the r.h.s. is reevaluated after changing
the estimate for the activity ingestion rate. The
process is repeated until the evaluation of the
r.h.s. and the urine activity excretion rate differ
by less than 0.1%. Table I lists the individual
daily activity ingestion rates as calculated by
this method.
RESULTS AND DISCUSSION
If the loss of '"Cs via perspiration and
insensible losses is neglected during the
counting interval and a 24-hr urine sample is
begun immediately after counting, then there
exists a mechanism to calculate the uptake
during the 24-hr sampling period withoutfull
knowledge of the uptake interval.
Of the 68 urine samples collected in April
1978, only 26 samples were determined to be
of sufficient volume to be considered 24-hr
samples. The measured daily excretion rates,
measured body burdens and calculated daily
activity ingestion rates are presented in Table
1. The total error on the body burden
measurement is estimated to be +25% while
the error on the excretion values exclusive of
sample fluctuations is +!0%. The mean daily
ingestion rate as calculated from the body
burdens and excretion rates in Table | is
2100 Bqd™! (57 nCid™').