The principal difference between our method and the method of Gilbert et al.
(1976) are:
{a} our estimate of the vegetation ingestion rate based on energy
requirements (8 kg/day) is higher than their estimate based on the rumen
multiplied by the fraction transferred from the gastrointestinal tract to the
blood. For the Area 13 cow discussed above,
contents of a fistulated steer (6 kg/day), (b) our estimate of the average
fyy = Iyfgh = 565,000 pci/day x (3 x 1079)
concentration of plutonium in vegetation is lower than theirs, 55 pCi/g versus
103 pCi/g, and {c) we included an input for soil ingestion.
If we assume that the digestibility of Area 13 vegetation is 49 percent instead
of 36 percent, a ration of 6 kg/day would be adequate to meet the cow's maintenance energy requirements.
The weighted mean vegetation/soil ratio for Area 13
is about 0.15 or 50 percent higher than average.
If we use this ratio instead
of 0.1 and 6 kg/day instead of 8 kg/day, Ly = 0.495 wCi/day and Io = 0.565 0.495 = 0.07 pCi/day, or 127 g (soil}/day.
,
= 16.95 pCi/day.
where vy
and
is the rate at which
£
ingested Pu reaches the blood
(pCi/day)
is the fraction transferred from the gastrointestinal tract
to blood.
As this cow was left in the inner compound of Area 13 for 177 days before
sacrifice (Smith, 1977), the total 732Pu expected to have entered the blood
via ingestion would be 3,000 pCi.
Inhalation Rate
Standard Man's respiration rate is 20m3/day (LCRP, 1959) and his digestible
energy requirement for maintenance metabolism (no weight gain or loss) is
2,600 kcal/day (NRC, 1968). We assume that for man and cattle, respiration
rates are proportional to digestible energy maintenance requirements.
The DE
requirement (Equation (19)) for a 409-kg cow is 13,185 kcal/day. The cow's
estimated respiration rate is therefore 13,185 x 20/2,600 = 101 m3/day.
The cow’s plutonium inhalation rate (T,) can be estimated by
a
_ 20
DE
bo *
3600
La * &s
The transfer of inhaled ptutonium to blood is considerably more complicated
than indicated by Figure 2. Assuming resuspended particles to have an AMAD of
0,5 um, and applying the ICRP-19 lung model (Figure 4 and Table 11) to the
cow, we obtain the following expression for the rate at which inhaled plutonium
would be expected to enter the blood:
where, I, is the plutonium inhalation rate (pci/day),
DE is the digestible energy required (Equation1 for maintenance
metabolism (kcal/day),
43
20m3/day divided by 2600 kcal/day = 7.69 x 10°77 m°/kcal for
Standard Man,
L, is a mass loding factor (100 ug(soil)/m3(air)) as recommended
by Anspaugh (1974),
C. is the average plutonium concentration (pCi/g) in the soil of
thie area grazed by the cow.
For the 409-kg cow of the Area 13 inner enclosure, where cC. = 550 pCi/g, T =
5.56 pCi/day.
Ingestion Versus Inhalation
>> I_, but the accumulation
It is obvious from the preceding discussion that I.
of plutonium in organs or tissues other than the gastrointestinal tract or
Lungs and the excretion of plutonium in milk depends (Figure 2 and Equation a7)
The rate at which ingested
on the rate (r.) at which plutonium reaches blood.
plutonium reaches the blood is simply the plutonium ingestion rate (1)
648
Tha = 90-0021 Ty + 0.0833 2p yy Any, +3 x 107 yy
(22)
dy,/dt = 0.18 E - Ayyy
(23)
dy,/dt = 0.225 kay, - Age
(24)
dyg/dt = 0.75 [1,(0.2079 + 0.12/4,) + 0.667 Atyy]
(25)
21
(21)
where, Tp, is the rate (pCi/day) at which inhaled plutum enters the
blood,
0.0021 is the fraction of inhaled plutonium transferred directly
te blood from the upper respiratory tract,
06,0833 is the fraction of plutonium deposited in the Lungs and then
transferred to blood,
¥Y, 1s the amount (pCi) present in the lung at time t,
My = I1n(2}/500 days is the lung clearance rate,
ys, 1s the amount (pCi) present in the lymph at time t,
Ay = in(2)/1000 days is the lymph clearance rate,
y,
ig the amount (pCi) present in the gastrointestinal tract at
time
t,
0.18 is the fraction deposited in the lungs and cleared with a
500-day half-life,
0.225 is the fraction transferred from lung to Lymph and then cleared
to blood with a 1000-day half-life,
0.75 day is the average residence time of plutonium in the gastrointestinal tract,
0.2079 ts the fraction transferred directly from the upper respiratory
tract to the gastrointestinal tract,
649